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Re: [linux-lvm] How well tested is the snapshot feature?



On Fri, Jun 07, 2002 at 08:35:44AM -0700, Dale Stephenson wrote:
> 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.

I'm not sure what this kupdated interaction that you mention could be.
Both brw_kiovec and kcopyd stay well away from both the filesystem
and the buffer cache.

> 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.

Which kernel are you using ?  I've found that 2.4.18 can be easily
persuaded to deadlock by having two processes making GFP_NOIO requests
for memory whilst the system is short of free memory. 2.4.19-pre9
works fine.

- Joe



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