Dell Unity: Understanding Dynamic Pools (Mapped RAID) (Dell Correctable)

• Dynamic pool technology was introduced in 4.2 Dell Unity OE code.
• Dynamic pools are also known as Mapped RAID and Extent Based RAID.
• From Unity OE 4.2 through 5.1, dynamic pools are supported on all-flash physical hardware only.
• In OE version 5.2, support for dynamic pools on hybrid systems was added.
• In Unity all-flash models running OE version 4.2 or later, all new pools created in the Unisphere GUI are dynamic pools, and new pools created in the Unisphere CLI and REST API are dynamic pools by default. For hybrid systems running 5.2 or later, this is the default pool type when creating pools in Unisphere.
• Dynamic Pools are also known as Extent Pools.
• Dynamic pools replace the previous pool technology (known as traditional pools) as the default pool type for all-flash systems running OE version 4.2 and greater and hybrid flash systems running OE 5.2 and greater.

• Drives are not wasted, because there are no fixed spares required.
• All drives in the system can be added to a pool. This prolongs the life of the drives in the pool, since the load is spread across additional drives.
• Rebuild times are faster than with traditional pools since spare capacity for a dynamic pool is spread across multiple drives rather than concentrated on a single hot spare drive. More drives contribute to the rebuilding process when a drive fails.
• Pools can usually be expanded based on desired capacity. For example, you can add one drive at a time to a dynamic pool, providing provisioning flexibility and cost savings.

• A group of drives of the same drive type which have been combined into a hidden Dynamic Pool object. 
• Each drive within a Dynamic Pool can only be part of a single drive partnership group. 
• A drive will never change the drive partnership group.
• Each drive partnership group can only contain a single drive type, though different sizes of a particular drive type can be mixed within the group. 
• The maximum number of drives contained within a drive partnership group is 64.

• When a drive partnership group is full, a new group must be started with the minimum number of drives for the RAID width + 1 drive worth of spare space.

• Using different sized of the same drive type in a DPG is allowed, however, if a DPG contains x 400GB drives and only 1 800GB drives, half the extents on the 800GB will not be used until enough drives of the same size are present and fulfill the RAID width.

• As each drive partnership group can only contain a single drive type, flash, SAS, and NL-SAS drives will be placed into their own drive partnership groups. Furthermore, different speed spinning drives can be added to the same dynamic pool but will be placed into their own drive partnership groups for performance reasons. For example, 10K RPM and 15K RPM drives residing in the same pool will be placed into different drive partnership groups regardless of the drive size as shown in the figure below.

• A drive extent is a portion of a drive.
• The size of an extent is fixed for each drive type; SAS Flash 3 or SAS Flash 4.
• The number of extents per drive depends on the drive's type and size.
• When a dynamic pool is created, each drive in the pool will be partitioned into drive extents.
• A drive extent can either be:

Spare Space Extent

• Dynamic Pools do not require dedicated Hot Spares. It uses spare space reserved within each pool and can only be used to replace a failing or failed drive. 
• The number of spare space extents reserved within a Dynamic Pool directly depends on the size of the drives and the number of drives within the Pool.
• For every 32 drives of the same type within a Dynamic Pool, 1 or 2 drives worth of space is allocated as spare space depending on the OE code as described previously. 
• The amount of spare space reserved always ensures that the drive with the largest usable capacity within the Pool can be replaced with the spare space extents remaining within the Pool.
• Spare space within the drive partnership group must be replenished after a rebuild completes as now there is insufficient spare space in that group.
• If a free drive exists within the system, and it is the same size or larger and the same drive type as the failed drive, it will be consumed by the dynamic pool to replace the missing spare space extents. Once the failed drive is replaced, it is left free within the system.
• If no free drives exist within the system, once the failed drive has been replaced, the new drive becomes consumed by the dynamic pool and the rebalance of spare space extents will occur.

• After spare space extents are reserved within the Dynamic Pool, RAID extents are created with the remaining drive extents.
• A RAID extent is drive extents which complete the stripe width for a RAID type. For example, if RAID 5 was selected as the RAID type, and 4+1 was selected as the stripe width for the Pool, the RAID extent would contain 5 drive extents (4+1). 
• The RAID extent provides RAID protection for user data stored within the Dynamic Pool and is later used to provide usable capacity to the Pool for storage resource creation. 
• A single RAID extent cannot contain two drive extents from a single drive for protection purposes. The software will ensure that no RAID extents will contain 2 drive extents from the same drive (controlled by the drive extent pool).
• RAID extents must contain drive extents from only a single drive partnership group. 

• Dynamic Pool created with 6 drives, assuming that RAID 5 (4+1) has been selected. 
• In this example, a number of extents have already been reserved as spare space extents, and the first 3 RAID extents are shown. 
• Each RAID extent in this example contains 5 drive extents, due to the 4+1 stripe width. 
• The 5 drive extents are selected from the drives within the drive partnership group, and no 2 extents are selected from the same drive. 
• For ease of illustration, the drive extents selected are in order across the drives within the Pool. 
• Within a real system, the Dynamic Pool algorithm will select drive extents from different drives seemingly at random within the drive partnership group. 

