Autoraid 12H : How much diskspace can i allocate to LUNs ?

[Lothar Krueler]

Hi there,
if often read about allocating 50-60% diskspace to LUNs for keeping most space as possible in raid level 1/0.
I never understood : 50-60% of what ?
Please take a look at my example:

Vendor ID = HP
Product ID = C5447A
Array serial number = 0000000xxxxxx
----------------------------------------------------
Array State = READY
Server name = zeus
Array type = 3
Mfg. Product Code = IJMTU00004

--- Disk space usage --------------------
Total physical = 260497 MB *
Allocated to LUNs = 71680 MB *
Used as Active Hot spare = 34732 MB *
Used by non-included disks = 0 MB *
Used for Redundancy = 50995 MB *
Unallocated (avail for LUNs) = 103090 MB *
-----------------------------------------


Vendor ID = HP
Product ID = C5447A
Array serial number = 000000xxxxxx
----------------------------------------------------
Raid 0/1 blocks = 90770944
Raid 0/1 block length = 512
Raid 0/1 capacity = 44321 MB *

What??s the maximum space in MB i can allocate to LUNs with getting the maximum of raid 0/1 capacity ?

Thanks in advance
Lothar

[Steve Labar]

The "Unallocated (avail for LUNs) " is the space the 12H uses for RAID 1/0 space. The "Allocated to LUNs" is what you have assigned already. You have allocated approx 72GB and still have approx 103GB. Which means you started with about 175GB useable space. 50-60% of that would put you in the 85-105GB range. Since you have already allocated 72GB, you still have 13-33GB left to maintain your 50-60% for maximum efficiency.

Good Luck.

Steve

[Lothar Krueler]

Hi Steve,
thanks for your explaination. But max. 103G of 260G Physical Space for raid 0/1 seems not enough for me. After your response I thought about this a while an now i think that the amount of unallocated and allocated space must be the space available in raid 5. In best case it's nearly 80 %of physical Space (without Active Hot Spare). I think the 100% value is approximately the physical space decremented by the active hot spare. Probably the 12H itself occupies some space for handling the disks etc. In raid 0/1 this means i can get <= 50% of that. The active hot spare can also be used for 0/1 capacity, so i can add half of the hot spare capacity to the 50%. This leads to 50% of Physical Space decremented by the space the 12H needs itself. How much could this be ? Now i think that i can allocate nearly 120 G without loosing raid 0/1 capacity.
Let me know, what you and other experts think about this, please!

Best regards
Lothar

[Ted Ellis_2 · Accepted solution]

couple of things:

1. The active spare disk space, until needed, is actually used for RAID 0/1. Only when a disk fails will you lose that space for RAID 0/1 support, so unless you actually need the capacity for LUNs, which would also assume that you need much more than the 50-60% rule anyway. So if you don't need it, keep the active spare.

2. The Raid 0/1 capacity of 44 GB is what your system has to support RAID 0/1 I/O. The higher the number the better able the 12H is at supporting all I/O as RAID 0+1. You would have approx. 120 GB on your array that could be used and still keep almost all of your I/O Raid 0/1.

Something to keep in mind is that the 12H does its Raid work dynamically... so if you go beyond the 50-60% mark, only the I/O that can not be supported is converted along Raid5.... so most of your I/O would still crunch at 1/0. It is a little strange, but user friendly. The number of Luns, disks, and disk sizes all contribute to how much space the 12H needs to do its "Auto" features, so that is why you do not get a precise number to set. The controllers also use more overhead than other arrays due to this "Auto"Raid feature, so performance is a bit degraded there.

In short... keeping things around 120 GB to Luns will keep you close to the line, but going a bit higher will not result in a dramtic drop off in performance....

Ted

[Lothar Krueler]

Hi Ted,
it really sounds like know what you're talking about! Thank you. I think the question is cleared now and all of this replies together can help understanding the space management of the 12H.
Best regards
Lothar

[David Bell_1]

Ted,

If you happen to see this message, I was under the impression that the AutoRAID model 12H did not use the space for the active hot spare. It was simply "dedicated space" unless a failure occurred. It was not until the VA7XXX that the hot spare space was utilized for RAID 0+1. Is this inaccurate?

Thanks,

Dave

[Ted Ellis_2]

we have 2 12H units and my manual for them specifically indicates that the hot spare, when on, is:

1. actually spread across all disks in the array and
2. used to support RAID 0/1 I/O until a disk fails and it is needed

hope that helps. We have never had a disk fail or a way to actually verify this, but I will trust the manual until proven otherwise. Hope that helps

Ted

[Ted Ellis_2]

these babies may not be the best performers, but they are solid all around. We had one dropped so badly in a shipment that the frame torqued out of alignment badly enough to prevent it from fitting in our racks. Would you believe it still worked 100%... not a single component failed.

Ted