[CentOS] md raid1 - no speed improvement

Les Mikesell lesmikesell at gmail.com
Sun Mar 23 15:28:26 UTC 2008


Steve Snyder wrote:
> On Sunday 23 March 2008 7:11:41 am Kieran Clancy wrote:
> [snip]
>> So the total cached read bandwidth seems limited to about 2250 MB/s,
>> which is slightly higher than the cache read bandwidth for /dev/md2,
>> but I'm not too worried about that. More concerning is that I am
>> still getting ~80MB/s from each disk simultaneously on the buffered
>> reads. Given this I would expect /dev/md2 to give buffered read
>> speeds of at least 120MB/s (if not 150MB/s).
> 
> The value of RAID 1 isn't speed. it's fault tolerance.  I would expect 
> what you're seeing, a slight performance *loss* (relative to a single 
> disk) due to the additional overhead of the RAID management.
> 
> http://www.linuxhomenetworking.com/wiki/index.php/Quick_HOWTO_:_Ch26_:_Linux_Software_RAID#RAID_1

Raid1 is supposed to let the disks read independently but I'm not sure 
if it works that way when there is only a single read happening.  I'd 
expect a bigger difference when reading many small files simultaneously 
     and the drive with the head nearest to the right place is used for 
each access.

-- 
   Les Mikesell
    lesmikesell at gmail.com




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