[CentOS] 4 X 500 gb drives - best software raid config for a backup server?

Sat Feb 21 17:05:44 UTC 2009
Les Mikesell <lesmikesell at gmail.com>

Chan Chung Hang Christopher wrote:
>    
>> We were talking about RAID1; RAID5/6 is a different area. Linux software 
>> RAID1 is a safeguard against disk failure; it's not designed for speed 
>> increase. There is a number of things that could be improved in Linux 
>> software RAID; read performance of RAID1 is one of them - this _is_ why 
>> some hardware RAID1 adapters indeed are faster than software.
>> Read http://kernelnewbies.org/KernelProjects/Raid1ReadBalancing - since 
>> the 2.6.25 kernel a simple alternating read is implemented, but that 
>> does not take the access pattern into account.
>>   
> I have not read that yet but that is odd since I have been blasted by 
> others before for doubting md raid1 doing multiple disk reads.

I know current centos alternates because I had a box with bad memory 
corrupt a raid1 filesystem and after fixing it and fsck'ing the disk, 
errors would slowly re-appear as they were hit on the alternate disk 
that previous fsck runs had not seen.

To really speed things up, you would want to try to avoid seeks.  A 
simple alternating pattern may let the heads be in different places on 
small reads, but for a large file you will end up doing the same head 
motions on both drives (wasting the same time) as the reads alternate.

-- 
   Les Mikesell
    lesmikesell at gmail.com