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XFS and block-level snapshots

To: xfs@xxxxxxxxxxx
Subject: XFS and block-level snapshots
From: Kamil Kisiel <kamil@xxxxxxxxxxxxx>
Date: Fri, 6 Jun 2008 11:33:17 -0700
Sender: xfs-bounce@xxxxxxxxxxx

I had a question about XFS integrity and performing block-level snapshots.

We currently have a 2TB (but growing soon..) volume mounted by a Linux host with kernel 2.6.23 over iSCSI from our SAN. Our SAN unit has the capability to perform block-level snapshots, which is done at regular intervals.

I know that it is recommended to perform an xfs_freeze before performing a snapshot. However, the control of the snapshots is independent from the OS, which currently has no knowledge of their occurrence. I'm curious as to the repercussions of this. I understand that in all likelyhood, the integrity of files which are currently being written will not be preserved. However, even with an xfs_freeze this is not guaranteed, as an application may require additional disk transactions to maintain the file in a valid state (it is not necessarily atomic, depending on the application).

As far as metadata transactions are concerned, the journal should make these atomic, so there should not be any problem there?

Basically, I'd like to know what is the worst that could happen, and why an xfs_freeze is necessary in this scenario.

Kamil Kisiel
HPC Systems Engineer, Zymeworks Inc.
201-1401 West Broadway,
Vancouver, BC, V6H 1H6, Canada
Tel: (604) 678-1388 ext. 135
Fax: (604) 737-7077

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