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Active Hot spare on 12H

Kevin Dillon
Occasional Advisor

Active Hot spare on 12H

Are there ANY circumstances under which it would NOT be beneficial to set Active Hot Spare to ON?
Is there any performance hit?
I fully understand the benefits in terms of continuity of service/ performance following a disk failure.

Regards to all.
Kevin.
7 REPLIES
David Navarro
Respected Contributor

Re: Active Hot spare on 12H

Hi, when you use hot spare in 12H, you're doing two things:
first you have a extra disk, which will assume the rol of another disk in case of failure.
Second, you're hot spare disk is used for giving additional space for best performance, that is, you will have more disk space in raid 1 instead raid 5.

I hope it help you
David Navarro
Respected Contributor

Re: Active Hot spare on 12H

Sorry, I forgot to explain, that if you do not configure that disk in active hot spare, you will have more space for create new LUN's, but, probably at worst performance.
Kevin Dillon
Occasional Advisor

Re: Active Hot spare on 12H

Thanks David

Let me get this right....

regardless of whether Active Hot Spare is enabled or not, I will have the space equivalent to it available for allocation (or unallocated for Raid01 use!).

In your second message where you refer to "worse performance" - are you referring to the case where I elect to use this space for LUNs rather then leaving it unallocated?

Thanks
Kevin.
David Navarro
Respected Contributor

Re: Active Hot spare on 12H

Yes, When you configure HS (Hot Spare), you loose capability for allocation, that is, this disk is not used for obtain more space, but is used for redundancy, you will have more disk in RAID 01. If you do not do so, and you space allocated is the same, you will have the same level of redundancy/performance, and you can allocate more effectuve space, but you loose the extra security.
In fact, without HS you can do the same and the same funtionality. If you enable HS you will have just a reserved space for extra security without loose of performance.

Curt Thompson
Respected Contributor

Re: Active Hot spare on 12H

Hello Kevin,

Maybe I can clean this up a bit. From the Model 12 Users guide:

'Active hot spare reserves capacity to perform a rebuild in the event of a disk failure'.

'However, unlike conventional disk arrays that let the hot spare remain idle until it is needed, the disk array uses the Active Hot Spare for RAID 0/1 storage until the spare is needed'.

So, Active Hot Spare contains both benefits of disk failure protection while maximizing performance. To answer your original question,

'Are there ANY circumstances under which it would NOT be beneficial to set Active Hot Spare to ON?'

the only answer that comes to mind is a reduction of disk space to configure new LUN's
for filesystem addition/expansion. The amount lost equals the size of the largest disk installed in the array (spread out over all the disks, of course).

Good Luck,
Curt
A. Clay Stephenson
Acclaimed Contributor

Re: Active Hot spare on 12H

Hi Keven:

The only case is when you need to squeeze the last bit of space out of your AutoRAID. When you even get close to this point, you have turned your AutoRAID into a dog. My rule of thumb is no more than 60% allocated as LUNS to keep this puppy in RAID 0/1 all the time.

Regards, Clay
If it ain't broke, I can fix that.
Bill McNAMARA_1
Honored Contributor

Re: Active Hot spare on 12H

> Are there ANY circumstances
> under which it would NOT be > beneficial to set Active
> Hot Spare to ON?
Yes, when you need more disk space and are not really all too interested in performance.. ie archive system rather than mail system.

> Is there any performance hit?
No, quite the reverse. Turning hot spare on ensures that when a disk fails that the autoraid will restore your data to N+1 again, but in a slower Raid5 level.

> I fully understand the
> benefits in terms of
> continuity of service/
> performance following a
> disk failure.
You get this with hot spare on or off anyway, just in a different wrapping. Raid5 is slower, but better on space.
The active host spare is not an unused disk, it is used for a more expensive in terms of disk space raid.

The only disadvantage of using the HS is the space restriction.

The advantages are numerous,
. N+1 state restored as fast as possible on failure.
. Enough disk space to complete the restore without adding a replacement disk.
. Data stored in a faster raid level during normal operation.

Later,
Bill
It works for me (tm)