Re: Hardware purchase question

Mitch Pirtle wrote:
> On Mon, 13 Dec 2004 09:23:13 -0800, Joshua D. Drake
> <jd(at)commandprompt(dot)com> wrote:
>
>>RAID 10 will typically always outperform RAID 5 with the same HD config.
>
>
> Isn't RAID10 just RAID5 mirrored? How does that speed up performance?
> Or am I missing something?
>
> -- Mitch

Hi Mitch,

Nope, Raid 10 (one zero) is a mirror is stripes, no parity. with r10
you get the benefit of a full mirror which means your system does not
need to calculate the XOR parity but you only get 50% disk usage. The
mirror causes a slight write hit as the data needs to be split between
two disk (or in this case, to striped pairs) but reads can be up to
twice as fast (theoretically). By adding the stripe you negate the write
hit and actually gain write performance because half the data goes to
mirror A, half to mirror B (same with reads, roughly).

Raid 10 is a popular choice for software raid because of the reduced
overhead. Raid 5 on the otherhand does require that a parity bit is
calculated for every N-1 disks. With r5 you get N-1 disk usage (you get
the combined capacity of 3 disks in a 4 disk r5 array) and still get the
benefit of striping across the disks so long as you have a dedicated
raid asic that can do the XOR calculations. Without it, specially in a
failure state, the performance can collapse as the CPU performs all that
extra math.

hth

Madison