Re: Raid over 48 disks

[Thiemo Nagel <thiemo.nagel@ph.tum.de>]

>> 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