Re: I/O block when removing thin device on the same pool

[Lars Ellenberg <lars.ellenberg@linbit.com>]

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.

> Obviously any pause, even a few seconds is unacceptable.  Having a
> background kernel worker thread doing the delete, as you describe, is
> the way to go.  But there are complications to do with
> transactionality and crash protection that have prevented me
> implementing it.  I'll think on it some more now I know it's such a
> problem for you.
> 
> - Joe

Thanks,

    Lars