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RE: Advice needed on move to Sun 15K (losing spindles)

From: Stephen Lee <>
Date: Fri, 11 Oct 2002 10:46:12 -0800
Message-ID: <>

I've cussed and discussed the topic of one big stripe versus multiple small stripes with different people and have yet to come across anyone who has conducted a real test of various scenarios. If you stripe across all disks, then you have the advantage of guaranteed, perfectly balance I/O -- there's certainly something to be said for that! But, then you have a mix of reads and writes going across all drives too. A good argument can be made for taking those parts of the database that tend to be only one kind of operation -- for example, archive logs are writes -- and putting them in their own area. So the drives handling the writing of archived logs are doing only one kind of operation (or are they?!), but you subtract from the drives allocated for other operations. But then there is the issue of: Just exactly how do hard drives work? For example, when doing a large "write only" operation (like creating an archived log) is the drive really doing this neat and tidy write only, one track after the next, each track right beside the other? Or does the drive actually write a little bit, read a little bit (like a check sum or verify operation), then write some more. And when writing, does it do this smooth, nicely contiguous write, all in one operation? Or does it write a little bit (like an OS buffer full), then move to a different track to update an allocation table (then perhaps read the allocation table), then perhaps go pick up a timing mark, etc.?

I suspect some of the answer is dependent on the number of drives and controllers available. (And I must say, that when I read your original question, I wondered why on earth would an organization ready to drop a bundle on a 15K be scrounging for drives -- if I interpreted your post correctly. Is this a Dilbert sort of thing?)

The only time I have striped across all drives was the only time I was in a position to make that decision. This was a few years ago, and it was when I did Solaris/AIX admin. It was on a Sparc 4500 with 6 250Mhz CPU's. Since we did not have an Oracle DBA, and I didn't have the time or inclination to devote to setting up and maintaining and "official" OFA compliant structure, I just made one giant (considered giant at the time) 250 Gb filesystem that would hold all things Oracle and be done with it. I made two 30-drive (8.4 Gb drives) stripes and mirrored them using Solstice Disk Suite. There were 10 wide scsi controllers. Each controller had a 6-drive JBOD attached to it. An eleventh controller had an additional JBOD to be used for hot spares. As you might guess, with a I/O pipe this big, there was no way the 6 CPU's could generate enough I/O to bog things down or even cause a hint of an I/O wait.

So the stripe across all drives does work. In my case, I had 60 drives on 10 controllers to work with. Could this have been made more efficient by making a collection of smaller stripes? I have never found anyone who could answer that. The Disk Suite folk can tell you that there is an optimal striping configuration for Disk Suite if we leave Oracle out of the picture. But with Oracle in the picture, who knows?

One configuration that sounds reasonable is to put data files with random reads and writes on one stripe, put even numbered redo logs on a stripe, put odd numbered redo logs on a stripe, put archived logs on a stripe. The reasoning (or arm-chair theory) behind the even/odd redo logs is that at a log switch, one file system can be doing writes, while the other is doing reads for the log archiving. This is sorta kinda the way we do things at our shop here with some modifications depending on the app -- like maybe dedicate a stripe to servicing the outrageous temp requirements of a "data warehouse" (more correctly, a data landfill).

If you have only a few drives, my inclination (with no proof whatsoever) is that the one big stripe approach might be a good idea. Thus far, all I have ever gotten on this subject is a lot of "religion" and very few proven facts.

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Author: Stephen Lee

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