RE: VLDB ASM & SAN Striping Question
Date: Wed, 6 Jun 2012 12:18:02 -0500
Message-ID: <C5533BD628A9524496D63801704AE56D75B286AB5B_at_SPOBMEXC14.adprod.directory>
Understood - in my scenario I only used a small number of disks as an example. In real life, the numbers would be much larger - I would always want *many* disks/spindles forming a logical unit - which now brings us back to the Oracle doc where it says:
"Oracle ASM can be used on top of previously striped storage devices. If you use such a configuration, then ensure that you do not introduce hot spots by defining disk groups that span *logical devices which physically may be using the same resource (disk, controller, or channel to disk) rather than other available resources.* "
The assumption in that statement is that a SAN administrator (or array administrator) can purposefully create the LUN (logical device) across disks and controllers and that Oracle recommends doing so. (Hence my questions) :)
Chris Taylor
"Quality is never an accident; it is always the result of intelligent effort."
-- John Ruskin (English Writer 1819-1900)
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-----Original Message-----
From: Matthew Zito [mailto:matt_at_crackpotideas.com]
Sent: Wednesday, June 06, 2012 11:40 AM
To: Taylor, Chris David
Cc: oracle-l_at_freelists.org
Subject: Re: VLDB ASM & SAN Striping Question
Well, so - now we're stepping into territory beyond what I know *specifically* about the EVA and more generally about these types of arrays. So take everything I say with a grain of salt.
First, most of these arrays, when you create a disk group, will typically auto-allocate disks based on some sort of balanced controller/shelf mechanism. So, you tell it to make a disk group out of 4 disks when there are 4 shelves hooked up to the array, one disk will come from each shelf, and each shelf maps to 1 controller, or something like that. I think the EVA puts some number of shelves per controller, but whatever.
So - you could just say, "4 disk group", and it *should* go out and do the best it can do stripe things across controllers and shelves. That being said, the admin utility almost certainly can show you disks and their controllers, though it may not actually allow you to select specific disks.
All that being said, your approach specifically subverts the architectural approach of your storage array, which wants you to make very large disk groups striped across many disks, and let the array worry about placement. Every LUN is striped across all of the disks, and there's even some sort of rebalancing capability, though I've never worked with it specifically. In addition, you're going to need to dedicate disk space for redundancy, so starting to make three-disk groups means you lose 1/3 of your space to redundancy. HP often recommends disk groups with 50+ disks in it, and then carving LUNs out of that.
Matt
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