Re: RAIDframe performance vs. stripe size

[Greg Oster <oster%cs.usask.ca@localhost>]

On Thu, 10 May 2012 18:59:42 +0200
Edgar Fuß <ef%math.uni-bonn.de@localhost> wrote:

> I don't know whether I'm getting this right.
> 
> In my understanding, the benefit of a large stripe size lies in
> parallelisation:

Correct.

> Suppose the stripe size is such that a file system
> block fits on a single disc, i.e. stripe size = (file system block
> size)*(number of "effective" discs). Then, if one (read) transfer
> makes disc A seek to block X, there is a good chance that the next
> transfer can be satisfied from disc B != A, making disc B seek to
> block Y in parallel. Or will this (issuing requests to different
> discs in parallel) not happen on NetBSD?

Unless something issues the command to disc B to fetch block Y in
parallel, it won't happen.  That is, if you simply make a request for
block X from disc A, there's nothing in the RAIDframe code that will
have it automatically go looking to B for Y.

What you're typically looking for in the parallelization is that a
given IO will span all of the components.  In that way, if you have n
components, and the transfer would normally take t amount of time, then
the total time gets reduced to just t/n (plus some overhead) because
you are able to do IO to/from all components at the same time.
RAIDframe will definitely do this whenever it can.

Later...

Greg Oster