ADSM-L

Re: Recommendations for TSM on NT

2000-05-15 11:40:20
Subject: Re: Recommendations for TSM on NT
From: Kelly Lipp <lipp AT STORSOL DOT COM>
Date: Mon, 15 May 2000 09:40:20 -0600
Generally speaking, and I usually am, one should probably avoid RAID5 for
*SM, unless, has been pointed out, you build multiple RAID5 sets.  Think
about what's happening when data is being written from a client to a storage
pool: data is written to the pool, the recover log is written and the
database is written.  All essentially at the same time.  In a large RAID5
set this means moving all of the heads in all of the drives three times at
the same time.  The laws of physics still prevail.  This is going to be
slow.

The gains seen in example below were due to the additional cache.  Sure,
this helps, but sooner or later, all of these writes have to complete.
RAID5 is good in mostly read intensive applications.  *SM is a mostly write
intensive beast so RAID5 is not optimal.

We fight this battle with our STORServer product.  Our configuration uses
JBOD with the database and recovery log mirrored on separate volumes.  The
storage pool volumes are built on the remainder of the disks where the db
and recover log are located (if there is room) and on separate disks.  In a
RAID5 config, one can tolerate the failure of a single drive.  In our
configuration we can certainly tolerate a single disk failure.  Depending on
which second disk fails, we may tolerate that as well.  And perhaps a third
and a fourth.  Obviously if we're unlucky and lose both disk where the db is
we're down (or the log, or the OS).  But how unlucky can one get?

Kelly J. Lipp
Storage Solutions Specialists, Inc.
PO Box 51313
Colorado Springs CO 80919
(719)531-5926
Fax: (719)260-5991
www.storsol.com
lipp AT storsol DOT com

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