Re: Software RAID question

From: Juhan Leemet (juhan_at_logicognosis.com)
Date: 08/13/04 

Date: Fri, 13 Aug 2004 18:06:47 -0200

On Wed, 11 Aug 2004 20:37:51 +0000, Dances With Crows wrote:
> On 11 Aug 2004 19:56:44 GMT, Davide Bianchi staggered into the Black Sun
> and said:
>> On 2004-08-11, Clayton <ctav01@NOSPAMcomcast.net> wrote:
>>> from what I've read, software RAID is better than hardware
>> Where did you get this impression? Hardware is obviously better, but
>> it's more expensive than software.
>
> This depends on the RAID controller and your definition of "better".
> The advantage of a hardware RAID is that the controller itself handles
> parity checking without bogging down the CPU. The disadvantage of
> hardware RAID is that the controller's parity checking can be slower
> than having a fast CPU do the checking. Software RAID-1 with a 1GHz
> processor is usually faster than hardware RAID-1 with a cheap controller
> card. Can't remember the stats for RAID-5 but RAID-5 is more
> CPU-intensive than RAID-1.

Picking some small nits... There really isn't any parity calculation for
RAID1 (mirroring), so the I/O paths become the limiting factors. Any
decent hardware RAID will have multiple channels and should be able to do
the parallel writes (virtually) simultaneously. I'd be surprised (but it
happens all the time) if RAID1 were slower in hardware.

RAID5 is really where the differences in performance come in. A good
hardware RAID5 will have enough CPU in it to quickly calculate parities
and will also have multiple channels to do simultaneous reads and writes.
Even so, in some cases a RAID5 update requires a read/modify/write of disk
contents, so it must be slower than regular I/O. Doing all that in
software can be REALLY slow. I am using software RAID5 on some of my
server storage and I have noticed up to 5x slower writing speeds. Reading
is supposed to be better than 1x but maybe not as fast as RAID0 or RAID1.

BTW, I've heard of some "hardware RAID" that seems to depend on special
drivers. I suppose you will always need some way to configure the internal
processor and disks. Beware that some of these might be some kind of
WinRAID? They might not be easy (possible?) to run with Linux?

>>> eventually swap out the 60 for a 120 (or larger), will the array
>>> resize itself?
>> Of course not. You have to resize it. But if you want to do things
>> right get disks [that are] all the same size.
>
> In most hardware RAID-5 setups, if you have 3 disks of 10, 20, and 30G,
> you can only create an array of ((3-1)*10)G in size. If you use
> software RAID, you can partition the non-Redundant portions of the 20
> and 30G disks and use them as normal. HTH,

Yes. Some RAID setups suggest that you can add space to a RAID5
metadevice, but if you read the "fine print" you might discover that this
is just single disk space, without any redundancy. I have not heard of any
RAID5 that can dynamically reconfigure disk sizes, or even add disks (that
would alter the way the parity is computed across disks). Plan on doing
backup and restore (onto the new configuration).

Also if you use extra space on a RAID drive for other partitions, you
might be confounding the performance of the RAID by simultaneous accesses
to the other partition(s), whacking the head around on the disk. 

-- 
Juhan Leemet
Logicognosis, Inc.

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