Re: dm-cache with zero hit rate

["Steinar H. Gunderson" <sgunderson@bigfoot.com>]

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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