Ubuntu: apt upgrades failing with “Cannot initiate the connection to ports.ubuntu.com”

While doing a distro upgrade with

# do-release-upgrade

I kept getting failures half-way stating

Cannot initiate the connection to ports.ubuntu.com:80

The errors showed several IPv6 addresses that couldn’t be reached. My router supports IPv6, but not my ISP. Somehow I was expecting that the router would be doing the translation or DNS resolution between the two but this wasn’t the case.

Disabling IPv6 on the router didn’t to much. I have a recollection that some of the services I am running on my Ubuntu server require IPv6 enabled or else the OS breaks. So it can’t be disabled for the whole OS.

Luckily you can configure apt to only use IPv4:

# apt-get -o Acquire::ForceIPv4=true update

This will automatically refresh the sources and next time you run apt it will complete the upgrade. If not, your problem lies somewhere else.

Ubuntu: apt error message “Key is stored in legacy trusted.gpg keyring”

After upgrading to Ubuntu 22.04 running apt shows an error message saying “Key is stored in legacy trusted.gpg keyring“:

# apt update

All packages are up-to-date.
W: https://apt.syncthing.net/dists/syncthing/InRelease: Key is stored in legacy trusted.gpg keyring (/etc/apt/trusted.gpg), see the DEPRECATION section in apt-key(8) for details.

The key needs to be exported from the legacy keyring and then imported back to the current system.

List the keys and find the key ID of the repository that is showing the error. In this case it is Syncthing.

# apt-key list

pub   rsa2048 2014-12-29 [SC]
      37C8 4554 E7E0 A261 54E7  6E1E D26E 6ED0 0065 5A3E
uid           [ unknown] Syncthing Release Management <release@syncthing.net>
sub   rsa2048 2014-12-29 [E]


Copy the last 8 characters of the key (00655A3E) and export it.

# apt-key export 00655A3E | gpg --dearmour -o /usr/share/keyrings/syncthing.gpg

Update the source file for the repository adding the exported key.

# vim /etc/apt/sources.list.d/syncthing.list

deb [arch=amd64 signed-by=/usr/share/keyrings/syncthing.gpg] https://apt.syncthing.net/ syncthing stable #Syncthing

Confirm that the error message is no longer showing.

# apt update

Hit:5 https://apt.syncthing.net syncthing InRelease                                                     
Reading package lists... Done
Building dependency tree... Done
Reading state information... Done
All packages are up-to-date.

Finally, remove the old signature.

# apt-key del 00655A3E

Linux: Booting in single-user mode

Sometimes it might be necessary to start in single-user mode to do some administration work, or even reset an existing password.

Normally this can be achieved via the GRUB boot loader.

CentOS / RedHat (with root account enabled)

Switch on your system.

Press Esc until the GRUB menu shows up.

This will bring up the GNU GRUB menu. If the CentOS/RedHat logo/boot messages show up you will need to restart (Ctrl-Alt-Del) and try again.

Select the OS/boot you want to edit. Normally the first line. Press e to edit it.

CentOS Linux (3.10.0-1160.53.1.el7.x86_64) 7 (Core)
CentOS Linux (3.10.0-1160.45.1.el7.x86_64) 7 (Core)
CentOS Linux (3.10.0-1160.42.2.el7.x86_64) 7 (Core)
CentOS Linux (3.10.0-1160.41.1.el7.x86_64) 7 (Core)
CentOS Linux (3.10.0-1160.36.2.el7.x86_64) 7 (Core)
CentOS Linux (0-rescue-d0401f7cdedb4955a0a262b3e0054323) 7 (Core)

Use the ↑ and ↓ keys to change the selection.
Press 'e' to edit the selected item, or 'c' for command prompt.

You will need to find the entry for the kernel. Normally starts with linux16.

linux16 /vmlinuz-3.10.0-1160.53.1.el7.x86_64 root=/dev/mapper/centos_centos7-root ro rd.lvm.lv=centos_centos7/root rd.lvm.lv=centos_centos7/swap rhgb quite LANG=en_US.UTF-8

At the end of the line you can choose to add one of the following:


So that it looks like this:

linux16 /vmlinuz-3.10.0-1160.53.1.el7.x86_64 root=/dev/mapper/centos_centos7-root ro rd.lvm.lv=centos_centos7/root rd.lvm.lv=centos_centos7/swap rhgb quite LANG=en_US.UTF-8 single

Type Ctrl-X to exit and the system will start in single- user mode.

If required, remount the root filesystem:

# mount -o remount,rw /

[If there are other filesystems you need to mount from fstab:]

# mount --all

CentOS / RedHat (without root account enabled)

It might be that your system didn’t have a root account enabled, in which case the above steps will fail. There is a workaround.

Switch on your system.

Press Esc until the GRUB menu shows up.

This will bring up the GNU GRUB menu. If the CentOS/RedHat logo/boot messages show up you will need to restart (Ctrl-Alt-Del) and try again.

Select the OS/boot you want to edit. Normally the first line. Press e to edit it.

CentOS Linux (3.10.0-1160.53.1.el7.x86_64) 7 (Core)
CentOS Linux (3.10.0-1160.45.1.el7.x86_64) 7 (Core)
CentOS Linux (3.10.0-1160.42.2.el7.x86_64) 7 (Core)
CentOS Linux (3.10.0-1160.41.1.el7.x86_64) 7 (Core)
CentOS Linux (3.10.0-1160.36.2.el7.x86_64) 7 (Core)
CentOS Linux (0-rescue-d0401f7cdedb4955a0a262b3e0054323) 7 (Core)

Use the ↑ and ↓ keys to change the selection.
Press 'e' to edit the selected item, or 'c' for command prompt.

You will need to find the entry for the kernel. Normally starts with linux16.

linux16 /vmlinuz-3.10.0-1160.53.1.el7.x86_64 root=/dev/mapper/centos_centos7-root ro rd.lvm.lv=centos_centos7/root rd.lvm.lv=centos_centos7/swap rhgb quite LANG=en_US.UTF-8

At the end of the line add this:


So that it looks like this:

linux16 /vmlinuz-3.10.0-1160.53.1.el7.x86_64 root=/dev/mapper/centos_centos7-root ro rd.lvm.lv=centos_centos7/root rd.lvm.lv=centos_centos7/swap rhgb quite LANG=en_US.UTF-8 rd.break

Type Ctrl-X to exit and the system will start emergency mode in read-only mode.

Remount the root filesystem as read/write:

# mount -o remount,rw /sysroot

Switch to the sysroot jail.

# chroot /sysroot

Reset the password, or do any required tasks.

If you have SELinux enforcing mode enabled it will protect the OS from any changes. After you change the password type the following to overcome this:

# touch /.autorelabel


# reboot -f

Ubuntu / Debian

Switch on your system.

Press and hold the Shift key.

In some instances pressing the Esc key several times (instead of holding it) achieves the same result. Just be aware that if you press it too many times it will bring you to the GRUB CLI. You can type normal and you will get to the menu described below.

This will bring up the GNU GRUB menu. If the Ubuntu logo/boot messages show up you will need to restart (ctrl-alt-del) and try again.

