HOT HOT HOT… Hot Spare that is! VNX VNX2

The other day I had a customer purchase a brand new DAE for his VNX.. awesome.. A full shelf of 25 drives.  900 GB SAS drives 2.5″ form factor.  Well do some quick math.. you have 5 R5 groups (4+1)

But… what a sec.. What about hot spare? You can run parity and have R5 for protection.. but you still need to be in compliance with your hot spare policy.  This customer has the older 3.5″ DAE (15 slots) and the newer drives are 2.5 ” .. what to do..

Will you have a valid hot spare on hand?

After some online research:

Based on the discussion and the reference white papers for both VNX and VNX2 – drive size isn’t of importance. The other factors are: drive type and density. VNX2 is global and won’t take into consideration the drive speed, so you could potentially have slower speed drive of same type for a replacement. This is unknown to the Admin as the policy is set differently.

https://community.emc.com/thread/123197?start=0&tstart=0  — A great discussion about this and a fantastic resource for EMC related issues. The following is take from the above thread.
Hot spare algorithm
The appropriate hot spare is chosen from the provisioned hot spares algorithmically.  If there were no hot spares provisioned of appropriate type and size when a drive fails, no rebuild occurs.  (See the Drive Rebuilds section.)  The RAID group with the failed drive remains in a degraded state until the failed drive is replaced; then the failed drive’s RAID group rebuilds.
The hot spare selection process uses the following criteria in order:
  1. Failing drive in-use capacity – The smallest capacity hot spare drive that can accommodate the in-use capacity of the failing drive’s LUNs will be used.
  2. Hot spare location – Hot spares on the same back-end port as the failing drive are preferred over other like-size hot spares.
  3. Same Drive type – Hot spare must be of the same drive type.
Failing drive in-use capacity
It is the in-use capacity of the failing drive’s that determines the capacity of the hot spare candidates.  Note this is a LUN-dependent criterion, not a raw drive capacity dependency.  This is measured by totalling the capacity of the drive’s bound LUNs.  The in-use capacity of a failing drive’s LUNs is not predictable.  This rule can lead to an unlikely hot spare selection.  For example, it is possible for a smaller capacity hot spare to be automatically selected over a hot spare drive identical to, and adjacent to the failing drive in the same DAE.  This occurs because the formatted capacity of the smaller, hot spare (the highest-order selection criteria) matches the in-use capacity of the failing drive’s LUNs more closely than the identical hot spare drive.
Note that a hot spare’s form factor and speed are not a hot spare criteria within the type.
For example, a 3.5” format 15K rpm drive can be a hot spare for a failing 2.5” 10K rpm SAS drive.
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For the VNX2

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Bottom line this is good to know because the customer had open slots in their existing 15 slot 3.5 ” DAE and if drive form factor did matter they would need to buy another DAE for the 2.5″ drives!

Here is the Hot Spare drive matrix. It illustrates the Failed drive and compatible spare.

mcx_hs_matrix

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