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



On Wed, 27 Feb 2002, Stephenson, Dale wrote:

> Anselm Kruis writes:
> 
> [Of the VFS lock patch]
> > 
> > It deadlocks for XFS on smp under heavy writeload. I use a writeable
> > snapshot patch and replay the log afterwards.
> > 
> So if you don't have the patch, you don't have a deadlock?  I've been trying
> to track down a problem with multiple snapshots of the same volume, but the
> presence or absence of the lock patch doesn't seem to make a difference.

I don't have the deadlock without the VFS patch.

> 
> > 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.
> > 
> Except that it's a lot more steps :->.

Or a small script. :-)

  I only wrote a writable snapshot
> patch because I wanted to use XFS with snapshots and they didn't have the
> nouuid,norecovery mount options at the time.  Is there any difference
> between doing the replay and just mounting norecovery?

I think so. The filesystem might be inconsistent if you don't replay the
log.


---
 Anselm Kruis                                 Tel. +49 (0)89-356386-74
 science + computing ag		              FAX  +49 (0)89-356386-37
 Ingolstädter Str. 22             mailto: A Kruis science-computing de
 D-80807 München                 WWW: http://www.science-computing.de/

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