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[oracle-l] Re: The Holy War: Disks

From: Paul Drake <>
Date: Tue, 27 Jan 2004 14:20:30 -0800 (PST)
Message-ID: <>

I have a site with a CX400 in use, with W2K3 Server, but its not yet in production, so there isn't much IO to look at. I might be able to run a few tests there.

Its a small oltp database and its clone.

> I have just been given ~1tb of disk on a new SAN.
> The engineer wanted
> to give me 3 huge (maxed out) disks, 2 350GB and a
> third with the
> remainder. I argued for 6 disks similarly sized.
> My fellow DBA supported my argument. The engineer
> and the dw architect
> wanted 3 disks.
> I am going to have i/o problems no matter what.
> Concatenating 10
> physical disks into 1 logical disk is going to have
> as much i/o latency
> percentagewise as 6 physical disks concatenated into
> 1 logical disk.
> Each disk has 1 bus, so to speak. These buses are
> concatenated together
> into 1 "device". (I'm being told that "device" is a
> unix term and it
> doesn't apply in Windows).

A LUN is a LUN is a LUN.
It is apportioned within the management software the same way.

> So, concatenating 10
> disks (and buses)
> together for 1 high speed disk is going to result in
> having even more
> data on the other side of the "straw".

Lisa, did you hike the max IO_size for the operating system? It was 256 KB on w2k by default, I did not check it on w2k3 server. Connor McDonald had a referereference to the setting some time ago. I think I have an article related to hiking this in the regsitry, a max size of 1 MB was possible.

Is the oracle server multipathed over multiple Fibrechannel host bus adapters, or at least over multiple ports? If not, its likely that even a 2 Gbps connection will be the rate limiting factor, if you have 30 drives mounted on 3 different buses. You probably wanted multipathing for availability purposes anyways.

> I strongly feel if I have 3 disks instead of 6, my
> options for
> alleviating i/o contention are very limited.

If your max IO size is 256 KB, then having more mount points should increase throughput. Datawarehouse - you'll likely want 1 MB reads
(db_block_size * db_file_multiblock_read_count = 1 MB).

> Any i/o balancing would be
> messier and more difficult. We are going with 6
> disks instead of 3 with
> the understanding that when we add more disk to this
> server, we'll
> evaluate performance of the 6 disks and reconsider.

what is the degree of parallelization that will be used? I'm assumming that you'll only have a 4-way box, that may appear as an 8-way box with SMT enabled.

> As far as I know, the Clariion SANs don't have the
> whizbang
> functionality of the Symmetrix that allows moving
> datafiles at the
> physical level within the SAN to alleviate i/o
> hotspots. I also don't
> buy the argument that the SAN cache should alleviate
> i/o problems.

As far as large table scans (and other multiblock IO), its more a matter of IF read-ahead is effective.

> This
> is a data warehouse that has the potential to become
> enormous and it
> will blow the size of any SAN cache during data
> loads, guaranteed.

good article in SysAdmin magazine several months ago, comparing cache-centric vs. throughput-centric external storage units. This unit should be good in the non-cache-centric arena.

> Is anyone in this type of environment? What have
> your experiences been?
> Any and all comments are welcome.

> Thank you
> Lisa Koivu
> Senior Monkey
> Cendant Timeshare Resort Group
> Orlando, FL, USA


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Received on Tue Jan 27 2004 - 16:20:30 CST

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