linux-kernel - Re: dm-cache not writing out cache metadata at reboot?

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Date:	Thu, 9 May 2013 13:36:16 -0700
From:	"Darrick J. Wong" <darrick.wong@...cle.com>
To:	Mike Snitzer <snitzer@...hat.com>
Cc:	Joe Thornber <thornber@...hat.com>,
	device-mapper development <dm-devel@...hat.com>,
	linux-kernel@...r.kernel.org
Subject: Re: dm-cache not writing out cache metadata at reboot?

On Wed, May 08, 2013 at 06:05:26PM -0400, Mike Snitzer wrote:
> On Wed, May 08 2013 at  5:48pm -0400,
> Darrick J. Wong <darrick.wong@...cle.com> wrote:
> 
> > Hi,
> > 
> > So I've been watching the hit/miss counters in dmcache and I've noticed a
> > couple of things that look like errors to me:
> > 
> > First, I noticed that if I reboot the system, neither cache_postsuspend nor
> > cache_dtr get called.  This might simply be expected behavior, but it means
> > that the in-memory superblock structure doesn't get written out to disk upon
> > reboot.  Just to be sure, I put a printk into __commit_transaction.  It prints
> > out for 'dmsetup info' and 'dmsetup remove' but nothing at reboot.
> 
> We don't have reboot notifiers that auto-magically tear down an
> artbitrary DM stack.  Typically the device shutdown includes unmounting
> filesystems, stopping LVM (which tears down DM devices, etc).
> 
> So given that we don't have any userspace LVM2 support for dm-cache yet
> I'm not surprised by this.  In fact it is expected.
>  
> > Second, cache_status calls dm_cache_commit, which writes out a superblock to
> > the metadata device.  However, there's no call to save_stats to copy the
> > current values of the counters out to the disk's copy prior to calling
> > dm_cache_commit.  Therefore, we seem to be writing out stale copies of
> > superblock fields.
> > 
> > The second one seems fixable with the attached patch
> 
> I'll defer to Joe on this but I think sync_metadata() is pretty heavy to
> be doing every 'dmsetup info'.  BTW, with just dm_cache_commit() the
> superblock fields aren't stale; only the on-disk hints are.

Hrmm, how about this: dmsetup info will call save_stats so that the superblock
gets written with the freshest hit/miss counts, and I'll create a new dmsetup
message command that actually flushes everything out?

--D
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