I've got a bit of a strange problem with snapshot sizes. First, some
background:
For ages our DBA backed up all the company databases to a directory NFS
mounted from a NetApp filer. That directory would then get dumped to tape.
About a year ago, I built an OpenSolaris (technically Nexenta) machine with 24
x 1.5TB drives, for about 24TB of usable space. I am using this to backup OS
images using backuppc.
I was also backing up the DBA's backup volume from the NetApp to the (ZFS)
backup server. This is a combination of rsync + snapshots. The snapshots were
using about 50GB/day. The backup volume is about 600GB total, so this
wasn't bad, especially on a box with 24TB of space available.
I decided to cut out the middleman, and save some of that expensive NetApp
disk space, by having the DBA backup directly to the backup server. I
repointed the NFS mounts on our DB servers to point to the backup server
instead of the NetApp. Then I ran a simple cron job to snapshot that ZFS
filesystem daily.
My problem is that the snapshots started taking around 500GB instead of 50GB.
After a bit of thinking, I realized that the backup system my DBA was using
must have been writing new files and moving them into place, or possibly
writing a whole new file even if only part changed.
I think this is the problem because ZFS never overwrites files in place.
Instead it would allocate new blocks. But rsync does a byte-by-byte
comparison, and only updates the blocks that have changed.
Because it's easier to change what I'm doing than what my DBA does, I decided
that I would put rsync back in place, but locally. So I changed things so that
the backups go to a staging FS, and then are rsync'ed over to another FS that
I take snapshots on. The only problem is that the snapshots are still in the
500GB range.
So, I need to figure out why these snapshots are taking so much more room than
they were before.
This, BTW, is the rsync command I'm using (and essentially the same command I
was using when I was rsync'ing from the NetApp):
rsync -aPH --inplace --delete /staging/oracle_backup/ /backups/oracle_backup/
This is the old system (rsync'ing from a NetApp and taking snapshots):
zfs list -t snapshot -r bpool/snapback
NAME USED AVAIL REFER MOUNTPOINT
...
bpool/snapb...@20100310-182713 53.7G - 868G -
bpool/snapb...@20100312-000318 59.8G - 860G -
bpool/snapb...@20100312-182552 54.0G - 840G -
bpool/snapb...@20100313-184834 71.7G - 884G -
bpool/snapb...@20100314-123024 17.5G - 832G -
bpool/snapb...@20100315-173609 72.6G - 891G -
bpool/snapb...@20100316-165527 24.3G - 851G -
bpool/snapb...@20100317-171304 56.2G - 884G -
bpool/snapb...@20100318-170250 50.9G - 865G -
bpool/snapb...@20100319-181131 53.9G - 874G -
bpool/snapb...@20100320-183617 80.8G - 902G -
...
This is from the new system (backing up directly to one volume, rsync'ing to
and snapshotting another one):
r...@backup02:~# zfs list -t snapshot -r bpool/backups/oracle_backup
NAME USED AVAIL REFER MOUNTPOINT
bpool/backups/oracle_bac...@20100411-023130 479G - 681G -
bpool/backups/oracle_bac...@20100411-104428 515G - 721G -
bpool/backups/oracle_bac...@20100412-144700 0 - 734G -
Thanks for any help,
Paul
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