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AutoRaid arraydsp question

Chris Brezil
Occasional Visitor

AutoRaid arraydsp question

I am somewhat new to the HP-UX world and am coming into a situation where an HP consultant has set up a machine here with an AutoRaid 12H. The AutoRaid has 12 9GB drives installed. Active Hot Spare is enabled on our system. I have been doing some research about the workings of the AutoRaid unit and while I believe I understand it in general, I am trying to understand some of the statistics that arraydsp has produced from our system. I have been unable to find out any of this information in the AutoRaid SysAdmin guide, so I figured I'd ask the experts.

I have run the arraydsp command on seperate days and gotten the following results:

Disk space usage
Total physical - 104198 MB
Allocated to LUNs - 78000 MB
Used as Active Hot spare - 8683 MB
Used by non-included disks - 0 MB
Used for Redundancy - 17061 MB
Unallocated - 454 MB
(All of the above was the same on both days - as I would expect since we made no changes)

On Day1
Raid 0/1 blocks - 19055872
Raid 0/1 capacity - 9304 MB

2 days later
Raid 0/1 blocks - 19513600
Raid 0/1 capacity - 9528 MB

How is the amount of Raid 0/1 storage calculated and why would it have changed over the course of the 3 days? I am assuming that since we only have 454 MB of unallocated storage for the AutoRaid to use for Raid 0/1 that it is also using the Active Hot Spare for this as I believe I read somewhere that it would. However, the sum of these two values is only 9137MB, which is less than the total Raid 0/1 storage indicated on either day. Where did this 'extra' storage come from? Also, if the AutoRaid uses the Active Hot Spare for Raid 0/1 storage and one of the disks in the raid does fail, how is the failed disk rebuilt onto the spare if the spare is being used for 0/1 storage?
4 REPLIES
Antoanetta Naghiu
Esteemed Contributor

Re: AutoRaid arraydsp question

Active Hot Spare capacity and Unallocated are Raid 0/1, but Raid 0/1 space can be new logical drive with its capacity before begins to fill up with data and being converted to Raid 5.
In your case, the question is: Why your Raid 0/1 capacity got increased? Did you re-create any logical drive in the next couple of days?
What hapened in the array in the last two days?
Chris Brezil
Occasional Visitor

Re: AutoRaid arraydsp question

We have made no changes to any of the logical drives over the time period that I ran arraydsp. This is what really confused me.

Also one additional question. As Raid 0/1 involves mirrored storage, does the amount of space shown (i.e. 9304 MB) indicate that there is 9304 MB of Raid 0/1 storage available (that actually 18608 MB is used when mirroring is accounted for) or is this a total figure including the mirroring so I would have to divide it by half. I assumed the former, but just wanted to make certain.
Antoanetta Naghiu
Esteemed Contributor

Re: AutoRaid arraydsp question

Re: AutoRaid arraydsp question

Wow. Where to start. The autoraid users guide is the best source for the information you seek. Also look at:
www.hp.com/essd/model12H_autoraid_tool.html
(netscape browser works best) for a great capacity planning tool.
But let me try to clarify a bit here too. The array automatically tries to reserve 10% of its total capacity as raid 1/0 space for the write working set. If you allocate ALL available space to luns and hotspare and you fill the array up completely with data it is possible your applications, making heavy write demands, may exceed the 10% raid 1/0 minimum and force the array to service write io's in raid5 mode (performance degrades). The amount of raid 1/0 space available can be guaranteed by leaving space unallocated/avail for luns. Hotspare space is always reserved for mechanism failures but is used for raid 1/0 performance until it is needed. And just in general as an array fills up it begins migrating older timestamped data into raid 5..but brings it back to raid 1/0 as it is requested by the host..so raid 1/0 blocks can be constantly varying as data is added and removed from all configured luns. And the bottom line is you dont have much control over the autoraid beyond adding mechanisms, assigning hotspare space, and creating luns!. Hope that helps!