Skip to main content

How to automatically import a ZFS pool built on top of iSCSI devices with systemd

When using ZFS on top of iSCSI devices one needs to deal with the fact that iSCSI devices usually appear late in the boot process. ZFS on the other hand is loaded early and the iSCSI devices are not present at the time ZFS scans available devices for pools to import.

This means that not all ZFS pools might be imported after the system has completed boot, even if the underlying devices are present and functional. A quick and dirty solution would be to run zpool import <poolname> after boot, either manually or from cron.

A better, more elegant solution is instead to hook into systemd events and trigger zpool import as soon as the devices are created.
This solution has been tested with ZOL 0.6.4 and Centos 7 as of 2015/12.

First you will need to figure out the systemd disk ids that you wan systemd to notify you about.
This can be done by running systemctl list-units --all --full | grep disk from a shell with all disks mounted and/or connected. Disk ids are in the first column and the last part of the disk id is the same as the name found under /dev/disk/by-id/.

iSCSI disks helpfully report iSCSI_Storage under the last column.

Once you have figured out the systemd device id you need to create a custom service unit like the following (/etc/systemd/system/itank-pool.service):

IMPORTANT: Replace the device ids referenced by After and WantedBy with your device id.

Now try rebooting your server or systemctl reload, export the pool and restart the iscsi service. You should notice that just after the iSCSI devices appear systemd will run the zpool import service (one shot).

Comments

Popular posts from this blog

Indexing Apache access logs with ELK (Elasticsearch+Logstash+Kibana)

Who said that grepping Apache logs has to be boring?

The truth is that, as Enteprise applications move to the browser too, Apache access logs are a gold mine, it does not matter what your role is: developer, support or sysadmin. If you are not mining them you are most likely missing out a ton of information and, probably, making the wrong decisions.
ELK (Elasticsearch, Logstash, Kibana) is a terrific, Open Source stack for visually analyzing Apache (or nginx) logs (but also any other timestamped data).

From 0 to ZFS replication in 5m with syncoid

The ZFS filesystem has many features that once you try them you can never go back. One of the lesser known is probably the support for replicating a zfs filesystem by sending the changes over the network with zfs send/receive.
Technically the filesystem changes don't even need to be sent over a network: you could as well dump them on a removable disk, then receive  from the same removable disk.

A not so short guide to ZFS on Linux

Updated Oct 16 2013: shadow copies, memory settings and links for further learning.
Updated Nov 15 2013: shadow copies example, samba tuning.

Unless you've been living under a rock you should have by now heard many stories about how awesome ZFS is and the many ways it can help with saving your bacon.

The downside is that ZFS is not available (natively) for Linux because the CDDL license under which it is released is incompatible with the GPL. Assuming you are not interested in converting to one of the many Illumos distributions or FreeBSD this guide might serve you as a starting point if you are attracted  by ZFS features but are reluctant to try it out on production systems.

Basically in this post I note down both the tought process and the actual commands for implementing a fileserver for a small office. The fileserver will run as a virtual machine in a large ESXi host and use ZFS as the filesystem for shared data.