[dm-devel] dm-cache with zero hit rate
Steinar H. Gunderson
sgunderson at bigfoot.com
Fri Aug 2 13:58:28 UTC 2013
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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Homepage: http://www.sesse.net/
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