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

Fri Feb 20 14:52:47 UTC 2009
Chan Chung Hang Christopher <christopher.chan at bradbury.edu.hk>

> "md1 will read from both disk" is not true in general.
> RAID1 md reads from one disk only; it uses the other one in case the 
> first one fails. No performance gain from multiple copies.
>   
I beg to differ. I have disks in a raid1 md array and iostat -x 1 will 
show reads coming off both disks. Unless you do not have the multipath 
module loaded, md will read off both disks. Now whether md will read 
equally off both disks, that certainly will not be true in general.
> You can easily see this for yourself by setting up a RAID1 from e.g. 
> sda1 and sdb1 - /proc/mdstat is:
>
> Personalities : [raid1]
> md1 : active raid1 sdb1[1] sda1[0]
>        104320 blocks [2/2] [UU]
>
> and then comparing the output of hdparm -tT :
>   
ROTFL.

How about using the proper tool (iostat) and generating some disk load 
instead?

> To get performance gain in RAID1 mode you need hardware RAID1.
>   

Bollocks. The only area in which hardware raid has a significant 
performance advantage over software raid is raid5/6 given sufficient 
cache memory and processing power.