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Space occupation in Autoraid 12H.

Massimo Carozzo
Occasional Advisor

Space occupation in Autoraid 12H.

This info request follows similar posting within forums>general where Bill McNamara gave me some useful hints on the matter suggesting to post here my questions.
My problem is to understand how 12H firmware distribuite the available space along the physical disk drives in a disk array.
This because I'm planning to increase capacity of my Autoraid 12H adding 4x18.2Gb disk drives to the existing 4x4.3Gb disk drives.
At the moment, executing

arraydsp -a $ID

command I have this result:

--- Disk space usage --------------------
Total physical = 16383 MB *
Allocated to LUNs = 7524 MB *
Used as Active Hot spare = 4095 MB *
Used by non-included disks = 0 MB *
Used for Redundancy = 4764 MB *
Unallocated (avail for LUNs) = 0 MB *
-----------------------------------------

Is there anyone that can give me an idea about how the different space items are allocated?

At the moment I have roughtly 6.5Gb of data:
supposing I wish to leave the present hot spare disk drive unallocated, even after adding 4x18.2Gb disk drive, how will be the future hypothetic space distribution in the disk drives of the new configured disk array supposing I will create subdivide the new space in two new LUNS?

My question are based on the fact that, reading 12H manual, I didn't succeed to understand which kind of reasoning leads 12H Autoraid to distribuite the disk space how he really do.
Many thanks.
Massimo
3 REPLIES
Bob Inglis
Trusted Contributor

Re: Space occupation in Autoraid 12H.

Massimo,

A very good question, unfortunatly there isn't a easy answer. I'll try to simplify a little.

The AutoRAID takes about 10% of the "Total Physical Capacity" plus the "Active Spair" and any "Unallocated Space" for RAID 0/1. The remaining space is used for RAID 5.

The overhead in RAID 5 depends upon the number of disks installed as the AutoRAID stripes across all available disks. With only 4 disk installed the "Redundancy" or parity information is about 25%. 5 disks = 20%. 10 disks = 10%

It gets really complicated when you mix different capacity drives.

When you add the first larger drive the AutoRAID has nowhere to mirror the upper portion, so it just uses the portion which is the same as the other drives.

When you add two larger drives all it can do is mirror them because RAID 5 requires at least 3 drives. And now the "Active Hot Spare"
size has to be increased, which results in a very small gain in usable capacity.

When you add 3 larger drives it can start using them in RAID 5. (33% overhead)

And four would be 25% overhead

There is a great planning tool available at: http://www.hp.com/essd/capacity.html

I tried 4 - 4.3's and 4 - 18's and got 46 gig usable capacity with "Hot Spare" enabled.

Adding 6 - 9gig drives gave me 45 gig.


Plan for the future and tomorrow will take care of itself.
Massimo Carozzo
Occasional Advisor

Re: Space occupation in Autoraid 12H.

Thank Bob.
Indeed the way which autoraid spread the space across the disks is very complicate.
Attached there is an Excel sheet where I tried to figure the present and the future situation (4 disks sheet and 4+4 disks sheets).
The present situation is correctly replicated by Excel sheet (is it a lucky event?)but the future, as also you calculated at the site you suggest, is not 46 Gb but something more.
Do you think it respects sufficently the reality?

Is my supposition correct when I draw a new configuration where I imagine that Active Hot Spare areas will be redistribuited automatically as well as the striping areas and Raid 0/1 overhead control area?

Is it correct that this redistribution take place when, after disks insertion, I reconfigure the new disks making them 'allocated' and creating one or more new
LUNS?

Last but not least: the present unique LUN will be modified as regard space allocation with the addition of the new LUNS or will remain unmodified without a specific reconfiguration action (e.g. delete and recreate)?

I beg your pardon in advance for this quantity of questions?
Best regards.

Massimo.
Bob Inglis
Trusted Contributor

Re: Space occupation in Autoraid 12H.

I think the planning tool will give you a good approximation but it may be a little conservative in it's result.

Immediatly after inserting the second larger drive the AutoRAID will increase the "Active Hot Spare" space. (auto include = enabled)

Shortly after inserting an additional disk the AutoRAID will start "Ballancing". Durring this process the AutoRAID is moving RAID 5 data to RAID 0/1 (unallocated space is used as RAID 0/1) and restriping the remaiing RAID 5 data to incorporate the additional disk (increasing the stripe).

This is all done automaticly by the AutoRAID.
Nothing more than inserting the new drives is required. Your existing Data will be redistributed but it will be transparent to the OS. The LUNs will seem unchanged.
Plan for the future and tomorrow will take care of itself.