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VA7100 controller settings.

 
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Don Watson
Occasional Advisor

VA7100 controller settings.

I need to configure a VA7100 array. I'm using cvui, and have set the array for RAID1+0, however there are several configuration parameter options that I don't understand. The help doc does not explain the trade-offs of choosing one opton over another, and some of the defaults don't make sense. For example:

1. Hot spare - Using RAID1+0 I have in effect 1 hot spare for each primary disk already. Why would I want to reserve a disks worth of space for yet another hot spare? Is this just to cover the time between a disk failing and getting it replaced, so that mirroring is never interrupted? Is there any other downside to setting this to none?

2. Optimize Policy - It sounds like Optimize Policy is only applicable in an AutoRaid setup. Does this setting have any relevance at all for RAID1+0?

3. Pre-fetch - Why would the default be set to off? Why would anyone want to disable read ahead? Is there a performance penalty, or some other reason why you might not want this enabled?

15 REPLIES 15
Vincent Fleming
Honored Contributor

Re: VA7100 controller settings.

Hi;

To answer your questions...

1. Yes, it is to cover you from when a drive fails until you replace the drive. It's an added safety feature.

2. Optimize Policy is really only valid in AutoRAID mode.

3. There are a number of parameters that actually do nothing (such as turning write caching off!), but are there for compatibility reasons. I can't find my list of things, but I think Pre-fetch is one of those.

Good luck!
No matter where you go, there you are.
Don Watson
Occasional Advisor

Re: VA7100 controller settings.

Ok, I'm with you on Hot Spare and Optimize Policy. I also read in the doc where some of the parameters aren't really functional, and are there for compatibility with O/S's that require certain settings, however, Pre-Fetch is not documented that way. Not saying it really is functional, but it's not listed as being one of the useless compatiblity options.
Vincent Fleming
Honored Contributor

Re: VA7100 controller settings.

I've been reading up for you...

Apparently the pre-fetch does indeed do something; disabling read-ahead can increase performance in highly random, small-block read I/O situations. Mostly the option to turn it off is used for benchmarking, so you normally won't want to turn it off as in the real-world, I/O patterns like that would be very rare.

Good luck!
No matter where you go, there you are.
Vincent Farrugia
Honored Contributor

Re: VA7100 controller settings.

Hello,

The scope of having a hotspare is simple. It's true; in RAID 1+0, you have mirroring, and effectively another disk for all disks. What hotspare does is... whenever the machine senses a disk failure, it rebuilds all the data to the hotspare immediately. If you had no hotspare, the system would still function and no data would be lost, however, if you lose another disk, data in that LUN would be lost. Hotspare enables the machine to rebuild the LUN to the hotspare disk if a failure occurs, thus bringing the machine in a non-urgent state again.

HTH,
Vince
Tape Drives RULE!!!
Don Watson
Occasional Advisor

Re: VA7100 controller settings.

Thanks for the replies. Very helpful.

One last question, if I turn on "Hot Spare", allocate my LUN's, and then at some later time decide I need to use that hot spare disk space, can I just turn off the hot spare option and have it return the space for LUN allocation? If so, would this happen on the fly or require any downtime?

I'm guessing I can do this, but would really like to know for sure up front.
Vincent Fleming
Honored Contributor

Re: VA7100 controller settings.

Yes, you can change your spare setting at any time.

Changing to no spares will behave as you expect - the space will be able to be allocated as a LUN without any downtime.

Good luck!
No matter where you go, there you are.
Don Watson
Occasional Advisor

Re: VA7100 controller settings.

Ok. But I guess I lied about the "one more question" thing.

What exactly is "automatic"? Doesn't the concept of hot spare imply reserving an amount of space equal to the size of at least a disk?

Also, I checked my available space for allocating LUN's (with hot spare set to none), then set hot spare to "largest disk", checked the available space again and it was exactly the same as before. How is that possible?

If a good explanation of hot spare is documented somewhere, a link would be a fine answer. Thanks.
Vincent Farrugia
Honored Contributor

Re: VA7100 controller settings.

Hello,

For starters, follow this link:

http://www.hp.com/cgi-bin/cposupport/get_doc.pl?SNI=hpsurestor30307&LC=information_storage&Tfile=lpg35046

For advanced, follow this link, pg. 52:

http://www.hp.com/cposupport/manual_set/lpg28817.pdf

You can also read the rest of the manual of course :-)

HTH,
Vince
Tape Drives RULE!!!
Don Watson
Occasional Advisor

Re: VA7100 controller settings.

Thanks, but the first link is quite vague and leaves many questions unanswered. When I try the second link I get an error that the file can't be downloaded.

I have actually read every manual I can get ahold of, including every one that HP support has directed me to, but nothing I can find so far fully explains hot spare usage, or even gives a useful definition of how each option works for each of the different RAID levels.