Hi,
I have been trying to figure out the best chunk size for raid10 before
migrating my server to it (currently raid1). I am looking at 3 offset
stripes, as I want to have two drive failure redundancy, and offset
striping is said to have the best write performance, with read
performance equal to far. Information on the internet is scarce so I
decided to test chunking myself. I used the script
http://rabbit.us/pool/misc/raid_test.txt to iterate through different
chunk sizes, and try to dd the resulting array to /dev/null. I
deliberately did not make a filesystem on top of the array - I was just
looking for raw performance, and since the FS layer is not involved no
caching/optimization is taking place. I also monitored the process with
dstat in a separate window, and memory usage confirmed that this method
is valid.
I got some pretty weird results:
http://rabbit.us/pool/misc/raid_test_results.txt
From all my readings so far I thought that with chunk size increase the
large block access throughput decreases while small block reads
increase, and it is just a matter of finding a "sweet spot" balancing
them out. The results, however, clearly show something else. There are
some inconsistencies, which I attribute to my non-scientific approach,
but the trend is clearly showing.
Here are the questions I have:
* Why did the test show best consistent performance over a 16k chunk? Is
there a way to determine this number without running a lengthy
benchmark, just from knowing the drive performance?
* Why although I have 3 identical chunks of data at any time, dstat
never showed simultaneous reading from more than 2 drives. Every dd run
was accompanied by maxing out one of the drives at 58MB/s and another
one was trying to catch up to various degrees depending on the chunk
size. Then on the next dd run two other drives would be (seemingly
random) selected and the process would repeat.
* What the test results don't show but dstat did is how the array resync
behaved after the array creation. Although my system can sustain reads
from all 4 drives at the max speed of 58MB/s, here is what the resync at
different chunk sizes looked like:
32k - simultaneous reads from all 4 drives at 47MB/s sustained
64k - simultaneous reads from all 4 drives at 56MB/s sustained
128k - simultaneous reads from all 4 drives at 54MB/s sustained
512k - simultaneous reads from all 4 drives at 30MB/s sustained
1024k - simultaneous reads from all 4 drives at 38MB/s sustained
4096k - simultaneous reads from all 4 drives at 44MB/s sustained
16384k - simultaneous reads from all 4 drives at 46MB/s sustained
32768k - simultaneous reads from 2 drives at 58MB/s sustained and
the other two at 26MB/s sustained alternating the speed
between the pairs of drives every 3 seconds or so
65536k - All 4 drives started at 58MB/s sustained gradually
reducing to 44MB/s sustained at the same time
I repeated just the creation of arrays - the results are consistent. Is
there any explanation for this?
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