Re: Raid over 48 disks

[Bill Davidsen <davidsen@tmr.com>]

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?

-- 
Bill Davidsen <davidsen@tmr.com>
  "Woe unto the statesman who makes war without a reason that will still
  be valid when the war is over..." Otto von Bismark