Select Advanced Options on the GRUB menu.

			GNU GRUB version 2.04

 Ubuntu 20.04.4 LTS
*Advanced options for Ubuntu 20.04.4 LTS
 History for Ubuntu 20.04.4 LTS
 UEFI Firmware Settings

And select the recovery mode option. Normally the latest kernel installed on your system.

			GNU GRUB version 2.04

 * Ubuntu 20.04.4 LTS, with Linux 5.13.0-37-generic
** Ubuntu 20.04.4 LTS, with Linux 5.13.0-37-generic (recovery mode)
 Ubuntu 20.04.4 LTS, with Linux 5.13.0-35-generic
 Ubuntu 20.04.4 LTS, with Linux 5.13.0-35-generic (recovery mode)

This will boot the system and show a series of options. Select root.

Recovery Menu (filesystem state: read only)

	resume			Resume normal boot
	clean			Try to make free space
	dpkg			Repair broken packages
	fsck			Check all file systems
	grub			Update grub bootloader
	network			Enable networking
	root			Drop to root shell prompt
	system-summary	System summary


This message will show. Press Enter.

Press Enter for maintenance
(or press Ctrl-D to continue)

If your / volume is ZFS it will be already read and write. Other filesystems might start in read only mode. If so, remount:

# mount -o remount,rw /

[If there are other filesystems you need to mount from fstab:]

# mount --all

Changing an user’s password

# passwrd <username>

Adding a new user

In the rare event of not having an user, you can add one and give it sudo privileges.

# useradd <username>

# usermod -a -G sudo <username>

# passwd <username>

Other OS

You can find description on how to get to single-user mode for other Linux distros in this Microsoft’s article.


ZFS ‘Failed to start Mark current ZSYS boot as successful’ fix

On Ubuntu 20.04 after installing the NVIDIA driver 510 metapackage the system stopped booting.

It will either hang with a black screen and blinking cursor on the top left or show the following error message:

[FAILED] Failed to start Mark current ZSYS boot as successful.
See 'systemctl status zsys-commit.service' for details.
[  OK  ] Stopped User Manager for UID 1000.

Attempting to revert from a snapshot ends up with the same error message. This wasn’t the case on another separate system that had the same upgrade.

The “20.04 zys-commit.service fails” message is quite interesting and it seems that the overall cause is a mismatch of user/kernel zfs components.

These are the steps I followed to fix it. Many thanks to Lockszmith for his research in identifying the issue and finding a fix. He created two posts raising it, links provided here.




Restart Ubuntu and boot in recovery mode


*Advanced options for Ubuntu 20.04.3 LTS

[Select the first recovery option in the menu]
*Ubuntu 20.04.3 LTS, with Linux 5.xx.x-xx-generic (recovery mode)

[Wait for the system to load the menu and select:]

[Press Enter for Maintenance to get the CLI]

Check the reason for the error.

# systemctl status zsys-commit.service
 Feb 17 11:11:24 ab350 systemd[1] zsysctl[4068]: level=error msg="couldn't commit: couldn't promote dataset "rpool/ROOT/ubuntu_733qyk": couldn't promote "rpool/ROOT/ubuntu_733qyk": not a cloned filesystem"

Attempting to promote it manually fails:

# zfs promote rpool/ROOT/ubuntu_733qyk

cannot promote `rpool/ROOT/ubuntu_733qyk` : not a cloned filesystem

Uninstall the NVIDIA drivers.

# dkms uninstall nvidia/510.47.03
# dkms remove nvidia/510.47.03 --all

Make sure you can connect to the internet. You can temporarily assign a DHCP address to one of the network interfaces.

# dhclient -v eno1
# ip address

Update the system and install a 3rd party ZFS set of tools.

# apt update
# apt upgrade
# apt autoremove

[Add 3rd party PPA for zfstools]
# add-apt-repository ppa:jonathonf/zfs
# apt update 

[Upgrade ZFS]
# apt upgrade

[If ZFS isn't upgraded, do it manually]
# apt install zfs-initramfs zfs-zed zfsutils-linux

It might take a bit to update. Reboot normally.

# reboot

It should boot normally.

If this doesn’t work for you, reboot in recovery mode again and promote the filesystem manually.

# zfs promote rpool/ROOT/ubuntu_733qyk

Sort graphical drivers

Revert to NVIDIA metapackage 470 (if this is what broke your system). Reboot, and fix resolution settings.

Upgrading back to 510 will bring the error back and make it even more difficult to fix. Don’t!

Things will only work if zfs and zfs-kmod match versions.

$ zfs --version
[boot in recovery mode]
# apt reinstall zfs-initramfs zfs-zed zfsutils-linux
# zfs promote rpool/ROOT/ubuntu_733qyk

[reboot in normal mode]
[Configure the 470 drivers]

Reverting to previous ZFS version

The system should now be back to normal, but you might want to revert to the mainline ZFS version despite the bug. After all, this was a hack to promote the filesystem and get it back to work.

# add-apt-repository --remove ppa:jonathonf/zfs

[Check that is has been removed]
$ apt policy

# apt update

# apt remove zfs-initramfs zfs-zed zfsutils-linux
# apt install zfs-initramfs zfs-zed zfsutils-linux

[Check the right version is installed]
# apt list --installed | grep zfs

# apt autoremove

[Pray harder]
# reboot

With that I managed to bring my system back to a working condition, but updating the drivers a second time made it fail again and I couldn’t fix it. A clean install of 20.04.3 doesn’t seem to exhibit this problem. Not sure what is the reason behind it but there are a few bugs open with Canonical regarding this.

I hope that 22.04 will bring a better ZFS version.

Linux / Unix: Comparing differences between folders

I had to check the file changes between two Backintime snapshots recently. You can always use rsync for that, but there is a more straightforward way by using diff.

$ diff -qr directoyr-1/ directory-2/

-q will display only the files that differ.

-r will make the comparison recursive.

There is a GUI application called Meld that provides similar functionality, but the diff approach will work anywhere and requires memorising less flags than rsync.

Raspberry Pi : Configuring a Time Capsule/Backintime server

In this post, I am setting up a Time Capsule and Backintime server. I am using a Raspberry Pi that has Ubuntu installed, with a USB disk that has been configured into a ZFS pool.

Setting up backup users

You are going to have to create users for each of the services/users that will be connecting to the server. You want to keep files and access as isolated as possible. As in a given user shouldn’t have any visibility or notion of other users’ backups. We are also creating accounts that can’t login into the system for Time Machine, only authenticate.

Check if there is an entry for nologin in:

$ cat /etc/shells

If there is no entry add it:

# vim /etc/shells
# /etc/shells: valid login shells

Create a generic user for the backups, or dedicated accounts for each user to increase security:

Generic user example:

# useradd -s /usr/sbin/nologin timemachine
# passwd timemachine

Dedicated user example:

# useradd -s /usr/sbin/nologin timemachine_john
# passwd timemachine_john

Note that useradd doesn’t create a home

If required, the default shell can be changed with:

# usermod -s /usr/sbin/nologin timemachine_john

Setting up backup user groups

If more than one system is going to be backed up it is advisable to use different accounts for each.

