ADSM-L

Re: Advice needed for EMC CX500 setup

2005-06-08 10:57:18
Subject: Re: Advice needed for EMC CX500 setup
From: Richard Rhodes <rrhodes AT FIRSTENERGYCORP DOT COM>
To: ADSM-L AT VM.MARIST DOT EDU
Date: Wed, 8 Jun 2005 10:56:32 -0400
At the recent EMC conference at a session about Clariions, they indicated
several things about Clariions that I thought I would share.  From my
notes . . . .

1)  Raid 0+1 is better for writes, but doesn't help with reads.  It will
only/mostly use just one copy for reads.  It's not as smart as a
dmx/symm, which will perform reads from both copies.  Raid 5
random reads are just as good as 0+1 random reads.

2)  Raid 3.  The way I heard it explained, r3 was recommended for
big block sequential processing (like tsm staging pools) for ATA
drives.  The reasoning was that ATA drives don't currently have command
tag queueing.  Since they can only do one i/o op at a time, r3 fits
real well with this.  They recommended this for up to around 10
concurrent data streams to the raidset.   The quote went something
like this:  ATA drives are brain dead, and raid 3 optimizes their
brain-deadness.

3)  Raid 5 on Fiber Channel drives is  very good with large block
sequential I/O.  Raid 3 is not needed here.

4)  Raid5 definitely has the write performance penality, which
requires 4 i/o's per random write (not writing a full strip).   This
is hidden behind the write cache and performed later.   They gave
a rule of thumb of using raid5 for random I/O up to a  write/read
ratios of around 25-30%.  That is, a random
access pattern where there is 70% reads and 25% writes.

5)  Leave the raidset strip size at the default (I think it's
64k, or 128 blocks).  They have performed all
kinds of internal testing and this is the optimum size for Clariion raid 5
raidsets.  Period.  The internal processing logic of the Clariion is
optimized for this strip size - don't mess with it unless you reallllllly
know what you are doing.

6)  15k rpm fc drives give around a 30% increase in small block
random iops than 10k drives.


Rick


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