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Re: How many of you use S.A.M.E?

From: Mark Brinsmead <>
Date: Fri, 2 Feb 2007 21:13:29 -0700
Message-ID: <>


   It is, in general, possible to decide at least approximately where data will go on the disk. The most obvious method to do this is with partitioning. When you create a disk partition (either to support a filesystem or provision a raw partition) you specify the range of blocks/tracks that will be used. This is a common sysadmin task, but many DBAs never have do this themselves. Perhaps that is why you are unaware of the technique.

   No sane person has ever (to my knowledge) tried to place specific data on specific blocks, but many people can and do arrange to put certain classesof data in designated
regions of a disk. That doesn't mean nobody has ever asked me to put specific data on specific block, just that those who did probably weren't sane. But I digress.

   A very long time ago, and with certain operating systems, you could contrive to do this sort of thing. Used to be, some DBAs would even do things like try to place index blocks on one specific disk surface, and the "corresponding" data blocks on the physically opposite disk surface (and track and block). But that was quite some time ago -- in many cases, you can't even determine how many surfaces a device actually has, and the question of how many blocks there are in each track is almost completely meaningless... (Unless you are prepared to take into account that the answer is usually different for each track, anyway.)

   That said, in many cases, you can still specify things like "online redo goes on the outer 5%; indexes on the next 20%; ...". More or less, anyway.

   In reality, though, today's storage devices are getting more and more "intelligent" (or sometimes deceitful), that is making this harder and harder to do. Sometimes, at least...

   Personally, I do not have any experience with EMC "Meta Devices", but based on the description I could definitely imagine them holding some unpleasant surprises for people laying out databases unwarily... This can include errors like mistaking "meta devices" for physical disks and doing RAID-0 striping across them, or even naively multiplexing online redologs across metadevices that happen to be stored on the same physical devices.

   While you don't need to know precisely where a given block is located on a disk, if you do want to plan your I/O you do need to know how your database files map onto physical devices...

On 2/1/07, amonte <> wrote:
> Hi Brandon
> Have you used EMC Meta Devices before?
> By the way have you tried by chance what Loaiza suggested, putting data in
> specfic physical sectors of a hard drive? I am really curious how can that
> be achieved. S.A.M.E is simple but putting data as he says makes life
> impossible dont you think so?
> Regarding query tuning, it is good, perfect. However the real world
> experience has taught me that, if you have 50 lousy or even experienced
> old developers and 4 DBAs you can spend 100% of DBA time tuning queries and
> it will still be endless. My opinion is we have to get some sort of balance,
> not be too biased to any.

-- Mark Brinsmead
   Senior DBA,
   The Pythian Group

Received on Fri Feb 02 2007 - 22:13:29 CST

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