It is possible to isolate users by assigning them individual datasets, but that might create storage silos.

An alternative is to create individual users that belong to the same backup group. The backup group can access the backintime dataset, but not each other’s data.

Create the group.

# addgroup backupusers

Assign main group and secondary group (the secondary group would be the shared one).

# usermod -g timemachine_john -G backupusers timemachine_john

Although not required, you could force the UID and GID to be a specific one.

# usermod -u 1012 timemachine
# groupmod -g 1012 timemachine

Time Capsule

Install netatalk

Install netatalk from the repositories.

# apt install netatalk

Allow access to all the appropriate accounts to the directory where the backups are going to be written to:

# chown :timemachine_john /backups/timecapsule/
# chmod 775 /backups/timecapsule/

Edit the settings of the netatalk service so that that share can be seen with the name of your choice and work as a Time Capsule server.

# vim /etc/netatalk/AppleVolumes.default

Enter the following:

/backups/timecapsule "pi-capsule" options:tm

Note that you can give the capsule a name with spaces above.

Restart the service:

# systemctl restart netatalk

Check that netatalk has been installed correctly:

# afpd -V

afpd 3.1.12 - Apple Filing Protocol (AFP) daemon of Netatalk
afpd has been compiled with support for these features:

          AFP versions: 2.2 3.0 3.1 3.2 3.3 3.4 
         CNID backends: dbd last tdb 
      Zeroconf support: Avahi
  TCP wrappers support: Yes
         Quota support: Yes
   Admin group support: Yes
    Valid shell checks: Yes
      cracklib support: No
            EA support: ad | sys
           ACL support: Yes
          LDAP support: Yes
         D-Bus support: Yes
     Spotlight support: Yes
         DTrace probes: Yes

              afp.conf: /etc/netatalk/afp.conf
           extmap.conf: /etc/netatalk/extmap.conf
       state directory: /var/lib/netatalk/
    afp_signature.conf: /var/lib/netatalk/afp_signature.conf
      afp_voluuid.conf: /var/lib/netatalk/afp_voluuid.conf
       UAM search path: /usr/lib/netatalk//
  Server messages path: /var/lib/netatalk/msg/

Configure netatalk

# vim /etc/nsswitch.conf

Change this line:

hosts:          files mdns4_minimal [NOTFOUND=return] dns

to this:

hosts:          files mdns4_minimal [NOTFOUND=return] dns mdns4 mdns

Note that if you are running Netatalk 3.1.11 or above it is not necessary any more to create the /etc/avahi/services/afpd.service. Using this file will cause an error.

If you are running an older version go ahead, otherwise jump to the next section.

Create /etc/avahi/services/afpd.service as root

# vim /etc/avahi/services/afpd.service

and fill it up with:

<?xml version="1.0" standalone='no'?><!--*-nxml-*-->
<!DOCTYPE service-group SYSTEM "avahi-service.dtd">
        <name replace-wildcards="yes">%h</name>

Configure the AFP service

Edit the configuration file.

# vim /etc/netatalk/afp.conf
; Global server settings
mimic model = TimeCapsule6,106

path = /backups/timecapsule
time machine = yes

Check configuration and reload if needed:

# systemctl status avahi-daemon	

[restart if necessary]
# systemctl restart netatalk

[Make the service automatically start]
# systemctl enable netatalk.service

If you go to your Mac’s Time Machine preferences the new volume will be available and you can start using it.

netatalk troubleshooting

Some notes of things to check from the server side (Time Capsule server):


Backintime setup

Configuring Backintime

Prepare users

If you have disabled passwords and are only using keys, you will need to temporarily change the security settings to allow Backintime to exchange keys.

On the remote system/Pi/server:

# vim /etc/ssh/sshd_config
PasswordAuthentication yes
# systemctl restart ssh

Backintime uses SSH, so the user accounts need to be allowed to login. Therefore the default login shell needs to reflect this.

If not created already, assign the user a home directory. Finally, allow the user to read and write the folder containing the backups.

# usermod -s /usr/bin/bash backintime_john

# mkdir /home/backintime_john

# chown backintime_tuxedo:backintime_john /home/backintime_john/

# usermod -d /home/backintime_john/ backintime_john

# chown :backupusers /backups/backintime/

# chmod 775 /backups/backintime/

Permissions for some of the subfolders might be required in multi-user configuration after the first backup:

# chown :backupusers /backups/backintime*/backintime

# chmod 770 /backups/backintime*/backintime/system1/
# chmod 770 /backups/backintime*/backintime/laptop2/

Prepare keys

To simplify things these are the roles:

[Local system]
The client machine that is running Backintime and that you want to backup your data from.

[Remote system]
The SSH server that has the storage where your backup is going to be stored.

From the local system account you want to run backintime (either your user or root, depending on how you run Backintime) SSH into the remote system. In my case, a Raspberry Pi.

# ssh backintime_john@pi-capsule.local

After logging in check the host key.

$ ssh-keygen -l -f /etc/ssh/ssh_host_ecdsa_key.pub
256 SHA256:KjzU6aGqH6tXri/K87xz3H+cP35PMT7n+Ob6MIaBZb0 root@pi-capsule (ECDSA)

You can then log out from the remote machine.

From the local account, you want to run Backintime from generate a new SSH key pair.

# ssh-keygen

And then copy the public key to the Pi.

# ssh-copy-id -i ~/.ssh/id_rsa.pub backintime_david@pi-capsule
ECDSA key fingerprint is SHA256:KjzU6aGqH6tXri/K87xz3H+cP35PMT7n+Ob6MIaBZb0.
Number of key(s) added: 1

Note that the fingerprint is the same as the one displayed in the previous step.

Configure Backintime profile

You can now configure the SSH profile from Backintime and make the first run.

In the General tab:

Mode:               SSH

SSH Settings
Host:   pi-capsule
Port:   22
User:   backintime_david
Path:   /backups/backintime_david
Cipher:     [Leave as default]
Private Key:/root/.ssh/id_rsa

SSH private key:[empty in most cases]
Enable Cache Password

Host:       tuxedo
User:       root
Profile:    2

[Select appropriate setting after testing]

Exclude (example)

Older than 10 years
If free space is less than 50GiB
If free inodes is less than 2%

Smart remove
Run in background on remote Host
Keep last
14 days (7 days)
21 days (14 days)
8 weeks (6 weeks)
36 months (14 months)

Don't remove named snapshots
Enable notifications
Backup replaced files on restore
Continue on errors (keep incomplete snapshots)
Log level: Changes & Errors

After the first run has completed you can check which is the best performing cipher from the CLI.

# backintime benchmark-cipher --profile-id 2

After a few rounds, aes192-ctr came out as the best performing cipher for me.

Secure SSH

If you changed the SSH configuration at the beginning, after setting everything up, remember to secure SSH again on the server/remote system.

# vim /etc/ssh/sshd_config
PasswordAuthentication no
# systemctl restart ssh

Restoring restrictions to backup users

The login account is required for Backintime to be able to run rsync. It is worth doing a bit more research on how to harden/limit these accounts.


