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

[Joe Thornber]

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.  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.