#14 incremental snapshots


It should be very good to have read-only snapshots like in Solaris' ZFS, with some tool to send/receive them. If I have evms volume v1, I can create and activate first snapshot of this volume, say v1.1. Now I can dump this volume with dd to have offline snapshot, which may be restored. Sure, dd will copy all space of the volume including unused, which is sad. Much better if special utility will skip these zeroes.
But then, say after a day, I want to create second snapshot, v1.2. Then I want to "dump" incremental dump, the difference between v1.1 and v1.2, and save it as an a kind of incremental backup. So that original volume if crashed may be restored to state at the moment of activating v1-2 by restoring it from v1-1 dump and then applying this incremental dump v1.2-v1.1
in ZFS terminology it's like:
zfs snapshot v1@1
zfs send v1@1 > /backups/v1.1
Then, after a day:
zfs snapshot v1@2
zfs send -i v1@1 v1@2 > /backups/v1.2-1
And v1 may be restored after being destroyed by:
zfs receive vRestored </backups/v1.1
zfs receive vRestored </backups/v1.2-1
Because evms have all the track of modified blocks between volume and it's snapshot, it can be a great
improvement over existing technologies of incremental
backups, which usually involve scanning all files or at least directories on the volume, that can be a huge
task for, for example, imap storage.
Sergey Ivanov.


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