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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