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Re: VA7100 controller settings.

 
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Roger_22
Trusted Contributor

Re: VA7100 controller settings.

Don,

The confusion is about how the VA implements hot spares. Hot spare in the VA, called Active Hot Spare, is implemented as virtual spare space. That is, there is not a physical disk(s) associated with the spare space. The spare space is distributed across all the disks in the RAID group. Visualize a group of beakers that represent the disks. Data to the array is like water to the beakers. The water (data) is naturally distributed evenly to all the beakers. If a beaker was to fail, the water in that beaker can be re-distributed to the un-use capacity of the remaining beakers. Enabling Active Hot Spare merely assures that the beakers will have the reserved space available in case of a beaker failure.

So, why don???t you see a different in capacity for RAID 1+0 by enabling and disabling Active Hot Spare? The VA has one more trick to use. If there is insufficient free space in the beakers for the failed beaker???s water, the VA knows it can temporarily convert some of the inefficient RAID 1+0 capacity to the more efficient RAID 5DP capacity ??? thus freeing up sufficient capacity for the defective beaker???s water. Once the broken beaker is replaced, the water is re-distributed back to the new beaker and the RAID 5DP space is converted back to RAID 1+0.

As long as AutoRebuild is enabled, the VA will always attempt to recover from a disk failure ??? one of its prime directives ??? don???t lose data.

The beauty of this implementation is its protection power and its flexibility. It???s all controlled with the Enable Active Hot spare switch and difference between the capacity allocated to LUNs and the physical capacity of the array ??? the free space. By reducing the reserved LUN capacity, the array can withstand multiple sequential disk failures, or, in the case of your RAID 1+0 configuration, not convert any of the data to RAID 5DP first.
Don Watson
Occasional Advisor

Re: VA7100 controller settings.

Now we're getting somewhere.

Good explanation, but I'm still not clear about a couple of details. I get the part about "automatic" using a temporary RAID5DP conversion if a disk fails, but how does the "largest disk" option work, and under what circumstances would that be a better choice. What I'd love to find is a white paper describing the available scenarios with the trade-offs for each setting, but I've been unable to locate any such info.

If you are getting your facts from doc that's online I'd be perfectly satisfied with that rather than pestering everyone.

Thanks.
Don Watson
Occasional Advisor

Re: VA7100 controller settings.

One more point, I've got a VA 7100 rather than a 7400 if that matters.
David Bell_1
Honored Contributor

Re: VA7100 controller settings.

Don,

Not sure if you've seen all the white papers here. However, these cover the mechanics of RAID5DP. Hopefully, this will help you along,

Dave

http://www.hp.com/products1/storage/products/disk_arrays/midrange/va7100/infolibrary/index.html
Roger_22
Trusted Contributor
Solution

Re: VA7100 controller settings.

Hey, if you want any more answers you???ll need to assign some points! What, do you think I???m doing this for nothing?

The VA array allows any combination of supported disk capacities. ???Largest Disk??? just means it???s reserving the spare space for the largest disk in the array to fail ??? dah! The other missing option would be to reserve space for only the smallest disk to fail.

In RAID 1+0 mode, it really does not matter if you select any Active Hot Spare setting or not. If AutoRebuild is enabled, the array will continue to ???rebuild??? on each successive disk failure until it cannot make space (by converting data to RAID 5DP if necessary) to complete a rebuild. Technically, the last rebuild will stop where it is when it runs out of space and never complete ??? it???s just waiting for someone to add another disk.

The only real choice you have to influence the algorithm (beyond disabling AutoRebuild) is to decrease the reserved LUN allocation sufficiently so that on the first disk failure there is sufficient un-reserved capacity that the rebuild can complete without creating any tempory RAID 5DP space. This will keep everything in RAID 1+0 after the rebuild. For some workloads the RAID 5DP may cause performance issues. Remember, the fewer the number of disks in the Redundancy Group, the greater the percentage of the capacity needs to be converted to RAID 5DP to create a disk???s worth of free space. If you only have five disks in the Redundancy Group, than almost the total capacity of the RG needs to be converted to RAID 5DP to create sufficient space. If you have 20 disks in the RG (this requires a VA7400), then only a small percentage of the array capacity is converted to RAID 5DP.

The bad news is that there is no way to determine specifically (the array won???t tell) how much space to under allocate the array so that a rebuild won???t create RAID 5DP space. The good news is that it really doesn???t matter. Guessing is close enough. Cut back the LUN allocation by the capacity of the largest disk in the array. This may not be enough, but only a small amount will be in RAID 5DP if any (the explanation for that is yet another note)

I know of no white paper on this subject.

Oh, always enable the new pre-fetch algorithm. It is far more efficient at pre-fetching and with almost no downside.
Don Watson
Occasional Advisor

Re: VA7100 controller settings.

Sorry, I'm new to this forum and didn't get the points thing.

That's very good info, and does help me make decisions about the config settings.

Thanks for all your help.