I'm one of those people using LVM1 snapshots mostly successfully with XFS. However: 1) I'm using a process flag hack to keep XFS's filesystem freeze from snagging writes by kupdated, xfs_freeze, and fsync_dev/fsync_dev_lockfs. I've also removed the xfs_unmountfs_writesb() call from xfs_thaw(). I'm included the two patches I use, which apply against XFS 1.1. I've directed most of my discussion of this to the XFS list, as the locking mechanism is filesystem-specific. The only filesystem calls that the lvm driver makes with the VFS patch are fsync_dev_lockfs() and unlockfs(). The COW activity itself is done through brw_kiovec(), which I believe should not go through the file system at all (though it seems to cause lots of indirect file system activity through kupdated). device-mapper (LVM2) uses (with VFS enhancement) the very same fsync_dev_lockfs() and unlockfs() calls. However, the COW activity is not handled through brw_kiovec(), instead being transferred to device-mapper's kcopyd. I haven't worked with LVM2 yet, so it's certainly possible that kcopyd allieviates the pressure on kupdated. But in theory I would expect it to be susceptible to the same file system deadlocks experienced by LVM1. 2) I'm still seeing an occasional xfs_freeze deadlock. xfs_unmountfs_writesb() (from xfs_freeze) and kupdated get stuck on separate pagebuf locks. It occurs with multiple snapshots and streaming writes to the snapshot source over both samba and nfs. Using these patches, I haven't seen any oops problems, only the odd deadlock. You also have to mount the XFS snapshots with nouuid and norecovery options. Another approach that will work is to forget using the VFS patch entirely, and use writeable snapshots (available for 1.1, LVM2, and working patches have been posted to the list for 1.0.x). If the snapshot is writeable you can just mount with nouuid (or change the UUID of the snapshot), mount the snapshot and let XFS recovery do its stuff. This has the advantage of not running into any locking problems whatsoever. The disadvantage is that the filesystem isn't clean, so instead of having a snapshot of the filesystem at that point of time, you have the filesystem as if the power had suddenly failed at that point in time. Dale Stephenson steph@snapserver.com > -----Original Message----- > From: Joe Thornber [mailto:joe@fib011235813.fsnet.co.uk] > Sent: Friday, June 07, 2002 2:45 AM > To: linux-lvm@sistina.com > Subject: Re: [linux-lvm] How well tested is the snapshot feature? > > > On Fri, Jun 07, 2002 at 11:37:05AM +0200, Stephan Austermuehle wrote: > > On Fri, Jun 07, 2002 at 10:33:00AM +0100, Joe Thornber wrote: > > > > > Did you apply the VFS patch from the 1.0.3 release ? This makes > > > sure that the filesystem flushes a consistent state to the disk > > > before the snapshot is setup. > > > > Do you mean the VFS lock patch? Yes, it is applied. > > Curious, as far as I know people are using LVM1 snapshots successfully > with xfs. > > - Joe
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