Hi,
at work we have been experimenting with SATA-II port multipliers and SATA
controllers (FIS-based). For some reason the RAID-0 performance is
relatively awful.
We have one 4xeSATA controller card ADSA3GPX8-4E, twelve SpinPoint F1
disks that handle ~110MB/sec, and four port Addonics port multiplier
boxes. Ubuntu, kernel 2.6.24-19-generic x86_64, 2 x AMD2212 Opteron on an
Asus L1N64-SLI WS.
We did two sequential writing tests. One test with all 12 disks
behind the controller placed into one mdadm RAID-0, 512k..2048k
chunksize. We get ~475 MB/s.
The second test used four "mdadm --create"'s to reassign the 12 drives
into four RAID-0 with 3 disks in each. All four raids were simultaneously
written to. We get ~250 MB/s/raid and 1000 MB/s aggregate.
So the controller can handle at least 1000 MB/s.
My question is why is the single RAID setup more than half slower? Any
ideas?
Is there some kernel per-device bandwidth throttling?
I already tried: O_DIRECT, posix_fadvise(), madvise(), pdflush tuning,
different test programs, using a raw /dev/md{0,1,2,3} vs testing
the xfs filesystem. None of them get the single RAID performance over 475
MB/s. CPU load isn't a likely problem either. To check I wrote a threaded
program[1] that has threads writing in-sequence into the same output file.
All 4 cores loaded very evenly and are >=60% idle. But the rate is still
475 MB/s. Puzzling and frustrating :-)
Is there still something that a desperate person could try to tune to get
this lousy mdadm raid throughput improved?? :-)
Thanks in advance,
- Jan
[1] http://www.metsahovi.fi/~jwagner/tmp/wr-nexgen-threaded.c
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