RAID suggestions...

[Bruno Wolff III]

On Sun, Dec 31, 2006 at 19:48:23 -0700,
  "Ashley M. Kirchner" <ashley at pcraft.com> wrote:
> 
>    I'm looking at setting up a RAID system for doing video work.  My 
> two main requirements are speed and reliability.  Speed is an issue 
> because of the constant streaming of hundreds of megabytes, if not 
> gigabytes of data that is inherent with video.  And obviously 
> reliability, I want to make sure I don't lose any data of a drive 
> crashes on me.  This will also be a hardware RAID solution (as opposed 
> to software) and, depending on funds, I may try to make it a 
> hot-swappable system.
> 
>    Based on all the different RAID levels out there, what does the 
> collective suggest?  1? 3? 5? 0+1?

Raid 5 is generally good for mostly read situations, though you may want
to do some bench marking to see if you really need that. More spindles
are better than fewer spindles. Normally you would use raid 10 in preference
to raid 0+1, since raid 10 handles failures better. If are capturing video and
not publishing it, than raid 10 is usually better than raid 5 unless you have
a fair number of spindles.

I wouldn't be too quick to dismiss software raid. Generally unless you spend
good money to get a raid card with battery backed cache, you are better
off with software raid. In your case you might have a cpu bottleneck
depending on what processing you are doing of the video. In that case you
would be more inclined to go with hardware raid, but you would want to
look at the cost of buying more cpu versus the cost of a better raid card.

You should be able to rough out some numbers to see what you need to do
what you want. You should be able to figure out what data transfer rates
you need and then test the bandwidth for individual disks to get an estimate
of how many spindles you need. And you should be able to compare that
to the bandwidth supported by the controllers you are considering. If you
have really heavy demands, you may need multiple controllers.

Be careful with the write cache settings of your drives. If you are using
software raid, you can leave write caching enabled if you use write barriers.
A good raid card should properly handle write caching for you, but you
might want to do some sanity tests to make sure that writes aren't being
reported as complete when they are actually just in the drives' cache.