Using ZFS for File System 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, to copy a snapshot of the scratchpool file system to a new file system called slavepool on a new server, you would use the following command. This sequence combines the snapshot of scratchpool, the transmission to the slave machine (using ssh with login credentials), and the recovery of the snapshot on the slave using zfs recv:

root-shell> zfs send scratchpool@snap1 |ssh id@host pfexec zfs recv -F slavepool

The first part of the pipeline, zfs send scratchpool@snap1, streams the snapshot. The ssh command, and the command that it executes on the other server, 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 file system.

On the slave machine, the replicated file system 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 file system again, you 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 id@host pfexec zfs recv slavepool

This operation only succeeds if the file system on the slave machine has not been modified at all. You cannot apply the incremental changes to a destination file system that has changed. In the example above, the ls command would cause problems by changing the metadata, such as the last access time for files or directories.

To prevent changes on the slave file system, set the file system on the slave to be read-only:

root-shell> zfs set readonly=on slavepool

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

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

root-shell> zfs send scratchpool@snap1 |ssh id@host pfexec zfs recv -F slavepool

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

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