On Wed, 19 Dec 2007, Bill Davidsen wrote:

Thiemo Nagel wrote:

quoted

quoted

quoted

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.