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Re: [linux-lvm] Testing the new LVM cache feature



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.


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