[dm-devel] dm-cache mq policy warmup issue

[Vladimir Smolensky]

Okay, a little correction here. The problematic setup was with 2.3TB cache,
not 3TB. The mistake comes from the fact that at the time of the writing I
was dealing with another server which has 8x480GB ssd's in raid5, which is
little over 3TB. Obviously the number was floating in my head.





On Thu, Nov 14, 2013 at 4:37 PM, Joe Thornber <thornber at redhat.com> wrote:

> On Wed, Nov 13, 2013 at 04:13:43PM +0000, Mears, Morgan wrote:
> > On Mon, Nov 04, 2013 at 12:22:26PM -0500, Krishnasamy, Somasundaram
> wrote:
> > > Mike,
> > >
> > > I was doing some testing with latest dm-cache code (from 'for-next'
> branch). I
> > > am having difficulty in making the cache to warm-up. I used the below
> script
> > > to read range of blocks repeatedly to get them promoted. I assume after
> > > reading a block for mq->promote_threshold + 4 times, a block will be
> > > promoted. But it is not happening. I also tested the same with the
> > > sequential_threshold set to very high value.
> > >
> > > Cache warming-up was relatively easier in the previous dm-cache code.
> Is this
> > > something you have seen in your testing?. Is there anything different
> should I
> > > do?
> >
> > I'm also testing with the for-next branch of linux-dm.git and seeing the
> > same thing as Somu.  It's easy to reproduce; reading the same block on
> the
> > cached device over and over with dd does not result in that block being
> > promoted.
> >
> > I did notice that the "read miss" counter for the cached device
> increments
> > each time I execute dd, which I believe means this isn't a buffer cache
> > issue (I also used dd iflag=direct).  The migration_threshold value
> > shouldn't be an issue for this test, but I set it to a very high value
> > anyway; didn't make any difference.
> >
> > Are there any tests in device-mapper-test-suite that you would recommend
> > for testing whether promotions are occurring as expected?
> >
> > --Morgan
>
> Hi Morgan,
>
> I just added these tests, promotion is occuring, just not as much as I'd
> like:
>
>
> https://github.com/jthornber/device-mapper-test-suite/blob/master/lib/dmtest/tests/cache/promotion_tests.rb
>
> Ah, these tests all trigger promotion via a write, I'll do some that just
> read.
>
> - Joe
>
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