[linux-lvm] snapshots of busy ext2 file system corrupt

[Anselm Kruis]

On 26 Feb 2002, Urs Thuermann wrote:

> I tried creating a snapshot of a ext2 file system on a LV.  I expected
> a fsck on the snapshot device to give no errors, i.e. a consistent
> file system.  This is ok, as long as there is no (or little) load on
> the file system when creating the snapshot.
> 
> However, on a busy file system, the created snapshot has lots of
> errors found by fsck.
> 
> Is it this what the VFS lock patch is for?  If so, why hasn't it been
> integrated into the standard kernel?  Has it flaws in it?  Maybe
> performance wise?

It deadlocks for XFS on smp under heavy writeload. I use a writeable
snapshot patch and replay the log afterwards.

I think, the VFS patch has some principal problems. Creating a snapshot
with the VFS-lock patch applied is more or less equivalent to unmounting 
the file system, creating the snapshot of the device and remounting the
file system. That means that all ongoing write operations must be
suspended until the filesystem is in a "clean" state. This can take
some time. Up to 15 minutes from my observations and that is way too long.
I think the right way is: use a jornaling file system, take a snapshot,
make the snapshot writeable, replay the log, make the snapshot readonly
and dump it to tape or whatever you want. No races, no deadlocks, no
problems.

Anselm


---
 Anselm Kruis                                 Tel. +49 (0)89-356386-74
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