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

[Chris Mason]

On Tuesday, February 26, 2002 09:59:19 AM -0700 Andreas Dilger <adilger at turbolabs.com> wrote:

> On Feb 26, 2002  14:54 +0100, 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.
> 
> Yes.
> 
>> 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?
> 
> The VFS lock patch only applies to journaled filesystems.  There has not
> been any attempt to include support for sync_super_lockfs for ext2.  I
> _think_ that if you tried to mount the ext2 snapshot, it will still
> work, even though it is inconsistent.  Obviously it would be better to
> fix it so that ext2 snapshots are always consistent.

Yes, its very easy for the journaled filesystems (except XFS ;-) to 
do the snapshot locking, since snapshot locking is really just grab a 
lock on the log, finish transactions, let LVM do its thing, then release
the lock.

For ext2, the best we could do would probably be a hack based on 
locking the super.  As far as I know, nobody is planning on this,
especially given how easy it is to go from ext2->ext3.

-chris