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Date: Fri, 30 May 2014 15:54:49 +0200
From: Heinz Mauelshagen <heinzm@redhat.com>
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Subject: Re: [linux-lvm] Testing the new LVM cache feature
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To: "Richard W.M. Jones" <rjones@redhat.com>, Mike Snitzer <snitzer@redhat.com>
Cc: Zdenek Kabelac <zkabelac@redhat.com>, thornber@redhat.com, LVM general discussion and development <linux-lvm@redhat.com>

On 05/30/2014 03:46 PM, Richard W.M. Jones wrote:
> I have now set both read_promote_adjustment ==
> write_promote_adjustment == 0 and used drop_caches between runs.

Did you adjust "sequential_threshold 0" as well?

dm-cache tries to avoid promoting large sequential files to the cache,
because spindles have good bandwidth.

This is again because of the hot spot caching nature of dm-cache.


>
> I also read Documentation/device-mapper/cache-policies.txt at Heinz's
> suggestion.
>
> I'm afraid the performance of the fio test is still not the same as
> the SSD (4.8 times slower than the SSD-only test now).
>
> Would repeated runs of (md5sum virt.* ; echo 3 > /proc/sys/vm/drop_caches)
> not eventually cause the whole file to be placed on the SSD?
> It does seem very counter-intuitive if not.

Please retry with "sequential_threshold 0"

Heinz

>
> Rich.