Different Drive Speeds on same Array Controller

[Greg Woodroffe]

Hello People,

If I configure my server with 2x15K SCSI Drives (RAID1) for the Boot Partition and 3x10K SCSI Drives (RAID5) for the data partition on the same (single channel) array controller (641), will the RAID1 pack operate at 15K, or do all drives on the array controller drop to the lowest common denominator - even though the drives are configured as seperate RAID arrays?

Is this also the case if the arrays are on seperate channels (6402 etc)?

Thanks in advance.

Greg

[Gus Kwong · Accepted solution]

Hi Greg,

>"...will the RAID1 pack operate at 15K..."
Umm... I assume you mean the interface speed right? I know that RPM does not change and the buffer of the SCSI controller does not care how quickly data fill the space.

I think this link may explain better http://h20000.www2.hp.com/bizsupport/TechSupport/Document.jsp?objectID=tis14430

There will be no change in terms of the interface speed if they are the same kind (i.e. Ultra320).

If you have one SCSI channel for all your RAID sets then the issue would be more to do with the bandwidth utilisation of the SCSI bus.


I hope I didn't misunderstand your question.

Gus

[Prashant (I am Back)]

Hi,

Since the lowest configuration is 10K even though it is diff raid, the server will work on lowest one avaible that would be 10 k.

Regards,
Prashant S.

[Gus Kwong]

Ummm... Prashant, I believe rotation speed does not really change in a hard drive... If they are set for 15k, mixing them with 10k will not change the RPM. One thing for sure is if they start putting automatic transmission like a car in HDs then you will have the same mechanical problem too! Imagine changing clutch for your hard drive... but then again that could be fun too!

Gus

[Prashant (I am Back)]

Hi,

Logically yes, but i am looking for a document which states that. But as the information i have i know what i have just post.
:-)

Regards,
Prashant S.

[Doug de Werd]

I just confirmed this with the SA 641 Engineering team. The speed is limited to the lowest common speed within a RAID set, not to the SCSI bus. So if you have an array that has all 15K drives, it obviously runs at 15K speed. If you have another array - even if it's on the same SCSI bus/channel - that is all 10K drives, then that will run at 10K (and the other array will continue to run at 15K). And if you have a mix of 10K and 15K in a RAID set, it will run at 10K. So the fact that you have different drives speeds on the same bus doesn't matter. Each array will run at the speed of its lowest speed drive.

Thanks,
Doug

[Greg Woodroffe]

Thanks for all the helpful replies.

[Gus Kwong]

Hi guys,

This is rather interesting! I just wish there is a HP documentation on the web but I just could not find anything about variable rotation speed on the same RAID array.

There have been some similar discussions regarding mixture of rotation speed/RPM drives in a single RAID array where you can search from the HP forum --- some were even answered by HP-logo people, but again there were two versions. Here is what I've found...

DISCUSSION-1
============
http://forums1.itrc.hp.com/service/forums/questionanswer.do?threadId=173697

DISCUSSION-2
============
http://forums1.itrc.hp.com/service/forums/questionanswer.do?threadId=173375


Until someone kindly forward a link to an offical documentation, I think I would stick with my educated guess (and yes I could be wrong).

It is just a little bit hard to imagine parallel-SCSI drives (operating in a half-duplex mode and allow queuing) would need to be forced to slow down mechanically from 15000 round-per-mintue down to 10000 round-per-minute (or 7200rpm in some cases). If 15k SCSI drives can adjust the rotation-speed, then it will also need to slow down for 7200rpm drives (again, we are not talking about the interface speed, such as Ultra2 or Ultra3, but the actual physical rotation speed).

To me, it would sound much easier for the HDs to run at their own speed, but let the fast ones wait while the slower ones catching up.


Gus