EMC Symmetrix: Dynamic Hot Spares

There are two types of sparing strategies available on EMC Symmetrix Series of machines.

Dynamic Hot Sparing: Starting the Symmetrix 4.0, EMC had introduced dynamic hot spares in its Enginuity code to support customers against failing disk drives and reducing the probability of a data loss. Available there onwards on each version of Symmetrix, customers have been able to use this Hot Sparing technology. Today the Dynamic sparing is available on Symmetrix 4.0, Symmetrix 4.8, Symmetrix 5.0, Symmetrix 5.5, DMX, DMX2, DMX3, and DMX4 systems.

Permanent Spares: Was introduced starting the Symmetrix DMX3 products, now available on DMX4’s and V-Max systems. I believe, Enginuity code 5772 started supporting Permanent Spares to guard customers against failing disk drives to further help reduce any performance, redundancy and processing degradation on the Symmetrix systems with features that were not available with the Dynamic Hot Sparing.

Highlights of Permanent Sparing

Due to some design, performance, redundancy limitations and Symmetrix mirror positions, dynamic hot spares were becoming a bottleneck related to customer internal job processing, example: a failed 1TB SATA drive sync to dynamic spare might take more than 8 to 48 hours. While a similar process to remove the dynamic spare and equalize the replaced drive might take the same. During this time the machine is more or less in a lock down (Operational but not configurable).

Due to these limitations, a concept of Permanent spares was introduced on EMC Symmetrix systems, which would help fulfill some gaps the Dynamic hot spares technology has. Following are the criteria for Dynamic Hot Spares.

To read about EMC Symmetrix : Permanent Hot Spares

Some important things to consider with Dynamic Hot Sparing

Supported through microcode (Enginuity) version starting Symmetrix Family 4.0, support extended through all later releases of Enginuity until DMX-4 (5773).
Dynamic Hot Spares configured and enabled in the backend by an EMC CE.
No BIN file change is performed as the Dynamic Hot Spare gets invoked or removed upon a disk drive failure.
No BIN file change is allowed until the Dynamic Hot Spare is removed from the active used devices pool and inserted back into the Spares pool.
An EMC CE will need to attend site to replace the failed drive and put the dynamic hot spare back in the pool of devices available for sparing.
Enginuity does not check for performance and redundancy when the dynamic hot spare is invoked.
In the previous generation of Symmetrix systems, an exact match (speed, size, block size) was required with Dynamic hot spares. Starting I believe the 5772 (DMX3 onwards) version of microcode that requirement is not necessary. Now larger or smaller multiple dynamic spares can be spread across protecting multiple devices not ready, the one to one relationship (failed drive to dynamic spare) is not true any more.
Related to performance on DMX3 systems and above, if correct dynamic spares are not configured, customers can see issues around redundancy and performance. Example, A 10K drive can be invoked automatically against a failed drive that is 15K causing performance issues. Also a drive on the same loop as other raid group devices can be invoked as a hot spare, potentially causing issues if the entire loop was to go down.
Dynamic spares will not take all the characteristics of failed drives. Example, mirror positions.
While the Permanent Spare or Dynamic Hot Spare is not invoked and is sitting in the machine waiting for a failure, these devices are not accessible from the front end (customer). The folks back at the PSE labs, will still be able to interact with these devices and invoke it for you incase of a failure or a proactive failure or for any reasons the automatic invoke fails.
If a Permanent Spare fails to invoke, a Dynamic Hot Spare is invoked, if a Dynamic Hot Spare fails to invoke, the customer data stays unprotected.
Dynamic Hot Spare is supported with RAID-1, RAID-10, RAID-XP, RAID-5 and various configurations within each Raid type. Dynamic hot sparing does not work with RAID-6 devices.
As far as I know for the V-Max systems, Dynamic hot sparing is not supported.

Some important benefits of Dynamic Hot Sparing

Dynamic Hot Sparing kicks in when Permanent Sparing fails to invoke
Provides additional protection against data loss
No BIN file change is performed with Dynamic Hot Sparing

As a requirement to all the new systems that are configured now, sparing is required. Hope this provides a vision into configuring your next EMC Symmetrix on the floor.