On Tue, 18 Dec 2007, Thiemo Nagel wrote:

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

Kind regards,

Thiemo

# dd if=/dev/sdc of=/dev/null bs=1M count=1024
1024+0 records in
1024+0 records out
1073741824 bytes (1.1 GB) copied, 13.8108 seconds, 77.7 MB/s

With more than 2x the drives I'd think you'd have faster speed, perhaps 
the contoller is the problem?

I am using ICH8R (but the raid within linux) and 2 port SATA cards, each 
has their own dedicated bandwidth via PCI-e bus.

I have also tried (on 3ware controllers exporting as JBOD etc, sw RAID5) 
with 10 disks, I saw similar performance with read but not write.

Justin.