Re: Consistent live fsfs backups without copy

On May 2, 2006, at 3:51 AM, Phil Endecott wrote:
> Have a look at LVM snapshots. I have posted previously about the
> method
> on this list, and a search should find the threads. It seems to
> work well
> for me and seems to meet your requirements.

We were just discussing this method. It's definitely a promising
idea. We're reluctant, though - we've heard about LVM causing crashes
(i.e., [1]) and seen it cause data loss not so long ago.

The LVM HOWTO used to talk about using "xfs_freeze" to guarantee
consistency if you're using XFS. I don't see a similar tool for ext3
or anything telling me whether it's unnecessary or impossible.

Now it says "This facility does require that the snapshot be made at
a time when the data on the logical volume is in a consistent state -
the VFS-lock patch for LVM1 makes sure that some filesystems do this
automatically when a snapshot is created, and many of the filesystems
in the 2.6 kernel do this automatically when a snapshot is created
without patching." [2] I'd be a lot more comfortable if it gave me an
authoritative way to know if my filesystem gives me this important
guarantee on the kernel I'm currently running. Guess I'll be digging
through kernel source...

[1] http://lists.debian.org/debian-isp/2005/10/msg00063.html
[2] http://www.tldp.org/HOWTO/LVM-HOWTO/snapshotintro.html

-- 
Scott Lamb <http://www.slamb.org/>
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