On Wed, 19 Dec 2007, Bill Davidsen wrote:
Thiemo Nagel wrote:
Performance of the raw device is fair:
# dd if=/dev/md2 of=/dev/zero bs=128k count=64k
8589934592 bytes (8.6 GB) copied, 15.6071 seconds, 550 MB/s
Somewhat less through ext3 (created with -E stride=64):
# dd if=largetestfile of=/dev/zero bs=128k count=64k
8589934592 bytes (8.6 GB) copied, 26.4103 seconds, 325 MB/s
Quite slow?
10 disks (raptors) raid 5 on regular sata controllers:
# dd if=/dev/md3 of=/dev/zero bs=128k count=64k
8589934592 bytes (8.6 GB) copied, 10.718 seconds, 801 MB/s
# dd if=bigfile of=/dev/zero bs=128k count=64k
3640379392 bytes (3.6 GB) copied, 6.58454 seconds, 553 MB/s
Interesting. Any ideas what could be the reason? How much do you get from
a single drive? -- The Samsung HD501LJ that I'm using gives ~84MB/s when
reading from the beginning of the disk.
With RAID 5 I'm getting slightly better results (though I really wonder
why, since naively I would expect identical read performance) but that does
only account for a small part of the difference:
16k read 64k write
chunk
size RAID 5 RAID 6 RAID 5 RAID 6
128k 492 497 268 270
256k 615 530 288 270
512k 625 607 230 174
1024k 650 620 170 75
What is your stripe cache size?
# Set stripe-cache_size for RAID5.
echo "Setting stripe_cache_size to 16 MiB for /dev/md3"
echo 16384 > /sys/block/md3/md/stripe_cache_size
Justin.
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