Re: Bcache stuck at writeback of a key, consuming 100% CPU, not possible to detach

[Vojtech Pavlik <vojtech@suse.cz>]

On Mon, Aug 31, 2015 at 08:53:54AM -0800, Kent Overstreet wrote:

> > >  - with a large enough amount of data, the 30 second writeback_delay may be
> > >    insufficient; if it takes longer than that just to scan the entire keyspace
> > >    it'll never get a chance to sleep. try bumping writeback_delay up and see if
> > >    that helps.
> > 
> > That shouldn't be the case when the amount of dirty data is below a
> > gigabyte, or is it?
> 
> No - it has to scan the entire btree, cached _and_ dirty data - so the scanning
> gets expensive when you have lots of clean cached data and very little dirty
> data, so it's supposed to ratelimit to no more than one scan every 30 seconds
> (IIRC; that algorithm has gone through a couple different iterations). But if
> it's taking more than 30 seconds to complete one scan... well, you see the
> problem?

That makes sense. The cache device is ~200 GB. I'll try to play with
writeback_delay when the system is up and running again. Currently I'm
doing a full backup such that I can test things without any worries.

-- 
Vojtech Pavlik
Director SuSE Labs