DB2 LUW 8.2: RAID-5, Logging and Controller Question

Brian Stewart

DB2 LUW 8.2: RAID-5, Logging and Controller Question
Hello -

On AIX 5.3 (4 CPU) I have encountered a situation where there are two
controllers with four disks each which are both local to the system. Lets
call them hd0 through hd7. Root is mirrored on hd0 and hd1 on controller A
(disks hd0 - hd3) and DB2 with all its data is on hd2 - hd7 in a RAID-5
configuration on controller A and B (disks hd4 - hd7). So, the RAID-5
system is split between two controllers, one of which is also doing the
logging - the root vg on controller A.

Has anyone seen a situation where having the logging on the same controller
which is shared by the RAID-5 system is a performance problem? I am seeing
very high I/O on the RAID-5 device and high CPU I/O wait times during
certain queries.I have ruled out most everything else I can think of.

Any thoughts?

Much thanks in advance for any and all feedback.

Brian

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Mike Bell

Re: DB2 LUW 8.2: RAID-5, Logging and Controller Question
(in response to Brian Stewart)
The only thing I can think of is that you have to be running software raid
to split the raid over multiple controllers. Most of the high end
controllers can do hardware raid on the controller when all the disks are on
the same controller. The main cost of software raid is write processing
where you have to create the parity disk - that is not simple in normal
processing. Read processing is just grabbing stripes not cpu heavy at all.

The only time a query would be affected would be if it was generating writes
to temp or something like that.

NOTE - I am Z/os competent and much less then that on AIX.

Mike
HLS Technologies

-----Original Message-----
From: DB2 Data Base Discussion List [mailto:[login to unmask email] On Behalf
Of Brian Stewart
Sent: Monday, January 26, 2009 11:52 AM
To: [login to unmask email]
Subject: [DB2-L] DB2 LUW 8.2: RAID-5, Logging and Controller Question

Hello -

On AIX 5.3 (4 CPU) I have encountered a situation where there are two
controllers with four disks each which are both local to the system. Lets
call them hd0 through hd7. Root is mirrored on hd0 and hd1 on controller A
(disks hd0 - hd3) and DB2 with all its data is on hd2 - hd7 in a RAID-5
configuration on controller A and B (disks hd4 - hd7). So, the RAID-5
system is split between two controllers, one of which is also doing the
logging - the root vg on controller A.

Has anyone seen a situation where having the logging on the same controller
which is shared by the RAID-5 system is a performance problem? I am seeing
very high I/O on the RAID-5 device and high CPU I/O wait times during
certain queries.I have ruled out most everything else I can think of.

Any thoughts?

Much thanks in advance for any and all feedback.

Brian


________________________________


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< http://idug.org/lsAU >

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______________________________________________________________________

* IDUG 2009 Melbourne, Australia * 18-20 March * http://IDUG.ORG/Events *
______________________________________________________________________




IDUG.org was recently updated requiring members to use a new password. You should have gotten an e-mail with the temporary password assigned to your account. Please log in and update your member profile. If you are not already an IDUG.org member, please register at http://www.idug.org/component/juser/register.html