Re: Rép. : Question of a Newbie to POWER Systems

Jim

Rob has said, from his experience, that a 3-drive RAID parity set is dreadfully slow (in so many words). I hope he didn't mean a mirrored setup.

You mention here having a sufficiently large write cache - does that take care of the performance issue? Because I've heard it said that the 3-drive issue is no longer a problem.

Thanks
Vern

On 3/29/2011 1:52 PM, Jim Oberholtzer wrote:

True the math does work out better, but....

It still winds up being effectively three drives, and worse yet from a
write perspective, mirrored. A write to 3 mirrored pairs has to wait
for both of the drives in the mirrored pair to commit the write before
it is released by the storage subsystem to the OS. If you get a large
write that affects more than one of the mirrored pairs, you now have to
wait on all four, or possibly all six drives to complete the write
operation.

Read is some what quicker since it can come from either drive in the
mirrored set.

Since the original question was regarding disk performance a three drive
system (or three mirrored pairs) without much write cache, the system is
going to have problems with any moderate level of I/O.

Jim Oberholtzer
CEO/Chief Technical Architect
Agile Technology Architects, LLC


On 3/29/2011 1:08 PM, Porterfield, Sean wrote:

That math does work out better!
--
Sean Porterfield


-----Original Message-----
From: John Jones
Sent: Monday, March 28, 2011 13:35
To: Midrange Systems Technical Discussion
Subject: Re: Rép. : Question of a Newbie to POWER Systems

Since the OP said

"It contains 3 disks of 140 GB - Which are RAID-1 mirrored totalling 420GB effective."

I was assuming it was 3 pairs/6 physical units.


On Mon, Mar 28, 2011 at 11:49 AM, Porterfield, Sean

Since he said RAID1-mirrored, I think it's really 4 drives rather than 3.
If not, that would be the first thing to fix, because a disk is
unprotected.
--
Sean Porterfield


--

JJ
4 Out of 3 people have trouble with fractions.