Some examples of some issues and some troubleshooting steps you can apply.

Time Capsule can’t be reached / firewall settings

Make sure the server is allowing AFP connections from the Mac client.

# ufw allow proto tcp from CLIENT_IP to PI_CAPSULE_IP port 548

Time Capsule – Configuring Time Machine backups via the network on a macOS VM

The destination needs to be configured manually.

Mount the AFP/Time Capsule mount via the Finder.

In the CLI configure the destination:

# tmutil setdestination -a /Volumes/pi-capsule

The backups can then be started from the GUI.

You can get information about the current configured destinations via the CLI.

# tmutil destinationinfo
Name            : pi-capsule
Kind            : Network
Mount Point     : /Volumes/pi-capsule
ID              : 7B648734-9BFC-417F-B5A1-F31B8AD52F4B

Time Capsule – Checking backup status

# tmutil currentphase
# tmutil status

ZFS stalling on a Raspberry Pi

Check the recordsize property. Reduce it to the default 128 kiB.

Reduce ARC size to reduce the amount of memory consumed/reserved for ZFS.

Understanding rsync logs

The logs indicate the type of change rsync is seeing. A reference is available here:

XYcstpoguax  path/to/file
||||||||||╰- x: The extended attribute information changed
|||||||||╰-- a: The ACL information changed
||||||||╰--- u: The u slot is reserved for future use
|||||||╰---- g: Group is different
||||||╰----- o: Owner is different
|||||╰------ p: Permission are different
||||╰------- t: Modification time is different
|||╰-------- s: Size is different
||╰--------- c: Different checksum (for regular files), or
||              changed value (for symlinks, devices, and special files)
|╰---------- the file type:
|            f: for a file,
|            d: for a directory,
|            L: for a symlink,
|            D: for a device,
|            S: for a special file (e.g. named sockets and fifos)
╰----------- the type of update being done::
             <: file is being transferred to the remote host (sent)
             >: file is being transferred to the local host (received)
             c: local change/creation for the item, such as:
                - the creation of a directory
                - the changing of a symlink,
                - etc.
             h: the item is a hard link to another item (requires 
             .: the item is not being updated (though it might have
                attributes that are being modified)
             *: means that the rest of the itemized-output area contains
                a message (e.g. "deleting")

Some example output:

>f+++++++++ some/dir/new-file.txt
.f....og..x some/dir/existing-file-with-changed-owner-and-group.txt
.f........x some/dir/existing-file-with-changed-unnamed-attribute.txt
>f...p....x some/dir/existing-file-with-changed-permissions.txt
>f..t..g..x some/dir/existing-file-with-changed-time-and-group.txt
>f.s......x some/dir/existing-file-with-changed-size.txt
>f.st.....x some/dir/existing-file-with-changed-size-and-time-stamp.txt 
cd+++++++++ some/dir/new-directory/
.d....og... some/dir/existing-directory-with-changed-owner-and-group/
.d..t...... some/dir/existing-directory-with-different-time-stamp/ 

ZFS: Setting up ZFS storage on Ubuntu

If you are new to ZFS, I would advise doing a little bit of research first to understand the fundamentals. Jim Salter’s articles on storage and ZFS are very recommended.


The examples below are to create a pool from a single disk, with separate datasets used for network backups.

In some examples, I might use device names for simplicity, but you are advised to use disks IDs or serials.

Installing ZFS

Ubuntu makes it very easy.

# apt install zfsutils-linux

ZFS Cockpit module

If Cockpit is installed, it is possible to install a module for ZFS. This module is sadly no longer in development. If you know of alternatives, please share!

$ git clone https://github.com/optimans/cockpit-zfs-manager.git
# cp -r cockpit-zfs-manager/zfs /usr/share/cockpit

Configuring automatic snapshots

This service generates automatic snapshots every hour, and it can be configured to retain your preferred period.

# apt install zfs-auto-snapshot

The snapshot retention is set in the following files:


By default, the configuration runs the following snapshots and retention policies:

Period Retention
Hourly 24 hours
Daily 31 days
Weekly Eight weeks
Monthly 12 months

I configured the following snapshot retention policy:

Period Retention
Hourly 48 hours
Daily 14 days
Weekly Four weeks
Monthly Three months


# vim /etc/cron.hourly/zfs-auto-snapshot

# Only call zfs-auto-snapshot if it's available
which zfs-auto-snapshot > /dev/null || exit 0

exec zfs-auto-snapshot --quiet --syslog --label=hourly --keep=48 //


# vim /etc/cron.daily/zfs-auto-snapshot

# Only call zfs-auto-snapshot if it's available
which zfs-auto-snapshot > /dev/null || exit 0

exec zfs-auto-snapshot --quiet --syslog --label=daily --keep=14 //


# vim /etc/cron.weekly/zfs-auto-snapshot

# Only call zfs-auto-snapshot if it's available
which zfs-auto-snapshot > /dev/null || exit 0

exec zfs-auto-snapshot --quiet --syslog --label=weekly --keep=4 //
# vim /etc/cron.monthly/zfs-auto-snapshot

# Only call zfs-auto-snapshot if it's available
which zfs-auto-snapshot > /dev/null || exit 0

exec zfs-auto-snapshot --quiet --syslog --label=monthly --keep=3 //

Setting up the ZFS pool

This post has several use cases and examples, and I recommend it highly if you want further details on different commands and ways to configure your pools.


In my example there is no resilience, as there is only one attached disk. For me, this is acceptable because I have an additional local backup besides this filesystem.

It is preferable to have a second backup (ideally off-site) than a single one regardless of any added resilience you might set.

I create a single pool with an external drive. Read below for an explanation of the different command flags.

zpool create -f 
-o ashift=12 
-O compression=lz4 
-O acltype=posixacl 
-O xattr=sa 
-O relatime=on 
-O atime=off 
-O normalization=formD 
-O canmount=off 
-O dnodesize=auto 
-O sync=standard 
backup_pool scsi-SSeagate_Desktop_NA7HP4VK

Block size / ashift

Of the above values, the most important one by far is ashift.

The ashift property sets the block size of the vdev. It can’t be changed once set, and if it isn’t correct, it will cause massive performance issues with the filesystem.

Find out your drive’s optimal block size and match it to ashift.

It is set in bits.

bits sector size
9 512 bytes
10 1 kiB
11 2 kiB
12 4 kiB
13 8 kiB
14 16 kiB
15 32 kiB
16 64 kiB

recordsize is another performance impacting property, especially on the Raspberry Pi. Smaller sizes can improve performance when accessing random batches, but higher values will provide better performance and compression when reading sequential data. The problem on the Raspberry Pi has been that with a value of 1M the system load increased, eventually stopping the filesystem activity until the system was restarted.

The default value (128k) has performed without any noticeable issue.


lz4 compression is going to yield an optimum performance/compression ratio. It will make the storage perform faster than if there is no compression.

