Re: RAIDframe performance vs. stripe size

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

On Thu, 10 May 2012 17:46:38 +0200
Edgar Fuß <ef%math.uni-bonn.de@localhost> wrote:

> Does anyone have some real-world experience with RAIDframe (Level 5)
> performance vs. stripe size?
> My impression would be that, with a not to large number of components
> (4+1, in my case), chances are rather low to spread simultaneous
> accesses to different physical discs, so the best choice seems the
> file system's block size.
> On the other hand, the Carnegie Mellon paper suggests something around
>       1/2*(througput)*(access time),
> which would amount to more than a Megabyte with my discs.
> So, what are the real-world benefits of a large stripe size? My
> application are home and mail directories exported via NFS.

There have been various discussions of this on the mailing lists over
the years... 

The one issue you'll find is that RAIDframe still suffers from the 64K
MAXPHYS limitation -- RAIDframe will only be handed 64K at a time, and
from there it hands chunks of that out to each component.  So if you
have a 4+1 RAID 5 set, if you make the stripe 32 blocks (16K) wide,
that will allow a single 64K IO to hit all 4+1 disks at the same time.
Of course, you'll want to make sure that your filesystem is aligned so
that those 64K writes are stripe aligned, and you'll also probably want
to use fairly large block sizes (16K/64K frag/block) too...

Later...

Greg Oster