FreeBSD-Debian ZFS Migration

Since the Zettabyte File System (ZFS) is steadily getting more and more stable on non-Solaris and non-FreeBSD systems, I decided to put my data pool created for the previous entry to the test. In principle, it should be possible to migrate a pool from one operating system to another. Imagine the following scenario – a company is getting new hardware and/or new IT experts and needs to migrate to a different OS. In my case it was from FreeBSD to Debian and vice versa. All data volumes were located in a single pool, but depending on the size of the company, it might be several pools instead. Before even thinking of migrating it is first important to make sure that all I/O related to the pool(s) to be migrated was stopped. When the coast is clear we can “zpool export <pool>” and begin our exodus to another operating system.

From FreeBSD to Debian
After exporting the zdata pool I installed Debian Testing/Stretch onto the system-bound SSD drive. ZFS is not part of the base installation, hence all pool imports need to be done after the system is ready and the zfs kernel module is built from the zfs-dkms and spl-dkms packages. apt resolves all dependencies properly so the only weak link is potential issues with building ZFS on GNU/Linux. Should no problems occur, we can proceed with importing the ZFS pool. GNU/Linux is cautious and warns the user about duplicate partitions/volumes. Those will not be mounted, even if the pool itself is imported successfully. Thankfully, conflicts can be resolved instantly by using a transition partition/drive to move data around. Once that’s done, our ZFS pool is ready for new writes. Notice that the content of /usr/local/ will undergo major changes as FreeBSD uses it for storing installed ports/packages and their configurations. In addition, /var/db will contain the pkg sqlite database with all registered packages. While this does not specifically interfere with either apt or Debian (apt configurations are in /var/lib and cached .deb packages in /var/cache/apt/archives), it’s important to take notice of.

From Debian to FreeBSD
Here, the migration is slightly smoother. The “bsdinstall” FreeBSD installer is designed in a more server-centric fashion (and ZFS is integral to the base system) so the ZFS pool can be connected and imported even before the first boot into the new system. The downside is that FreeBSD does not warn about “overmounting” system partitions from the zdata pool so it’s relatively easy to bork the fresh installation. Also, /var/cache will contain loads of unwanted directories and /usr/src, /usr/obj, /usr/ports and /usr/local need to be populated anew just like during a brand new FreeBSD installation.

Either way, the migration process is not too difficult and definitely not horrendously time-consuming. Should the user/administrator have PostgreSQL, MySQL or other SQL-like databases in /var/db, extra steps might need to be taken to ascertain forward and backward compatibility of the database packages. In the end, it’s a matter of knowing what each OS places where. FreeBSD is structured in a very intuitive and safe (from an administrator’s point of view) way. Debian, just like any other GNU/Linux distribution is a bit more chaotic, hence more caution is required. Both are good in their own regard, hence my incentive for migration testing.

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