ZFS 0.8 doesn’t give many choices regarding compression but bear in mind that you can change the algorithm on a live system.

gzip will impact performance but yields a higher compression rate. It might be worth checking the performance with different compression formats on the Pi 4. With older Raspberry Pi models, the limitation will be the USB / network in most cases.

For reference, on the same amount of data these were the compression ratios I obtained:

backup_pool 1.34x
backup_pool/backintime 1.35x
backup_pool/timecapsule 1.33x

backup_pool 1.27x
backup_pool/backintime 1.30x
backup_pool/timecapsule 1.33x

All in all, the performance impact and memory consumption didn’t make switching from lz4 worthwhile.



It enables the POSIX ACLs and Linux Extended Attributes on the inodes rather than on separate files.

Access times

atime is recommended to be disabled (off) to reduce the number of IOPS.

relatime offers a good compromise between the atime and notime behaviours.


The normalization property indicates whether a file system should perform a Unicode normalisation of file names whenever two file names are compared and which normalisation algorithm should be used.

formD is the default set by Canonical when setting up a pool. It seems to be a good choice if sharing the volume via NFS with macOS systems and avoiding files not being displayed due to names using non-ASCII characters.

Additional properties

The pool is configured with the canmount property off so that it can’t be mounted.

This is because I will be creating separate datasets, one for Time Capsule backups, and another two for Backintime, and I don’t want them to mix.

All datasets will share the same pool, but I don’t want the pool root to be mounted. Only datasets will mount.

dnodesize is set to auto, as per several recommendations when datasets are using the xattr=sa property.

sync is set as standard. There is a performance hit for writes, but disabling it comes at the expense of data consistency if there is a power cut or similar.

A brief test showed a lower system load when sync=standard than with sync=disabled. Also, with standard there were fewer spikes. It is likely that the performance is lower, but it certainly causes the system to suffer less.


I am not too keen to encrypt physically secure volumes because when doing data recovery, you are adding an additional layer that might hamper and slow things down.

For reference, I am writing down an example of encryption options using an external key for a volume. This might not be appropriate for your particular scenario. Research alternatives if needed.

-O encryption=aes-256-gcm 
-O keylocation=file:///etc/pool_encryption_key 
-O keyformat=raw 

Pool options

Automatic trimming of the pool is essential for SSDs:

# zpool set autotrim=on backup_pool

Disabling automatic mount for the pool. (This applies only to the root of the pool, the datasets can still be set to be mountable regardless of this setting.)

# zfs set canmount=off backup_pool

Setting up the ZFS datasets

I will create three separate datasets with assigned quotas for each.

[Create datasets]
# zfs create backup_pool/backintime_tuxedo
# zfs create backup_pool/backintime_ab350
# zfs create backup_pool/timecapsule

[Set mountpoints]
# zfs set mountpoint=/backups/backintime_tuxedo  backup_pool/backintime_tuxedo
# zfs set mountpoint=/backups/backintime_ab350  backup_pool/backintime_ab350
# zfs set mountpoint=/backups/timecapsule  backup_pool/timecapsule

[Set quotas]
# zfs set quota=2T backup_pool/backintime_tuxedo
# zfs set quota=2T backup_pool/backintime_ab350
# zfs set quota=2T backup_pool/timecapsule

Changing compression on a dataset

The default lz4 compression is recommended. gzip consumes a lot of CPU and makes data transfers slower, impacting backups restoration.

If you still want to change the compression for a given dataset:

# zfs set compression=gzip-7 backup_pool/timecapsule

A comparison of compression and decompression using different algorithms with OpenZFS:


Querying pool properties, current compression algorithm and compress ratio

# zfs get all backup_pool
# zfs get compression backup_pool
# zfs get compressratio backup_pool
# zfs get all | grep compressratio

Changing ZFS settings

For reference, below are some examples of properties and settings that can be changed after a pool has already been created.

Renaming pools and datasets

If for any reason, a dataset was given a name that needs to be changed, this can be done with a command like this:

# zfs rename backup_pool/Test1 backup_pool/backintime_tuxedo

A zpool can be renamed by exporting and importing it.

# zpool export test_pool
# zpool import test_pool backup_pool

Attaching mirror disks

You can add an additional disk/partition and make the pool redundant in a RAID-Z configuration. Unfortunately, it doesn’t work to make it a RAID-Z2 or RAID-Z3.

# zpool attach backup_pool /dev/sda7 /dev/sdb7

Renaming disks in pools

By default, Ubuntu uses device identifiers for the disks. This should not be an issue, but in some cases, adding or connecting drives might change the device name order and degrade one or more pools.

This is why creating a pool with disk IDs or serials is recommended. You can still fix this if you created your pool using device names.

With the pool unmounted, export it, and reimport pointing to the right path:

# zpool export backup_pool
# zpool import -d /dev/disk/by-id/ backup_pool

There are additional examples in this handy blog post:


ZFS optimisation

ZFS should be running on a system with at least 4GiB of RAM. If you plan to use it on a Raspberry Pi (or any other system with limited resources), reduce the ARC size.

In this case, I am limiting it to 3GiB. It is a change that can be done live:

# echo 3221225472 > /sys/module/zfs/parameters/zfs_arc_max

To make it persistent between boots:

# vim /etc/modprobe.d/zfs.conf

[add this line]
options zfs zfs_arc_max=3221225472

# update-initramfs -u

You can check the ARC statistics:

$ less /proc/spl/kstat/zfs/arcstats

More on ZFS performance

Some other links with interesting points on performance:



Linux / Ubuntu / hdparm: Identifying drive features and setting sleep patterns

Preparing the storage

Install hdparm and smartmontools

Install hdparm and the SMART monitoring tools.

# apt install hdparm smartmontools

Identify the right hard drive

Make sure you identify the correct drive, as some of the commands will destroy data. If you don’t understand the commands, then check them first. You have been warned.

Identify the block size

Knowing the block size of the device is important. It will help optimising writes, and in the case of SSD or flash drives avoid write amplification and wear and tear.

[List details of all drives]

# fdisk -l

Sector size (logical/physical): 512 bytes / 4096 bytes
I/O size (minimum/optimal): 512 bytes / 4096 bytes

[List details of a specific drive]

# fdisk -l /dev/sda
Sector size (logical/physical): 512 bytes / 4096 bytes
I/O size (minimum/optimal): 4096 bytes / 4096 bytes
# smartctl --all /dev/sda
Sector Sizes:     512 bytes logical, 4096 bytes physical

Pay attention to the physical/optimal size. This is the one that matters.

SSDs will hide the true size of the pages and blocks. Even the same drive models might be built with different components, so getting it right is tricky.

Some suggest that 4kB is a generally good size for SSDs: https://www.intel.com/content/dam/www/public/us/en/documents/white-papers/ssd-server-storage-applications-paper.pdf

Use the drive’s sector physical size to match the ZFS ashift (block size).

Retrieve drive IDs

When setting ZFS pools or using disk tools it is best to avoid using device names as they can easily change their order. Using the drive ID or serial will ensure that no matter in which port or in which order the drives are plugged it will be the correct drive selected.

This matters with any disk accessing utility if you have several drives, or will be inserting external drives regularly.

