原文：http://breden.org.uk/2008/05/12/home-fileserver-backups-from-zfs-snapshots/
Backups are critical to keeping your data protected, so let’s discover how to use ZFS snapshots to perform full and incremental backups.
In the last article on ZFS snapshots, we saw how to create snapshots of a file system. Now we will use those snapshots to create a full backup and subsequent incremental backups.
Performing backups
Obviously we only created a small number of files in the previous ZFS snapshots article, but we can still demonstrate the concept of using snapshots to perform full and incremental backups.
We’ll write our backups to a backup target file system called ‘tank/testback’.
This backup target could exist within the same pool, like in our simple example, but would most likely exist in another pool, either on the same physical machine, or at any location addressable, using iSCSI or ssh with an IP address etc.
Full backup
Now let’s do a full initial backup from the ‘tank/test@1′ snapshot:
# zfs send tank/test@1 | zfs receive tank/testback
Let’s take a look at the file systems to see what’s happened:
# zfs list
NAME USED AVAIL REFER MOUNTPOINT
tank 766G 598G 28.0K /tank
tank/test 94.6K 598G 26.6K /tank/test
tank/test@1 23.3K - 26.6K -
tank/test@2 21.3K - 26.0K -
tank/test@3 21.3K - 26.0K -
tank/test@4 0 - 26.6K -
tank/testback 25.3K 598G 25.3K /tank/testback
tank/testback@1 0 - 25.3K -
Well, the command not only created the file system ‘tank/testback’ to contain the files from the backup, but it also created a snapshot called ‘tank/testback@1′. The reason for the snapshot is so that you can get the state of the backups at any point in time.
As we send more incremental backups, new snapshots will be created, enabling you to restore a file system from any snapshot. This is really powerful!
Let’s just take a look at the files in the full backup — it should contain the original files referenced from our initial snapshot ‘tank/test@1′.
# ls -l tank/testback
total 4
-rw-r--r-- 1 root root 15 May 12 14:50 a
-rw-r--r-- 1 root root 15 May 12 14:50 b
# cat /tank/testback/a /tank/testback/b
hello world: a
hello world: b
As we expected. Good
Incremental backups
Now let’s do an incremental backup, that will only transmit the differences between snapshots ‘tank/test@1′ and ‘tank/test@2′:
# zfs send -i tank/test@1 tank/test@2 | zfs receive tank/testback
cannot receive incremental stream: destination tank/testback has been modified since most recent snapshot
Oh dear! For some reason, doing the ‘ls’ of the directory, when we inspected the backup contents, has actually modified the file system.
I have no idea how this happens or why, but I have seen this problem, or phenomenon, mentioned elsewhere.
It appears that the solution is to set the backup target file system to be read only, like this:
# zfs set readonly=on tank/testback
Another possibility is to use the ‘-F’ switch with the ‘zfs receive’ command. I don’t know which is the recommended solution, but I will use the switch for now, as I don’t want to make the file system read only, as we have several incremental backups to perform:
# zfs send -i tank/test@1 tank/test@2 | zfs receive -F tank/testback
Let’s just take a look at the files in the full backup — it should contain the original files referenced from our initial snapshot ‘tank/test@2′ — i.e. just file ‘b’:
# ls -l /tank/testback
total 2
-rw-r--r-- 1 root root 15 May 12 14:50 b
# cat /tank/testback/b
hello world: b
Good, as expected
Now let’s send all the remaining incremental backups:
# zfs send -i tank/test@2 tank/test@3 | zfs receive -F tank/testback
# zfs send -i tank/test@3 tank/test@4 | zfs receive -F tank/testback
# zfs list
NAME USED AVAIL REFER MOUNTPOINT
tank 766G 598G 29.3K /tank
tank/test 94.6K 598G 26.6K /tank/test
tank/test@1 23.3K - 26.6K -
tank/test@2 21.3K - 26.0K -
tank/test@3 21.3K - 26.0K -
tank/test@4 0 - 26.6K -
tank/testback 93.2K 598G 26.6K /tank/testback
tank/testback@1 22.0K - 25.3K -
tank/testback@2 21.3K - 26.0K -
tank/testback@3 21.3K - 26.0K -
tank/testback@4 0 - 26.6K -
Here is the final state of the backup target file system after sending all the incremental backups.
As we would expect, it matches the source file system contents:
# cat /tank/testback/b /tank/testback/c
hello world: b
modified
hello world: c
Restore a backup
Now let’s restore all of our four backup target snapshots into four separate file systems, so we can demonstrate how to recover any or all of the data that we snapshotted and backed up:
# zfs send tank/testback@1 | zfs recv tank/fs1
# zfs send tank/testback@2 | zfs recv tank/fs2
# zfs send tank/testback@3 | zfs recv tank/fs3
# zfs send tank/testback@4 | zfs recv tank/fs4
Let’s look at the file systems:
# zfs list
NAME USED AVAIL REFER MOUNTPOINT
tank 766G 598G 33.3K /tank
tank/fs1 25.3K 598G 25.3K /tank/fs1
tank/fs1@1 0 - 25.3K -
tank/fs2 26.0K 598G 26.0K /tank/fs2
tank/fs2@2 0 - 26.0K -
tank/fs3 26.0K 598G 26.0K /tank/fs3
tank/fs3@3 0 - 26.0K -
tank/fs4 26.6K 598G 26.6K /tank/fs4
tank/fs4@4 0 - 26.6K -
tank/test 94.6K 598G 26.6K /tank/test
tank/test@1 23.3K - 26.6K -
tank/test@2 21.3K - 26.0K -
tank/test@3 21.3K - 26.0K -
tank/test@4 0 - 26.6K -
tank/testback 93.2K 598G 26.6K /tank/testback
tank/testback@1 22.0K - 25.3K -
tank/testback@2 21.3K - 26.0K -
tank/testback@3 21.3K - 26.0K -
tank/testback@4 0 - 26.6K -
Let’s check ‘tank/fs1′ - it should match the state of the original file system when the ‘tank/test@1′ snapshot was taken:
# ls -l /tank/fs1
total 4
-rw-r--r-- 1 root root 15 May 12 14:50 a
-rw-r--r-- 1 root root 15 May 12 14:50 b
# cat /tank/fs1/a /tank/fs1/b
hello world: a
hello world: b
Perfect, now let’s check ‘tank/fs2′ - it should match the state of the original file system when the ‘tank/test@2′ snapshot was taken:
# ls -l /tank/fs2
total 2
-rw-r--r-- 1 root root 15 May 12 14:50 b
# cat /tank/fs2/b
hello world: b
Perfect, now let’s check ‘tank/fs3′ - it should match the state of the original file system when the ‘tank/test@3′ snapshot was taken:
# ls -l /tank/fs3
total 2
-rw-r--r-- 1 root root 24 May 12 17:35 b
# cat /tank/fs3/b
hello world: b
modified
Perfect, now let’s check ‘tank/fs4′ - it should match the state of the original file system when the ‘tank/test@4′ snapshot was taken:
# ls -l /tank/fs4
total 4
-rw-r--r-- 1 root root 24 May 12 17:35 b
-rw-r--r-- 1 root root 15 May 12 18:58 c
# cat /tank/fs4/b /tank/fs4/c
hello world: b
modified
hello world: c
Great!
Conclusion
Hopefully, you’ve now seen the power of snapshots. In future posts, I will show what else can be done with snapshots.
For more ZFS Home Fileserver articles see here: A Home Fileserver using ZFS. Alternatively, see related articles in the following categories: ZFS, Storage, Fileservers.
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