[dm-devel] I/O block when removing thin device on the same pool

[Lars Ellenberg]

On Fri, Jan 29, 2016 at 04:04:21PM +0000, Joe Thornber wrote:
> On Fri, Jan 29, 2016 at 03:50:31PM +0100, Lars Ellenberg wrote:
> > On Fri, Jan 22, 2016 at 04:43:46PM +0000, Joe Thornber wrote:
> > > On Fri, Jan 22, 2016 at 02:38:28PM +0100, Lars Ellenberg wrote:
> > > > We have seen lvremove of thin snapshots sometimes minutes,
> > > > even ~20 minutes before.
> > > 
> > > I did some work on speeding up thin removal in autumn '14, in
> > > particular agressively prefetching metadata pages sped up the tree
> > > traversal hugely.  Could you confirm you're seeing pauses of this
> > > duration with currently kernels please?
> > 
> > There is 
> > https://bugzilla.redhat.com/show_bug.cgi?id=990583
> > Bug 990583 - lvremove of thin snapshots takes 5 to 20 minutes (single
> > core cpu bound?) 
> > 
> > >From August 2013, closed by you in October 2015,
> > as "not a bug", also pointing to meta data prefetch.
> > 
> > Now, you tell me, how prefetching meta data (doing disk IO
> > more efficiently) helps with something that is clearly CPU bound
> > (eating 100% single core CPU traversing whatever)...
> > 
> > Reason I mention this bug again here is:
> > there should be a lvm thin meta data dump in there,
> > which you could use for benchmarking improvements yourself.
> 
> There is no metadata dump attached to that bug.

Hm. Then we must have communicated via side channels
(irc, uploads) back then, I'm pretty sure we uploaded it somewhere.

> I do benchmark stuff
> myself, and I found prefetching to make a big difference (obviously
> I'm not cpu bound like you).  We all have different hardware, which is
> why I ask people with more real world scenarios to test stuff separately.


Thank you for suggestions (and re-opening that bug).
I'll have someone follow up in the bugzilla,
as soon as we have something to report.

I just checked, we still have some similar setups running regularly, and
according to log files of the one system I looked at, apparently snapshot
removals are typically between 2.5 and 4 minutes now, when they used to be 15
to 20 minutes most of the time.

But on first glance, I can see no correlation between kernel upgrades or other
changes and reduced removal times, so this may simply be a change of access
pattern on the origin :-/

I still can get you meta dumps, if you are interested,
or maybe have the guys try some things.

I would be communicating via a number of hops,
so no "direct access lab setup" right now.

Let me know what data you would be interested in most,
and I'll try to "make it happen", and relay it to you.

Cheers,

   Lars