14.4.1. Using ZFS for Filesystem Replication

Because zfs send and zfs recv use streams to exchange data, you can use them to replicate information from one system to another by combining zfs send, ssh, and zfs recv.

For example, if a snapshot of the scratchpool filesystem has been created and this needs to be copied to a new system or filesystem called slavepool. You would use the following command, combining the snapshot of scratchpool, the transmission to the slave machine (using ssh), and the recovery of the snapshot on the slave using zfs revc:

root-shell> zfs send scratchpool@snap1 |ssh mc@slave pfexec zfs recv -F slavepool

The first part, zfs send scratchpool@snap1, streams the snapshot, the second, ssh mc@slave, and the third, pfexec zfs recv -F slavepool, receives the streamed snapshot data and writes it to slavepool. In this instance, I've specified the -F option which forces the snapshot data to be applied, and is therefore destructive. This is fine, as I'm creating the first version of my replicated filesystem.

On the slave machine, the replicated filesystem contains the exact same content:

root-shell> ls -al /slavepool/
total 23
drwxr-xr-x   6 root     root           7 Nov  8 09:13 ./
drwxr-xr-x  29 root     root          34 Nov  9 07:06 ../
drwxr-xr-x  31 root     bin           50 Jul 21 07:32 DTT/
drwxr-xr-x   4 root     bin            5 Jul 21 07:32 SUNWmlib/
drwxr-xr-x  14 root     sys           16 Nov  5 09:56 SUNWspro/
drwxrwxrwx  19 1000     1000          40 Nov  6 19:16 emacs-22.1/

Once a snapshot has been created, to synchronize the filesystem again, you need to create a new snapshot, and then use the incremental snapshot feature of zfs send to send the changes between the two snapshots to the slave machine again:

root-shell> zfs send -i scratchpool@snapshot1 scratchpool@snapshot2 |ssh mc@192.168.0.93 pfexec zfs recv slavepool

Without further modification, this operation fails, because the filesystem on the slave machine can currently be modified, and you can't apply the incremental changes to a destination filesystem that has changed. It is the metadata that has changed. The metadata about the filesystem, like the last time it was accessed - in this case, it is our ls that caused the problem.

To prevent changes on the slave filesystem, you must set the filesystem on the slave to be read-only:

root-shell> zfs set readonly=on slavepool

Setting readonly means that you cannot change the filesystem on the slave by normal means, including the filesystem metadata. Operations that would normally update metadata (like our ls) silently perform their function without attempting to update the filesystem state.

In essence, the slave filesystem is nothing but a static copy of the original filesystem. However, even when configured to to be read-only, a filesystem can have snapshots applied to it. Now the filesystem is read only, re-run the initial copy:

root-shell> zfs send scratchpool@snap1 |ssh mc@slave pfexec zfs recv -F slavepool

Now you can make changes to the original filesystem and replicate them to the slave.

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