Re: RAID arrays and performance

On Tue, 4 Dec 2007, Gregory Stark wrote:
> Also, it's true, you need to preread more than 12 blocks to handle a 12-disk
> raid. My offhand combinatorics analysis seems to indicate you would expect to
> need to n(n-1)/2 blocks on average before you've hit all the blocks. There's
> little penalty to prereading unless you use up too much kernel resources or
> you do unnecessary i/o which you never use, so I would expect doing n^2 capped
> at some reasonable number like 1,000 pages (enough to handle a 32-disk raid)
> would be reasonable.

It's better than that, actually. Let's assume a RAID 0 striped set of
twelve discs. If you spread out twelve requests to twelve discs, then the
expected number of requests to each disc is one. The probablility that any
single disc receives more than say three requests is rather small. As you
increase the number of requests, the longest reasonably-expected queue
length for a particular disc gets closer to the number of requests divided
by the number of discs, as the requests get spread more and more evenly
among the discs.

The larger the set of requests, the closer the performance will scale to
the number of discs.

Matthew

--
All of this sounds mildly turgid and messy and confusing... but what the
heck. That's what programming's all about, really
-- Computer Science Lecturer