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Re: Story on IDE raids on tech-report.com & slashdot

On Sat, Dec 07, 2002 at 08:42:20PM +0100, Nicolas Bougues wrote:
> > This might be of interest for the ones that discussed IDE raid in the
> > past days in this list. 
> > 
> > 	IDE RAID Examined
> > 	http://slashdot.org/article.pl?sid=02/12/04/2245253
> > 	http://tech-report.com/reviews/2002q4/ideraid/index.x?pg=1
> You'll notice there that the 3ware board really sucks when it comes to
> write in RAID 5 mode. It's important to notice that until recently,
> 3ware had two versions of their boards : the "standard" one, and the
> "R5 Fusion" one, with "enhanced write performance", which was sold
> something like $100 more.

at a guess, i'd say that the difference is non-volatile write cache.
raid-5 write performance really sucks without it, but it flies when you
have a large (at least 32MB) write-cache.  for safety, it has to be
non-volatile write-cache (and if you don't care about safety, why are
you using RAID-5 rather than RAID-0?)

i did a whole lot of benchmarking last year, comparing different raid
configurations and filesystems (same machine, same raid controller, same
disks) while deciding on the optimum configuration for the mail server i
was building.  the RAID controller was a DOMEX scsi raid box hanging off
an adaptec 2940.

i found that, with a decent RAID controller and a large NV write-cache,
RAID-5 will outperform both RAID1 & RAID 0+1/1+0/10 while providing
significantly more disk space - for "n" disks, RAID5 = n-1 (or n-2 with
one hot spare), while RAID1 & RAID0+1 etc = n/2.

e.g. with 6 x 36GB drives: RAID5 = 180GB (or 144GB with a hot-spare
drive), while RAID1 = 108GB.

IME, for performance, size, and safety it's very hard to beat a good
RAID-5 setup with the filesystem formatted as XFS.  reiserfs performs
slightly better with lots-of-small-files-in-a-directory, but XFS beats
it as a better general-purpose filesystem.  

also, i trust XFS more than reiserfs because it had been in wide usage
(on SGI's IRIX) for several years before it was ported to linux...i.e.
longer & better testing in demanding production environments (SGI's
field of digital video production really stress-tests disks &
filesystems).  unfortunately, it's still not in the standard linux
kernel, you have to apply a patch to get XFS.


craig sanders <cas@taz.net.au>

Fabricati Diem, PVNC.
 -- motto of the Ankh-Morpork City Watch

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