$ ls -l /dev/disk/by-id/

lrwxrwxrwx 1 root root  9 Mar  9 13:16 usb-TOSHIBA_External_USB_3.0_20150612015531-0:0 -> ../../sda


You can also extract model and serial numbers with hdparm.

# hdparm -i /dev/sda


 Model=WDC WD10EZEX-08WN4A0, FwRev=01.01A01, SerialNo=WD-WCC6Y5FXAPHV

Even better, depending on the use of the drive, and if there is a plan to add mirror drives, is to partition the drive to ensure there is enough space if a different drive model is later added. Although I believe ZFS already does this and rounds down partitions using Mebibytes.

Test for damaged sectors

An additional and optional step is to test the hard drive for damaged sectors. This kind of test tends to be destructive so it is best if it is done before configuring the pools.

badblocks is a useful tool to achieve this.

It is installed by default, but if not you can do it manually.

# apt install e2fsprogs

A destructive test can be done with:

# badblocks -wsv -b 4096 /dev/sda

If you want to run the test while preserving the disk data you can run it in a non-destructive way. This will take longer.

# badblocks -nsv -b 4096 /dev/sda

ZFS has built-in checks and protection so in most cases you can skip this step.

Setting hard drive sleep patterns

Above I explained that using disk IDs is always a better idea. For simplicity, I will be using device names in several examples below, but I still advise using IDs or serials.

Check if the disk supports sleep

Check if the drive supports standby.

# hdparm -y /dev/sda

If supported the output will be:

 issuing standby command

Any other output might indicate that the drive doesn’t support sleep, or that a different tool/setting might be required.

Next, check if the drive supports write cache:

# hdparm -I /dev/sda | grep -i 'Write cache'

The expected output is:

           *    Write cache

The * indicates that the feature is supported.

An example of a complete hdparm output from a drive is shown below for reference. Different drives, with different features, will show different output, or even none at all.

# hdparm -I /dev/sda


ATA device, with non-removable media
        Model Number:       TOSHIBA MD04ACA500                      
        Serial Number:      55OBK0SPFPHC
        Firmware Revision:  FP2A    
        Transport:          Serial, ATA8-AST, SATA 1.0a, SATA II Extensions, SATA Rev 2.5, SATA Rev 2.6, SATA Rev 3.0
        Supported: 8 7 6 5 
        Likely used: 8
        Logical         max     current
        cylinders       16383   16383
        heads           16      16
        sectors/track   63      63
        CHS current addressable sectors:    16514064
        LBA    user addressable sectors:   268435455
        LBA48  user addressable sectors:  9767541168
        Logical  Sector size:                   512 bytes
        Physical Sector size:                  4096 bytes
        Logical Sector-0 offset:                  0 bytes
        device size with M = 1024*1024:     4769307 MBytes
        device size with M = 1000*1000:     5000981 MBytes (5000 GB)
        cache/buffer size  = unknown
        Form Factor: 3.5 inch
        Nominal Media Rotation Rate: 7200
        LBA, IORDY(can be disabled)
        Queue depth: 32
        Standby timer values: spec'd by Standard, no device specific minimum
        R/W multiple sector transfer: Max = 16  Current = 16
        Advanced power management level: 128
        DMA: sdma0 sdma1 sdma2 mdma0 mdma1 *mdma2 udma0 udma1 udma2 udma3 udma4 udma5 
             Cycle time: min=120ns recommended=120ns
        PIO: pio0 pio1 pio2 pio3 pio4 
             Cycle time: no flow control=120ns  IORDY flow control=120ns
        Enabled Supported:
           *    SMART feature set
                Security Mode feature set
           *    Power Management feature set
           *    Write cache
           *    Look-ahead
           *    Host Protected Area feature set
           *    WRITE_BUFFER command
           *    READ_BUFFER command
           *    NOP cmd
           *    DOWNLOAD_MICROCODE
           *    Advanced Power Management feature set
                SET_MAX security extension
           *    48-bit Address feature set
           *    Device Configuration Overlay feature set
           *    Mandatory FLUSH_CACHE
           *    FLUSH_CACHE_EXT
           *    SMART error logging
           *    SMART self-test
           *    General Purpose Logging feature set
           *    WRITE_{DMA|MULTIPLE}_FUA_EXT
           *    64-bit World wide name
           *    WRITE_UNCORRECTABLE_EXT command
           *    {READ,WRITE}_DMA_EXT_GPL commands
           *    Segmented DOWNLOAD_MICROCODE
                unknown 119[7]
           *    Gen1 signaling speed (1.5Gb/s)
           *    Gen2 signaling speed (3.0Gb/s)
           *    Gen3 signaling speed (6.0Gb/s)
           *    Native Command Queueing (NCQ)
           *    Host-initiated interface power management
           *    Phy event counters
           *    Host automatic Partial to Slumber transitions
           *    Device automatic Partial to Slumber transitions
           *    READ_LOG_DMA_EXT equivalent to READ_LOG_EXT
                DMA Setup Auto-Activate optimization
                Device-initiated interface power management
           *    Software settings preservation
           *    SMART Command Transport (SCT) feature set
           *    SCT Write Same (AC2)
           *    SCT Error Recovery Control (AC3)
           *    SCT Features Control (AC4)
           *    SCT Data Tables (AC5)
           *    reserved 69[3]
        Master password revision code = 65534
        not     enabled
        not     locked
        not     frozen
        not     expired: security count
                supported: enhanced erase
        more than 508min for SECURITY ERASE UNIT. more than 508min for ENHANCED SECURITY ERASE UNIT.
Logical Unit WWN Device Identifier: 500003964bc01970
        NAA             : 5
        IEEE OUI        : 000039
        Unique ID       : 64bc01970
Checksum: correct

An example of a complete smartctl output from a drive is shown below also for reference. As mentioned earlier, different systems will generate different outputs.

# smartctl --all /dev/sda
smartctl 7.1 2019-12-30 r5022 [aarch64-linux-5.4.0-1029-raspi] (local build)
Copyright (C) 2002-19, Bruce Allen, Christian Franke, www.smartmontools.org

Model Family:     Toshiba 3.5" MD04ACA... Enterprise HDD
Device Model:     TOSHIBA MD04ACA500
Serial Number:    55OBK0SPFPHC
LU WWN Device Id: 5 000039 64bc01970
Firmware Version: FP2A
User Capacity:    5,000,981,078,016 bytes [5.00 TB]
Sector Sizes:     512 bytes logical, 4096 bytes physical
Rotation Rate:    7200 rpm
Form Factor:      3.5 inches
Device is:        In smartctl database [for details use: -P show]
ATA Version is:   ATA8-ACS (minor revision not indicated)
SATA Version is:  SATA 3.0, 6.0 Gb/s (current: 3.0 Gb/s)
Local Time is:    Mon Mar  8 15:02:10 2021 UTC
SMART support is: Available - device has SMART capability.
SMART support is: Enabled

SMART Status not supported: Incomplete response, ATA output registers missing
SMART overall-health self-assessment test result: PASSED
Warning: This result is based on an Attribute check.

