[SunRescue] Re: Software raid

[Jonathan Katz]

Kevin wrote:
> Actually, you can run DiskSuite raid zero on one disk
> and get much better (and quieter) performance than
> without.  It must be the extra caching or some intelligent
> write queueing that smooths out the seeks.

I've heard other people mention this, too. Having never
experimented I've never done this and don't know how/why
it works *shrug*
 
> On a side note, using the "cat" device driver in FreeBSD
> to stripe two disks, I was able to achieve exactly twice
> the throughput of a single disk under careful measurement.
> I was impressed with that since it was a software only
> solution.

No "hard math" like calculating parity or ensuring atomic 
writes-- it just splits the write between two instances of
the sd driver IIRC.
 
> Of course those examples are with raid 0.
> Raid 5 is what kills you on performance.  If you have
> the disks, try raid0+1 (the optimum for high write
> counts anyway).  

That's what most of the client sites I work at, do. Then
again, until the T3 and A1000, Sun really didn't have a
decent hardware RAID solution (look at the A3500 and
RSM 2000-- until recently a kludgey UI and no real
guaranteed failover operation.) EMC just represents their
"disks" as 2GB or 4GB or 9GB "virtual disks" that you use
Veritas (or Disksuite) to stripe. A5x00's are just FC
JBODs which people need to use Disksuite or Veritas to
stripe. 

After reading up, I've found that larger stripe-sizes
increase performance left-and-right across the board.
Many times people will bitch about performance becuse
they used the default stripe values of 32k or 64k when
a size like 256k is more appropriate. As a bad indicator
of speed, just look at the speed of newfs on the stripe.
With a 64k stripe size when newfsing either a 70GB
stripe on my RSM2000 or just a Raid5 stripe on one of 
the RSM trays (7 4GB disks) the 32k stripe size would take
hours. with 256k it would take under 45 minutes. Read
and write operations (mostly larger then 3MB files) are
quite speedy in my setup at home.

Riad0+1 is faster than raid5-- no question, but many
folks don't setup either correctly (stripe size, mirroring
on the same controller, etc.) Most setups I see are
higher-end Oracle/Sybase setups where they would benefit
from a large stripe size. OH well...

-Jon