IBM SVC and how it affects IO

Posted on 2014-01-29
Last Modified: 2014-02-07
we are having our storage stack which is mainly IBM shark behind an SVC,we are just confused about few things?

1-our SAN team claims that type of RAID does does not matter if storage is behind SVC like ORACLE is not supposed to use RAID 5 and world rather go with 01,so is this claim correct
RAID won't matter if storage is behind SVC?

2-in another project I have increased the read ahead parameter for AIX which gave us great results with OS file sequential reads and backups,not sure why this never worked in this environments.Is it because of the SVC or because we are using CIO?

3-Will an SVC make extremely fast IO like flash show less response time,meaning is better for Flash not to use SVC?
Question by:it-rex
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Expert Comment

ID: 39819255
1) RAID on AIX won't really matter in this case, that's true.

You should keep in mind that there are at least two virtualization layers between AIX and the actual disk platter, the SVC firmware and the storage backend's various mechanisms (and perhaps a VIOS ...)

- The DS8K usually has its own RAID setup, multiple internal paths and, of course, cache.

- The SVC has its own cache (which in case of the DS8k efficiently collaborates with the backend cache) and has pooling options to distribute I/O across many backend LUNs even if they go to the same frontend LUN.

In fact, you'll never know where an actual I/O operation will land on the backend.

Best way is to use no RAID at all on the frontend server, let this server use its power for better purposes.

2) CIO implies DIO, and DIO disables read-ahead (and write-behind, too ...)

As for read-ahead in general:

Not only the SVC but also the DS8K try to detect sequential I/O and perform read-ahead on their own.
Configuring high read-ahead on AIX could/will trick these devices out so that they assume more sequential I/O than there actually is.

Example: AIX reads 2 consecutive blocks, read-ahead reads 6 more blocks - just in case... -> SVC sees sequential read of 8 blocks, reads another 16 blocks - you never know ... -> DS8K sees sequential read of 24 blocks, assumes that there is a really big sequential operation to come and reads 48 more blocks ...
So you can see that read ahead can/will lead to unneeded I/O,

3) I can't tell for sure because I don't have big experience with Flash yet, but it might indeed not run faster behind an SVC.

Weigh this up against the advantages of the SVC in regard to Easy Tier or Real Time Compression which can save valuable Flash space.

Moreover I think there are cooperation mechanisms between Flash and SVC  whose efficiency is similar to those between SVC and DS8K.

Here is a fine Redbook about SVC and Flash:
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Accepted Solution

Gerald Connolly earned 500 total points
ID: 39820258
@it-rex, you obviously don't understand what the SVC does?

The SVC adds another layer of virtualisation between the servers and the storage subsystems.
The physical storage like your Shark provides RAID LUNs to the SVC then slices and dices them up into the LUNs that your server sees, now this can mean a 1-2-1 mapping or the LUN a server sees could contain multiple storage LUNs ie the SVC presents a RAID-0 LUN of several Storage RAID-5 LUNs, or the opposite where a single Storage LUN is sliced up into several separate LUNs that are presented to one or more servers.

It also means that you could change the Shark for a say a HP 3Par without any problems with server hardware/software compatibility.

But yes your SAN team is right, it doesnt really matter as long as they are providing the right kind of performance and redundancy for your data - of course the SVC is in the Oracle support matrix isnt it?
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Author Comment

ID: 39820278
I stopped reading after
@it-rex, you obviously don't understand what the SVC does?
If you want to be professional and answer the question in a polite you be my guest ;if not please refrain and seek questions where the asker knows the answer.
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Expert Comment

by:Gerald Connolly
ID: 39820645
@it-rex, sorry it wasnt meant like that, Your first question implied you didnt understand what the SVC does otherwise why would you have framed the question in that way.

I don't consider i was impolite, maybe a bit OTT, and my reply was professional, giving the facts and an answer to your question.
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Author Closing Comment

ID: 39842928
thank you
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Expert Comment

ID: 39842958
And I thought you didn't even read that answer ...
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Author Comment

ID: 39843064
Of course I did
Please I need your urgent help on the aix lun Q I have just posted

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