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@xxxxxxxxxx> 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. Hmm, I wasn't aware that the lvm2 package had any teardown scripts. It doesn't seem to have any in RHEL5.8 or Ubuntu... > > 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. How often does dmsetup info run? I admit that it becomes slower with the patch, but I didn't think it was really in anyone's hot path. But given that there's a comment just prior that says: /* Commit to ensure statistics aren't out-of-date */ it feels like we ought at least to be calling save_stats() so that we update the on-disk statistics. Though, given that the metadata size should be about 10MB for a 100GB cache device, I don't mind flushing out 10MB of metadata to get the device info. Really the problem is that with both of these complaints active, the superblock counters and tables /never/ seem to get updated, even across multiple reboots. (I'm still digging for why I see such weird unreproduceable benchmark numbers.) --D -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
