Re: 3 disk raid-5 without parity

On Monday June 14, thunder7@xs4all.nl wrote:

I am trying to convince my boss our new database-server wants raid-0+1,
not raid-5, and I got an idea while reading endless articles about
raid-5 being slow when writing and management not listening.

You want numbers, not abstract arguments.

Configure your server with raid5 and do some performance measurements
- preferably with your database suite.
Then reconfigure with raid 0+1 and test again.

Show the numbers to your boss.  You get to choose which numbers to
show :-)

suppose you make a 3-disc raid-5 without parity:

data    disc1 disc2 disc3
A       A     A     B
B       B     C     C

How would that perform compared to raid-5 and raid-0+1?

Should be slightly better than raid1 of 2 drives, and slower than raid
0+1 on 4 drives.  How it compares with raid-5 depends largely on load
characteristics.  With only 3 drives, some loads will very often
provide raid5 with both data blocks in a stripe, and so no pre-reading
will be needed.

I hope to release a "raid10" module for 2.6 within a couple of weeks.
raid10 is basically a combination of raid1 and raid0 all in one module
with some interesting geometry possibilities.  This particular
geometry is one of the possibilities.

There is a problem with extending this: you need groups of 3 disks.
Then again, compared to raid-0+1 you need fewer disks.

There is no problem extending this. 
With 4 discs it would be
  A A B B
  C C D D
which is a lot like raid0 over raid1
With 5 disks is would be
  A A B B C
  C D D E E
or, if you wanted more redundancy:
  A A A B B
  B C C C D
  D D E E E

The more discs you have, the faster it should be.

Has this ever been implemented? Even better, benchmarked?

Implemented:  I'm fairly sure it has.  Not in Linux just yet.
Benchmarks: I don't know of any details.

NeilBrown

From: Neil Brown <redacted>
Date: Mon, Jun 14, 2004 at 09:45:21PM +1000

On Monday June 14, thunder7@xs4all.nl wrote:

quoted

I am trying to convince my boss our new database-server wants raid-0+1,
not raid-5, and I got an idea while reading endless articles about
raid-5 being slow when writing and management not listening.

You want numbers, not abstract arguments.

Configure your server with raid5 and do some performance measurements
- preferably with your database suite.
Then reconfigure with raid 0+1 and test again.

Show the numbers to your boss.  You get to choose which numbers to
show :-)

yes - unfortunately, corporate IT gets to decide which hardware to test
on. Guess what? They just got a shiny new SAN with 140 Gb disks,
configured in raid-5. Not quite the raid-0+1 over 36 Gb disks I had in
mind :-(
Anyway, that was just an introduction - nu use regurgling that on the
list.

I hope to release a "raid10" module for 2.6 within a couple of weeks.
raid10 is basically a combination of raid1 and raid0 all in one module
with some interesting geometry possibilities.  This particular
geometry is one of the possibilities.

I'm looking forward to that!

Kind regards,
Jurriaan
-- 
Anyone can build a fast processor. The trick is to build a fast system.
	Seymour Cray
Debian (Unstable) GNU/Linux 2.6.7-rc2-mm2 2x6078 bogomips load 2.49