Stripe Width

• When a dynamic pool is created in Unisphere, the user will select the RAID type desired, but the stripe width will be selected by the system automatically. 
• The stripe width selected by the system directly depends on the number of drives selected at Pool creation. 
• For example, when RAID 5 is selected during Pool creation in Unisphere, and 8 drives are selected, the stripe width is automatically set to 4+1 by the system. If the drive count was 14 or more for RAID 5, a 12+1 would be selected. 
• The selection of the larger stripe widths provides more usable capacity to the user. If you would like to force the system to choose a particular width, select only a specific number of drives at Pool creation, then expand the Pool with the remaining drives or use Unisphere CLI or REST API.
• In Unisphere, RAID 6 is the default and only option for the NL-SAS Capacity Tier. If RAID 1/0 or 5 is required for the Capacity Tier, Unisphere CLI or REST API can be used to create the pool or expand it with NL-SAS drives.
• Once the pool is created, the RAID type selected will be set for all current and future drive partnership groups within the tier of the pool. Once the RAID type is set for a tier, it is persisted for the lifetime of the pool and cannot be changed later.

• Within a Pool on a Dell Unity system, whether Traditional or Dynamic, exists one or more private RAID groups and a single private LUN created on each.
• The private RAID group is used to provide space to the private LUN, which provides space in the form of 256 MB slices to the user for storage resource allocation. 
• Within Dynamic Pools, a private RAID group is created using a combination of RAID extents.

Private LUNs

• A Dynamic Pool private LUN is created on a Dynamic Pool RAID Group
  • One private LUN per Dynamic Pool RAID Group
  • Similar to Traditional Private LUNs within a private RAID Group    
• The private LUN is divided into 256MB slices
  • Used to create Pool storage resources
• As with Dynamic Pool RAID Groups, the size of the private LUNs can vary within a Pool
  • Depends on how the Pool was created and expanded

Architecture of Traditional vs Dynamic Pool 



System Proactive Copy in Dynamic Pools

• If a drive is receiving errors above the Dell Unity OE's internal thresholds, a proactive copy operation may be started by the system.
• A proactive copy operation is the process of copying data from a drive going bad to a new location. Once the copy operation completes, the drive should be failed by the software. 
• If an unbound drive (spare) is available, it will be consumed by the dynamic pool. If not, the pool will use the spare space extents in the pool.
• During this operation, the system ensures the destination drive for each RAID extent does not already contain an extent from the same RAID extent. 

• If a drive were to fail before the proactive copy completes, a rebuild operation will occur for the uncompleted copied extents. The failed drive rebuild occurs by rebuilding the degraded RAID extents within the drive partnership group. 
• During the rebuild of a RAID extent, the remaining drive extents within the RAID extent are used to rebuild the missing drive extent to a spare space extent. 
• Once complete, the spare space extent becomes part of the RAID extent and the RAID extent is no longer degraded. As the various RAID extents needing to be rebuilt and spare space extents are spread across many drives within the drive partnership group, many drives are engaged to complete the rebuild operations.

Pool Expansion
Expanding a traditional pool, we are bound by the current RAID width and must add the same number of drives as that RAID group size.
In a dynamic pool, expanding the pool can be as minimal as a single drive depending on the current configuration.


Wear/End of Life

• In Dell Unity OE version 4.2 and later, system alert messages are sent at 180, 90, and 30 days before a Flash drive is expected to reach 100% wear. 
• At the 60 days mark, Dell is notified via call home that the drive may reach 100% wear, and a replacement is required. These alerts are generated for each individual drive.
• If a drive is close to the end of its usable life based on wear, a proactive copy operation may be started by the system to replace the worn drive. 
• This proactive copy does not copy drive extents within the drive to spare space extents within the Pool, but rather to a free drive within the system if available. 
• After the proactive copy operation completes, the drive is failed, an alert is generated, a call home is initiated, and the drive should be replaced.
• The proactive copy to spare space extents within the drive partnership group is not done because it would only increase the wear on the drives in the group, which may have the same amount of wear as the drive being replaced. If a valid spare is not available, no proactive copy operation is completed.

Hybrid Flash Systems
In Dell Unity OE 5.2 and later, changes have been made to the System Settings > Storage Configuration > Drives page for hybrid systems. Along with the traditional and dynamic pool information that is displayed for all-flash systems, hybrid systems also display the FAST Cache column. The FAST Cache column displays how many drives are currently configured in FAST Cache.



Other Useful KBAs for Dynamic Pools:
KB#000014378 Dell EMC Unity: Dynamic Pools best practices for initial drives configuration (User Correctable)

KB#000019530 Dell EMC Unity: Dynamic pools minimum drives count on creation wizard (User correctable)

KB#000055614 Dell EMC Unity : When a disk fails in a Dynamic pool, array performance can be impacted (Dell EMC Correctable)

KB#000010748 Dell EMC Unity: Dynamic pool enhancement/considerations over Traditional pools (User Correctable)

 KB#000010677 Dell EMC Unity: How to check the re-balancing of a Dynamic Pool (Dell EMC Correctable)

KB#000031834 Dell EMC Unity: Automatic copy back of Dynamic pool user extents (User Correctable)

KB#000019417 Dell EMC Unity: How to perform a rough calculation to determine usable space in a Dynamic Storage Pool. (User Correctable)

KB#000052749 Dell EMC Unity: Dynamic Pool is showing degraded state after mapped RAID rebuild is completed (User Correctable)


Reference Document:
Dell Unity: Dynamic Pools (delltechnologies.com)