To support high availability environments, providing an instant copy of the information on both the currently active machine and the hot backup is a critical part of the HA solution. There are many solutions to this problem, including Chapter 16, Replication and Section 14.1, “Using MySQL with DRBD”.

The ZFS filesystem provides functionality that allows you to create a snapshot of the filesystem contents and to then transfer the snapshot to another machine and extract the snapshot to recreate the filesystem. You can create a snapshot at any time, and you can create as many snapshots as you like. By continually creating, transferring and restoring snapshots you can provide synchronization between one ore more machines in a fashion similar to DRBD.

To understand the replication solution within ZFS, you must first understand the ZFS environment. Below is a simple OpenSolaris system running with two pools, the root pool and another pool mounted at /opt:

Filesystem             size   used  avail capacity  Mounted on
rpool/ROOT/opensolaris-1
                       7.3G   3.6G   508M    88%    /
/devices                 0K     0K     0K     0%    /devices
/dev                     0K     0K     0K     0%    /dev
ctfs                     0K     0K     0K     0%    /system/contract
proc                     0K     0K     0K     0%    /proc
mnttab                   0K     0K     0K     0%    /etc/mnttab
swap                   465M   312K   465M     1%    /etc/svc/volatile
objfs                    0K     0K     0K     0%    /system/object
sharefs                  0K     0K     0K     0%    /etc/dfs/sharetab
/usr/lib/libc/libc_hwcap1.so.1
                       4.1G   3.6G   508M    88%    /lib/libc.so.1
fd                       0K     0K     0K     0%    /dev/fd
swap                   466M   744K   465M     1%    /tmp
swap                   465M    40K   465M     1%    /var/run
rpool/export           7.3G    19K   508M     1%    /export
rpool/export/home      7.3G   1.5G   508M    75%    /export/home
rpool                  7.3G    60K   508M     1%    /rpool
rpool/ROOT             7.3G    18K   508M     1%    /rpool/ROOT
opt                    7.8G   1.0G   6.8G    14%    /opt

The MySQL data will be stored in a directory on /opt. To help demonstrate some of the basic replication functionality, there are also other items stored in /opt as well:

total 17
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/

To create a snapshot of the filesystem, you use zfs snapshot, and then specify the pool and the snapshot name:

root-shell> zfs snapshot opt@snap1

To get a list of snapshots already taken:

root-shell> zfs list -t snapshot
NAME                                         USED  AVAIL  REFER  MOUNTPOINT
opt@snap1                                       0      -  1.03G  -
rpool@install                               19.5K      -    55K  -
rpool/ROOT@install                            15K      -    18K  -
rpool/ROOT/opensolaris-1@install            59.8M      -  2.22G  -
rpool/ROOT/opensolaris-1@opensolaris-1       100M      -  2.29G  -
rpool/ROOT/opensolaris-1/opt@install            0      -  3.61M  -
rpool/ROOT/opensolaris-1/opt@opensolaris-1      0      -  3.61M  -
rpool/export@install                          15K      -    19K  -
rpool/export/home@install                     20K      -    21K  -

The snapshots themselves are stored within the filesystem metadata, and the space required to keep them will vary as time goes on because of the way the snapshots are created. The initial creation of a snapshot is really quick, because instead of taking an entire copy of the data and metadata required to hold the entire snapshot, ZFS merely records the point in time and metadata of when the snaphot was created.

As more changes to the original filesystem are made, the size of the snapshot increases because more space is required to keep the record of the old blocks. Furthermore, if you create lots of snapshots, say one per day, and then delete the snapshots from earlier in the week, the size of the newer snapshots may also increase, as the changes that make up the newer state have to be included in the more recent snapshots, rather than being spread over the seven snapshots that make up the week.

The only issue, from a backup perspective, is that snaphots exist within the confines of the original filesystem. To get the snapshot out into a format that you can copy to another filesystem, tape, etc. you use the zfs send command to create a stream version of the snapshot.

For example, to write out the snapshot to a file:

root-shell> zfs send opt@snap1 >/backup/opt-snap1

Or tape:

root-shell> zfs send opt@snap1 >/dev/rmt/0

You can also write out the incremental changes between two snapshots using zfs send:

root-shell> zfs send opt@snap1 opt@snap2 >/backup/opt-changes

To recover a snapshot, you use zfs recv which applies the snapshot information either to a new filesytem, or to an existing one.