General SMART Values:
Offline data collection status:  (0x80) Offline data collection activity
                                        was never started.
                                        Auto Offline Data Collection: Enabled.
Self-test execution status:      (   0) The previous self-test routine completed
                                        without error or no self-test has ever 
                                        been run.
Total time to complete Offline 
data collection:                (  120) seconds.
Offline data collection
capabilities:                    (0x5b) SMART execute Offline immediate.
                                        Auto Offline data collection on/off support.
                                        Suspend Offline collection upon new
                                        Offline surface scan supported.
                                        Self-test supported.
                                        No Conveyance Self-test supported.
                                        Selective Self-test supported.
SMART capabilities:            (0x0003) Saves SMART data before entering
                                        power-saving mode.
                                        Supports SMART auto save timer.
Error logging capability:        (0x01) Error logging supported.
                                        General Purpose Logging supported.
Short self-test routine 
recommended polling time:        (   2) minutes.
Extended self-test routine
recommended polling time:        ( 533) minutes.
SCT capabilities:              (0x003d) SCT Status supported.
                                        SCT Error Recovery Control supported.
                                        SCT Feature Control supported.
                                        SCT Data Table supported.

SMART Attributes Data Structure revision number: 16
Vendor Specific SMART Attributes with Thresholds:
  1 Raw_Read_Error_Rate     0x000b   100   100   050    Pre-fail  Always       -       0
  2 Throughput_Performance  0x0005   100   100   050    Pre-fail  Offline      -       0
  3 Spin_Up_Time            0x0027   100   100   001    Pre-fail  Always       -       9003
  4 Start_Stop_Count        0x0032   100   100   000    Old_age   Always       -       9222
  5 Reallocated_Sector_Ct   0x0033   100   100   050    Pre-fail  Always       -       0
  7 Seek_Error_Rate         0x000b   100   100   050    Pre-fail  Always       -       0
  8 Seek_Time_Performance   0x0005   100   100   050    Pre-fail  Offline      -       0
  9 Power_On_Hours          0x0032   084   084   000    Old_age   Always       -       6418
 10 Spin_Retry_Count        0x0033   253   100   030    Pre-fail  Always       -       0
 12 Power_Cycle_Count       0x0032   100   100   000    Old_age   Always       -       9212
191 G-Sense_Error_Rate      0x0032   100   100   000    Old_age   Always       -       482
192 Power-Off_Retract_Count 0x0032   100   100   000    Old_age   Always       -       104
193 Load_Cycle_Count        0x0032   100   100   000    Old_age   Always       -       9225
194 Temperature_Celsius     0x0022   100   100   000    Old_age   Always       -       37 (Min/Max 15/72)
196 Reallocated_Event_Count 0x0032   100   100   000    Old_age   Always       -       0
197 Current_Pending_Sector  0x0032   100   100   000    Old_age   Always       -       0
198 Offline_Uncorrectable   0x0030   100   100   000    Old_age   Offline      -       0
199 UDMA_CRC_Error_Count    0x0032   200   253   000    Old_age   Always       -       0
220 Disk_Shift              0x0002   100   100   000    Old_age   Always       -       0
222 Loaded_Hours            0x0032   085   085   000    Old_age   Always       -       6393
223 Load_Retry_Count        0x0032   100   100   000    Old_age   Always       -       0
224 Load_Friction           0x0022   100   100   000    Old_age   Always       -       0
226 Load-in_Time            0x0026   100   100   000    Old_age   Always       -       214
240 Head_Flying_Hours       0x0001   100   100   001    Pre-fail  Offline      -       0

SMART Error Log Version: 1
No Errors Logged

SMART Self-test log structure revision number 1
Num  Test_Description    Status                  Remaining  LifeTime(hours)  LBA_of_first_error
# 1  Short offline       Completed without error       00%      5617         -
# 2  Short offline       Completed without error       00%      4702         -

SMART Selective self-test log data structure revision number 1
    1        0        0  Not_testing
    2        0        0  Not_testing
    3        0        0  Not_testing
    4        0        0  Not_testing
    5        0        0  Not_testing
Selective self-test flags (0x0):
  After scanning selected spans, do NOT read-scan remainder of disk.
If Selective self-test is pending on power-up, resume after 0 minute delay.

More information about hdparm and smartctl is available on the following sites.








Configure the drive standby

Check the current standby configuration.

# hdparm -B /dev/sd[a-e]

 APM_level  = not supported

 APM_level  = 254

 APM_level  = not supported

 APM_level  = 254

 APM_level  = 254
Values Description
1 to 127 Power management is enabled. The lower the value the more aggressive the power management will be.
128 to 254 Power management is enabled but doesn’t allow spindown
255 The feature is disabled.
not supported The drive doesn’t support APM.

The status can be set manually:

# hdparm -B 127 /dev/sda

The IDE power mode status can be queried with:

# hdparm -C /dev/sd[ab]

 drive state is:  active/idle

 drive state is:  standby

For reference, several drives can be queried at the same time using different wildcards.

# hdparm -B /dev/sd?
# hdparm -C /dev/sd*
# hdparm -I /dev/sd[a-e]

Depending on the drive manufacturer and model you might need to query the settings with different flags. Check the man page.

[Get/set  the  Western  Digital Green Drive's "idle3" timeout value.]
# hdparm -J /dev/sd[a-e]

 wdidle3      = 300 secs (or 13.8 secs for older drives)

 wdidle3      = 8.0 secs

 wdidle3      = 300 secs (or 13.8 secs for older drives)

 wdidle3      = 300 secs (or 13.8 secs for older drives)

 wdidle3      = 300 secs (or 13.8 secs for older drives)

From the man page:

A setting of 30 seconds is recommended for Linux use. Permitted values are from 8 to 12 seconds, and from 30 to 300 seconds in 30-second increments. Specify a value of zero (0) to disable the WD idle3 timer completely (NOT RECOMMENDED!).

There are flags for temperature (-H for Hitachi drives), acoustic management (-M), measuring cache performance (-T), and others. Go on, read that man page. 🙂

The -S flag sets the standby/spindown timeout for the drive. Basically, how long the drive will wait with no disk activity before turning off the motor.

Value Description
0 Disable the feature.
1 to 240 Five seconds multiples (a value of 120 means 10 minutes).
241 to 251 Thirty minutes intervals (a value of 242 means 1 hour).

Note that hdparm might wake the drive up when is queried. smartctl can query the drive without waking it.

# smartctl -i -d auto -n standby /dev/sda

Making the hdparm configuration persistent

Information about all the options is available at https://manpages.ubuntu.com/manpages/bionic/man5/hdparm.conf.5.html and also in the default configuration file generated by hdparm.

Example values from the data gathered above:

# APM setting (-B)
apm = 127

# APM setting while on battery (-B)
apm_battery = 127

# on/off drive's write caching feature (-W)
write_cache = on

# Standby (spindown) timeout for drive (-S)
spindown_time = 120

# Western  Digital  (WD)  Green Drive's "idle3" timeout value. (-J)
wdidle3 = 300

hdparm.conf method

Edit the configuration file:

# vim /etc/hdparm.conf

And insert an entry for each drive. Select only settings/features/values that are supported by that drive, otherwise the rest of the options won’t be applied. Test, test, test!

