rsync+ZFS snapshot

[Matt McCutchen]

On Sun, 2008-07-20 at 18:06 +0530, Madan Kumar wrote:
> I am not so expert on rsync. I am using windows XP and Vista.
> I am able to take the backup using rsync.(It does not give the
> incremental)
> I need to take incremental also. After some googling (referring the
> link- http://www.jeffawaddell.com/2008_04_01_archive.html) I
> found that rsync + ZFS snapshot does the same job. It use rsync to
> make delta backups to the file server, and automatic ZFS snapshot to
> keep incrementals.
> Please give me some ideas  to use this  in my application... Please provide me some weblinks that can be benificial to use it.

If I understand that blog post correctly, the main advantage that the
author hoped to obtain from ZFS snapshots as compared to a hard-linked
backup tool like rsnapshot was that ZFS snapshots would use less space
to represent the difference in filesystem structure (potentially, a list
of changed inodes compared to an independent tree of directories).
Nowhere does he mention efficient representation of small changes to
large files.  In that post, "incremental" probably means keeping a full
copy of each changed file, not keeping just the updated portions as you
previously said you wanted.

By default, rsync updates a destination file by rewriting it using a new
inode, so the ZFS snapshot mechanism would have to be awfully clever to
avoid storing full copies of the old and new files.  With --inplace, ZFS
might be smart enough to store only the changed blocks but probably
wouldn't catch matched data at different offsets.  In any case, ZFS
doesn't seem to be available for Windows; you would need a Solaris or
Linux destination machine.  I still think your best bet is to use
rdiff-backup, rsync batch files, rdiff, or even plain diff (for text
files).

Matt
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