Hi Guys,
we spent some time over the past week looking at hammer vs jewel RBD
performance in HDD only, HDD+NVMe journal, and NVMe cases with the
default filestore backend. We ran into a number of issues during
testing and I don't want to get into everything, but we were eventually
able to get a good set of data out of fio's bandwidth and latency logs.
Fio's log sampling intervals were not uniform, but Jens has since
written an experimental patch for fio that fixes this which you can find
in this thread:
http://www.spinics.net/lists/fio/msg04713.html
We ended up writing a parser that can work around this by reading
multiple fio bw/latency log files and getting aggregate data even
assuming non-uniform sample intervals. This was briefly part of CBT,
but recently was included upstream in fio itself. Armed with this, we
were able to get a seemingly accurate view of hammer vs jewel
performance across various IO sizes:
https://docs.google.com/spreadsheets/d/1MK09ZXufTUCgqa9jVJFO-J9oZWMKn7SnKN7NJ45fzTE/edit?usp=sharing
The gist of this is that Jewel is faster than Hammer for many random
workloads (Read, Write, and Mixed). There is one specific case where
performance degrades significantly: 64-128k sequential reads. We
couldn't find anything obviously wrong with these tests, so we spent
some time running git bisects between hammer and jewel with the NVMe
test configuration (these tests were faster to setup/run than the HDD
setup). We tested about 45 different commits with anywhere from 1-5
samples depending on how confident the results looked:
https://docs.google.com/spreadsheets/d/1hbsyNM5pr-ZwBuR7lqnphEd-4kQUid0C9eRyta3ohOA/edit?usp=sharing
There are several commits of interest that have a noticeable effect on
128K sequential read performance:
1) https://github.com/ceph/ceph/commit/3a7b5e3
This commit was the first that introduced anywhere from a 0-10%
performance decrease in the 128K sequential read tests. Primarily it
made performance lower on average and more variable.
2) https://github.com/ceph/ceph/commit/c474ee42
This commit had a very large impact, reducing performance by another 20-25%.
3) https://github.com/ceph/ceph/commit/66e7464
This was a fix that helped regain some of the performance loss due to
c474ee42, but didn't totally reclaim it.
4) 218bc2d - b85a5fe
Between commits 218bc2d and b85a5fe, there's a fair amount of
variability in the test results. It's possible that some of the commits
here are having an effect on performance, but it's difficult to tell.
Might be worth more investigation after other bottlenecks are removed.
5) https://github.com/ceph/ceph/commit/8aae868
The new AioImageRequestWQ appears to be the cause of the most recent
large reduction in 128K sequential read performance.
6) 8aae868 - 6f18f04
There may be some additional small performance impacts in these commits,
though it's difficult to tell which ones since most of the bisects had
to be skipped due to ceph failing to compile.
This is what we know so far, thank for reading. :)
Mark
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