RAID Cards performance issue

[Christopher Chan]

Scott Balneaves wrote:
> On Tue, Dec 09, 2008 at 09:40:44AM +0800, Christopher Chan wrote:
>> Not surprisingly, raid5 beat the pants off raid10. Why? For six disks, 
>> on raid5, you have all six disks for input and output. On raid10, you 
>> need to make three mirrors and so you are effectively reduced to three 
>> disks for input and output.
> 
> Personally, I'd argue that's a broken RAID10, since, for reading at least, you
> should be able to read off BOTH sides of the mirror, typically doubling your
> read speed.

AFAIK, that sort of thing seems to be only done on Linux software raid. 
Not sure if any hardware raid card actually does the same thing as 
reading off all disks in a quasi raid0 way.

> 
> I've seen study after study indicating that, for most applications, RAID10
> outperforms RAID5, and my own personal experience is that way as well.  I
> suspect we're quickly edging into the realm of religious discussion, so we'll
> just leave things pat, and say, "Your Mileage May Vary, personal testing on
> your hardware setup will determine the best course of action".

I was just pointing out that performance is not necessarily an issue 
with raid5. A lot of factors can change the answer as to what is the 
fastest raid implementation I will get on this set of hardware and 
specifically those factors are whether there is sufficient processing 
power and cache memory on the raid card. This is not an argument for 
raid5 being the best solution there is.

A six disk raid5 array can take one disk going down. A six disk raid10 
array can take up to 3 disks going down if you are fortunate (or maybe 
not so fortunate) and in the tests in that Linux.com article, the raid10 
array was doing better with three spindles than the raid5 array with six 
spindles comparatively speaking. Like you said, it is a compromise. 
3ware raid10/raid1 sync/rebuild code is very good as they keep markers 
so you would not necessarily have to resync all data in a mirror. I am 
not sure if Linux md raid1 has caught on that count yet.

> 
> That being said, and since you're obviously up on RAID5, any suggestions for
> the original poster?

I am not up on raid5. Man, I stay away from it! That ten disk raid5 
array on a 3ware 9550 I used as a mail queue was given to me not only to 
handle a temporary issue but also to silence me on my criticism of 
raid5. That was when I learnt that the lack of cache memory was a 
primary reason why a 3ware 850x board has really poor performance when 
doing raid5. I would not touch raid5 with Linux software raid. Things 
have probably changed now though.

As for the OP, I asked the OP whether he was running the latest firmware 
from 3ware. Then there is whether he is using the 3ware board on a 
tested motherboard as listed on 3ware's website. After that it is try 
the latest driver from 3ware...all the general procedures one can take 
to try to find the problem. I am no longer in a high i/o required 
environment and so I have not touched a 3ware or any other raid card in 
a few years. You won't be getting any special insight from me on 
problems with anything newer than them 750x/850x 3ware boards.