[dm-devel] dm-cache with zero hit rate

[Amy Rupp]

Is it possible to use different input files (unique files, anyway) of the
same size, maybe even on different file systems if available?  The creation
of these could be scripted along with a dd - don't remove the file
immediately after the dd, though.  It might be even better if you could
issue multiple reads in parallel, though that will be hard to do if it's a
really small file.


On Fri, Aug 2, 2013 at 8:58 AM, Steinar H. Gunderson <sgunderson at bigfoot.com
> wrote:

> On Fri, Aug 02, 2013 at 09:23:44AM -0400, Mike Snitzer wrote:
> > Yes, it is surprising.  Curious to know if the promotions aren't
> > happening due to the IO scheduler somehow merging all your random small
> > IO.  We don't yet have a descrete counter to show the number of
> > migrations that were skipped due to sequential_threshold but that is
> > something we can add.
> >
> > But you can effectively disable the sequential_threshold by setting it
> > really high, e.g.:
> >
> > dmsetup message cache 0 sequential_threshold 16384
>
> I tried this, and it still doesn't appear to promote anything at all:
>
> cache: 0 23440891904 cache 913/8192 0 7021239 0 2049048 0 0 0 0 0 2
> migration_threshold 2048 4 random_threshold 8 sequential_threshold 16384
>
> It's only been running for a few minutes, though.
>
> FWIW, earlier I ran it on only one single partition, and then it worked.
> So it's not like my kernel is completely broken, at least.
>
> > Please write a file that is smaller than your specified
> > sequential_threshold, and then read it numerous times via direct IO,
> > e.g.:
> >
> > dd if=<your file> of=/dev/null iflag=direct bs=16K
>
> I did, with a 16 kB file (that should certainly be small enough, right?),
> executing the dd command 10000 times. Still nothing cached.
>
> /* Steinar */
> --
> Homepage: http://www.sesse.net/
>
> --
> dm-devel mailing list
> dm-devel at redhat.com
> https://www.redhat.com/mailman/listinfo/dm-devel
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://listman.redhat.com/archives/dm-devel/attachments/20130802/c1248721/attachment.htm>