Software RAID 5 or something else?

[Chris Tyler]

On Fri, 2009-01-23 at 12:39 -0500, aragonx at dcsnow.com wrote:
> 1) I want to get some redundancy in case of a drive failure.
> 
> 2) I want to increase my performance.  I have benchmarked my read and
> write performance to and from this server.  Using Samba, I seem to be able
> to get about 50Mb/sec reads and 40Mb/sec writes.  I am on a gig network
> and would like to be able to max out the cards (90Mb/sec is what I get at
> work).
> 
> So, the question is, what should I do?
> 
> 1)  Bite the bullet and get the hardware RAID controller.  Will this give
> me the performance I want?

RAID controllers with onboard memory and CPU might free up 2-3% of your
main CPU time. In all but the most demanding cases, this isn't
worthwhile. RAID controllers with ROM-based BIOS extensions and no CPU
use the main system CPU to do the work, so no real benefit over kernel
RAID, except that if the card fails it will be harder to get your data
back (!).


> 2)  Go with a software RAID 5.  Will I lose performance with this
> configuration?  If I use this but only get modest performance gains, that
> would be acceptable.

With 5 drives on 5 controllers, you could potentially see read
performance gains of 4x+ on long sequential reads (parallel reads).
Write performance will vary from less than 1/2 (read from 2 drives then
write to 2 drives for each sub-chunk modification) to more than 4x the
single-drive performance (large sequential writes), depending on usage
patterns.


-Chris