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?
I tested that and set 'writeback_delay' to 600. That should be 10
minutes. No change:
PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+
COMMAND
1721 root 20 0 0 0 0 R 100.0 0.000 1075:09
bcache_writeback
Vojtech
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