Unmount filesystems for snapshot? (EVA 3000)

[Rudy Williams]

Hello--

I am familiar with Business Copy on the HP VA series but new to the snapshot capability with Compaq EVA. Is it necessary to unmount filesystems before the snapshot? If not, why is that?

cjw

[Mike Naime · Accepted solution]

NO. you do not NEED to unmount file systems before the SNAP, CLONE, or SNAPCLONE. These are all designed to be done online. Please refrence the EVA docs for explanations of the differences.

http://h20000.www2.hp.com/bizsupport/TechSupport/DocumentIndex.jsp?contentType=SupportManual&docIndexId=3124&locale=en_US&prodTypeId=12169&prodSeriesId=321347

We use SNAP for backup purposes. We send data to a local "backup" drive on the EVA. This is then SNAPed and the SNAP is presented to the backup cluster for spooling to tapes.

[Bob_Vance]

I have to take a little issue with Mike's overly-sanguine (IMHO) view :>)

Anytime that you take a snapshot, the data should be in a consistent, preferably quiescent, state on the disk. Indeed, the general procedure for backing up a database is to:

. quiesce data (either shutdown the app and database or put the database into a "backup mode", designed for this purpose)
. take the snapshot
. resume the "normal" mode of the database
. mount and access the snapshot
Here, the database will be "quiescent" (as far as the disk/LUN goes) for just the few seconds it takes to create the snapshot.

If you simply take a snapshot of an active Unix filesystem, you will get copy of the data on the disks (LUNs, for Array) which could be in an inconsistent state, especially vis-a-vis any applications that were active on it, since some information is cached in system memory.

Normally, when you attempt to mount such a snapshot, you would get at least something like a "file-system inconsistent" error and be required to 'fsck it. Actually, it *would* be safest to 'umount' the FS before 'snap'ping it. However, depending on the FS in question, you can probably get by with just a 'sync' (after possibly quiescing certain apps) prior to the snap.

Therefore, I would answer by saying that "it depends" and YMMV and thereby avoid the issue ;>)

Keep in mind, however, that Mike's opinion should carry far more weight than mine.

bv

[Christian Schwartz-Søre]

An important issue to take into account when doing snaps or clones of any kind is to make sure you don't have any unwritten data in cache, which haven't been flushed to disk.
If data is flushed to disk first, then there is a better possibility of retaining consistent data in the snap.
The BC process doesn't flush data automatically.
If you script your process using SSSU you can incorporate a utiliy called "SYNC" found at www.sysinternals.com.
This utility flushes the cache.

Maintaining data consistency is important, seeing that corrupted data is of no use in a backup senario.

Regards
Christian