We reached a fairly good performance on a P400 controller (8 SATA 146GB
2,5" 10k rpm) with raid5 or raid6 Linux software raid: the writing
bandwidth reached about 140 MB/s sustained throughput (the hardware
raid5 gave a sustained 20 MB/s...). With a second, equal controller (16
disks) we reached (raid6 spanning all 16 disks) about 200 MB/s sustained.

The CPU load is negligible. Reading performance is about 20% better.

Best regards and my apologies for my bad English.

GT

P.S.: on a P800, 12 SATA 750GB 3,5" 7200 rpm, the hardware raid5 writing
performance was about 30 MB/s, software raid5 is between 60 and 80 MB/s.



Scott Marlowe ha scritto:
On Tue, Dec 9, 2008 at 5:17 AM, Mario Weilguni wrote:
Alan Hodgson schrieb:
Mario Weilguni wrote:
strange values. An individual drive is capable of delivering 91
MB/sec
sequential read performance, and we get values ~102MB/sec out of a
8-drive RAID5, seems to be ridiculous slow.
What command are you using to test the reads?

Some recommendations to try:

1) /sbin/blockdev --setra 2048 device (where device is the partition or
LVM volume)

2) Use XFS, and make sure your stripe settings match the RAID.

Having said that, 102MB/sec sounds really low for any modern controller
with 8 drives, regardless of tuning or filesystem choice.
First, thanks alot for this and all the other answers.

I measured the raw device performance:
dd if=/dev/cciss/c0d0 bs=64k count=100000 of=/dev/null

I get poor performance when all 8 drives are configured as one, large
RAID-5, and slightly poorer performance when configured as JBOD. In
production, we use XFS as FS, but I doubt this has anything to do with FS
tuning.
Yeah, having just trawled the pgsql-performance archives, there are
plenty of instances of people having terrible performance from HP
smart array controllers before the P800. Is it possible for you to
trade up to a better RAID controller? Whichever salesman sold you the
P400 should take one for the team and make this right for you.

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