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



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 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 */
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