# Drive A
/dev/disk/by-id/ata-WDC_WD40NMZM-59Y94S1_WD-WX41D296P1XX {
apm = 127
apm_battery = 127
write_cache = on
spindown_time = 120
#wdidle3 = 300

udev method

In my case, the above method works. I couldn’t get this one to work on my system, but it could be because of the OS. I am leaving it for reference in case it might be of help.

# vim /etc/udev/rules.d/69-disk.rules

Create an entry for each drive editing the serial number and hdparm parameters. Make sure that only supported flags are added or it will fail.

ACTION=="add", KERNEL=="sd[a-z]", ENV{ID_SERIAL_SHORT}=="S3R14LNUMB3R", RUN+="/usr/bin/hdparm -B 127 -S 120 /dev/%k"

You can also apply the same parameters to all rotational drives (all non-SSD ones) in one go.

ACTION=="add|change", KERNEL=="sd[a-z]", ATTRS{queue/rotational}=="1", RUN+="/usr/bin/hdparm -B 127 -S 120 /dev/%k"

NTP: Setting up an NTP server

Setting up an NTP server

chrony is the default service on newer OS releases (Red Hat 7.2 and later, any recent Ubuntu release).

chrony has several advantages over ntpd:

  • Quicker synchronisation.
  • Better response to changes in clock frequency (very useful for VMs).
  • Periodic polling of time servers isn’t required.

It lacks some features like broadcast, multicast, and Autokey packet authentication. When this is required, or for systems that are going to be switched on continuously ntpd is a better choice.

A more comprehensive comparison list is available here:


Locate a pool or set as close as possible to you from any public ntp servers.


Setting a chrony NTP server

chrony is installed by default on many distros. If you don’t already have it, install it.

Edit the configuration file.

# vi /etc/chrony.conf

Make the following changes.

# Edit the time sources of your choice
# iburts helps making initial sync faster

server 0.pool.ntp.org iburst
server 1.pool.ntp.org iburst
server 2.pool.ntp.org iburst
server 3.pool.ntp.org iburst

# Helps stabilising initial sync on restarts

driftfile /var/lib/chrony/drift

# Allows serving time even if above sources aren't available

local stratum 8

# Opens the NTP port to respond to client's requests
# Edit it with your client's subnet


# Enables support for the settime command in chronyc


Start and enable the service.

# systemctl start chronyd

# systemctl enable chronyd

Check the firewall configuration in the last section.

Chrony client configuration

driftfile /var/lib/chrony/drift
logdir /var/log/chrony
log measurements statistics tracking

Checking chrony

[Check if the service is running]
$ systemctl status chrony

[Display the system's clock performance]
$ chronyc tracking

[Display time sources]
$ chronyc sources

More information on chrony:



Setting an ntpd NTP server

Install ntpd for your distro if not already present.

# yum install ntp
# dnf install ntp
# apt install ntp

Syncing to the server’s own system clock

If the system is going to be isolated, with no internet connection, or any other time source available you can use its internal clock.

Edit /etc/ntp.conf.

# To point ntpd to sync with its own system clock
server prefer 
driftfile /etc/ntp.drift
tracefile /etc/ntp.trace

This will work in a network “island”, but it won’t be a correct time. It is best to sync from other time sources (next section).

Syncing to other NTP servers

# Edit the time sources of your choice
# iburts helps making initial sync faster
server 0.pool.ntp.org iburst
server 1.pool.ntp.org iburst
server 2.pool.ntp.org iburst
server 3.pool.ntp.org iburst

# Insert your own subnet address
# nomodify - Disallows clients from configuring the server
# notrap - Clients can't be used as peers for time sync
restrict netmask nomodify notrap

# Indicates where to keep logs
logfile /var/log/ntp.log

Start, enable and check ntpd status:

# systemctl start ntpd
# systemctl enable ntpd
# systemctl status ntpd

Remember that you will need to open your firewall to allow NTP queries. There are some instructions further down.

ntpd client configuration

driftfile /var/lib/ntp/drift

Checking ntpd

$ ntpq -p
$ date -R

More information on ntpd:



Remember that you might need to open your firewall for clients to connect to your server.

[Red Hat / CentOS]

# firewall-cmd --add-service=ntp --permanent
# firewall-cmd --reload


# ufw allow ntp


# ufw allow 123/udp

You might want to also modify the rule to limit access only to certain subnets or clients.

You can add lines to chrony and ntpd configurations to allow IPv6 traffic. You would need to add also additional firewall rules. IPv4 shown here for simplicity (and also because I don’t have the requirement). 🙂

SNFS/Xsan: Quantum SNFS metadata controller and Xsan client compatibility chart

In a previous life, I designed and built many SANs based on Xsan (I believe I started with Xsan 1.3). I then migrated to looking after SANs based on SNFS, either from 3rd party vendors, or Quantum.

I believe that the age of Fibre Channel is long over (although SNFS also works on Infiniband if I recall correctly). The advantages of block-level access have been eclipsed by the much higher bandwidth with Ethernet, at a fraction of the cost.

However, I want to share some of my documentation with the world. This is done under an Attribution-NonCommercial-ShareAlike 4.0 International (CC BY-NC-SA 4.0) license.

The information has been collected from Apple support articles (current and obsolete ones), ADIC’s and Quantum’s StorNext documentation, and personal experience.

Every Xsan 2.0 and above client has been included. Maybe one day I will add Xsan 1.x releases for historical purposes

Xsan 20.0 Xsan 5.0.1 Xsan 5 Xsan 4.1 Xsan 4 Xsan 3.1 Xsan 3 Xsan 2.3 Xsan 2.2 to 2.2.2 Xsan 2 to 2.1.1
11.0.1 10.13, 10.14, 10.15 10.12 10.11 10.10 10.9 10.8 10.7 10.6 10.5
SNFS 7.0.x
SNFS 6.4.0
SNFS 6.3.x
SNFS 6.2.x ? ? ?
SNFS 6.1.x ? ? ? ?
SNFS 6.0.5,, 6.0.6 ? ? ? ?
SNFS 6.0, 6.01, ? ? ? ?
SNFS 5.4.x ? ? ? ?
SNFS 5.3.2.x
SNFS 5.3.1
SNFS 5.3.0
SNFS 5.2.2
SNFS 5.2.1
SNFS 5.2.0
SNFS 5.1.x
SNFS 5.0.x
SNFS 4.7.x
SNFS 4.6
SNFS 4.3
SNFS 4.2.1
SNFS 4.2.0
SNFS 4.1.1 to 4.1.3
SNFS 4.0 to 4.1
SNFS 3.5.x
SNFS 3.1.2 to 3.1.5
SNFS controller and Xsan client compatibility chart.

There are some caveats with some of the supported configurations. Some releases were originally marked by Apple as incompatible and then reverted. In the same way, some configurations that were originally marked as working were then updated as not compatible.

Double-check official documentation before any deployment.

I hope you find this table useful. There are some additional Xsan curiosities I will post in the future.