Re: Proper method of snapshotting XFS with external log using LVM2

[David Chinner <dgc@sgi.com>]

On Thu, Aug 02, 2007 at 12:23:29AM +1200, Mario Becroft wrote:
> Hi David,
> 
> Thanks very much for your clear explanation of how to snapshot XFS
> filesystems with external log. It is good to know that I am doing it
> right.
> 
> I found that in the latest Linux kernel version 2.6.22.1, you cannot use
> the method I have been using, and which you confirmed is ok. The problem
> is that if you xfs_freeze the filesystem, then the LVM snapshot command
> hangs forever.

Ah, yes, I wondered if that still happened. The dm snapshot code is
a little naive - it just assumes that it is the only thing that can
freeze a device.

> (Back in kernel version 2.6.16 this did not happen.) I
> guess the snapshot command is also attempting to freeze the filesystem,
> or something, which doesn't work when it is already frozen.

It hangs on a lock that is held across the xfs_freeze -f ;
xfs_freeze -u sequence.

> I suppose
> how to fix this is a question for the linux-lvm mailing list.

IIRC from the last time this came, they'll tell you not to use
xfs_freeze.

As it is, the freeze_bdev() interface that both xfs_freeze and
dm-snapshot use is a complete pile of crud that we need to fix.
One of these days I'll get some time to fix it....

Cheers,

Dave.
-- 
Dave Chinner
Principal Engineer
SGI Australian Software Group