Mailinglist Archive: opensuse (1470 mails)

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Re: [opensuse] Re: any real-life experiences with bache / dm-cache and ssd ?
On Don, 2016-02-11 at 15:35 +0100, Per Jessen wrote:
On random read, it slows down quite a bit, I guess that is because
it's
actually reading from disk?
I'm surprised to see how slow "Mixed 70/30 random read and write with
8K
block size" is. Random read is reading from disk, but why is the
random
write so slow?

I think the fio.bash test script is a bit misleading in this case.

The interesting property to test is caching, meaning we expect repeated
access of the same files to show increasing performance because the
cache hit rate increases.

The following are repeated runs of the 70/30 random rw tests from
fio.bash on my bcache setup. The test files are created (but not cached
on the SSD) during the first run, and reused for subsequent runs (which
gradually causes more and more of the files to be cached):


# rm /home/8k7030test.*
# fio --size=128M --direct=1 --rw=randrw --refill_buffers --norandommap
--ioengine=libaio --bs=8k --rwmixread=70 --iodepth=16 --numjobs=16 -
-runtime=10 --group_reporting --name=8k7030test --directory=/home |
grep iops
read : io=64208KB, bw=6052.3KB/s, iops=756, runt= 10609msec
write: io=26704KB, bw=2517.2KB/s, iops=314, runt= 10609msec
# !fio
read : io=102888KB, bw=9474.1KB/s, iops=1184, runt= 10859msec
write: io=41888KB, bw=3857.5KB/s, iops=482, runt= 10859msec
# !fio
read : io=142144KB, bw=13066KB/s, iops=1633, runt= 10879msec
write: io=57912KB, bw=5323.3KB/s, iops=665, runt= 10879msec
# !fio
read : io=139864KB, bw=13272KB/s, iops=1659, runt= 10538msec
write: io=56968KB, bw=5405.1KB/s, iops=675, runt= 10538msec
# !fio
read : io=147000KB, bw=13905KB/s, iops=1738, runt= 10572msec
write: io=59616KB, bw=5639.5KB/s, iops=704, runt= 10572msec
# !fio
read : io=177832KB, bw=16286KB/s, iops=2035, runt= 10919msec
write: io=71288KB, bw=6528.9KB/s, iops=816, runt= 10919msec


It is also interesting to inspect the entire fio output. Among other
things, it shows the performance of all physical devices involved, as
well as the load distribution on each one of them.

Regards,
Olav

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