Applying ZFS snapshot to a non-ZFS FS

So this is a bit of a question of theory as well as specific (temporary use case)

• Two servers are to be in sync of each other. One On-Site, the other an Off-Site backup.

• However, the Off-Site should have the data duplicated and accessible if need be (not storing archive images of server1)

• server1 and server2 are connected over internet via VPN connection

• server1 uses ZFS Raid 10

• server2 uses ext4 Raid5 (temporary setup, will be replaced in future with ZFS and this use case vanishes)

Can you take a ZFS snapshot on server1, send it to server2 and have it be unpacked/applied to the raid5 array, essentially duplicating server1 via incremental snapshots?

I know that there are some other tools for duplication of filesystems, but i was wondering if we can use snapshots in a non zfs fs. (documentation leads me to believe this is not possible, but i do not know enough about this)

Answer

Yes, there are two theoretical options. Both use async replication so will have a nonzero RPO (although from your description that seems acceptable to some extent):

1. Use zfs send to create a stream on the source system, and then use some tool that can understand the contents of that stream and translate to POSIX filesystem primitives on the receiving system.

2. Take a snapshot on the source system and then use an FS-agnostic tool to copy stuff from that snapshot over.

The first one has the benefit of being the most performant option, because ZFS knows what parts of its pool have been changed and only has to look at / send those parts. However, I don’t know of any tool that can actually do this. (Prototypes have been built at ZFS developer hackathons, but there is not a big audience for this type of tool so they’ve never been made production quality AFAIK.)

The second one is less performant because it will have to inspect the data to see what changed, but it has the benefit that tools exist — although you may have to fight with it a little, you can use rsync for this. Also, its RPO might be higher since transferring the data will take a bit longer. The slightly tricky parts will be:

• Writing its metadata to a writable part of the pool on the source side, since the snapshot you’re copying will be read-only. (Look in the .zfs/ directory in the root of the filesystem you want to copy to find a readable copy of the snapshot.)
• Making the failover target not have intermediate state if the source system dies during an rsync run. Hopefully your target filer has the ability to snapshot before you start an rsync run, so that you can roll back to the “last good state” if the run fails. Otherwise, hopefully your data / application can tolerate some inconsistencies. (Or maybe there’s an rsync option that does this that I haven’t used before.)