Dynamic Disk Pooling (DDP)

Dynamic Disk Pooling (DDP) dynamically distributes data, spare capacity, and protection information across a pool of disk drives. DDP improves the time and performance of traditional RAID arrays. DDP will only be supported on 3956-CSA/XSA subsystems.

The TS7700 3956-CSA/XSA subsystem has a dynamic disk pools (DDP) configuration by default, supported by a microcode of 8.40.0.xx or higher. Previous TS7700 releases, such as CC9/CS9 and prior, used an RAID6 configuration. DDP is also known as Distribute RAID, or D-RAID.

Because RAID cannot keep up with increasing disk capacities, DDP was created to be more versatile by providing better rebuild times. In an RAID array when a drive fails, the remaining drives are read, parity recomputed, and the result is written to the spare drive. This is done from the initial logical block of the array to the last block in the array. This operation is time consuming because all data needs to be recomputed from the beginning of the array to the end of the array, and degrades performance, because although there are parallel reads, there is one single write to the spare drive. Thus, this single write becomes a bottleneck in the system.

In DDP, a disk pool is a set of drives that are logically grouped together in the storage subsystem, where data is distributed across all drives in the pool. The drives in each disk pool must be of the same drive type and drive media type, and they must be similar in size. Unlike RAID, there is no specific spare drive, rather, all drives have spare space that is reserved. When a drive fails, the remaining drives are read, the missing data is recomputed, and the result is written to multiple drives in their spare space. This operation is done on the pieces of data that are missing. The result is parallel reads and parallel writes, which significantly speeds up the rebuild time after a single drive failure.

Both RAID and DDP are techniques for striping data and parity information across a set of disks to provide fault tolerance, but how they operate to attain this goal is different.

The DDP design offers the following features:

Dual Drawer Configuration

24 DDM type (dual drawer): A pool that consists of two enclosure pairs that contain a total of 24 DDMs. The pair of enclosures consists of either a CSA and XSA or two XSAs.

Figure 1. Dynamic Disk Pool example for a two (dual) drawer configuration (CSA and XSA)

Single Drawer Configuration

12 DDM type (single drawer): A pool that consists of one enclosure that contains 12 DDMs. This type can be within a cache string that contains a single CSA with no XSA attached OR an XSA at the end of a cache string that has an odd number of enclosures (CSA + XSAs).

Figure 2. Dynamic Disk Pool example for a one (single) drawer configuration (XSA)

Traditional RAID Drive Rebuilds Eliminated

Data is automatically rebalanced across the drive pool.

Faster rebuild times after a drive failure.

Improved Performance

Automated load balancing.

Maintains performance under drive failure. Up to two drives can fail simultaneously at any time in the pool. For a pool that contains 12 drives, up to 2 DDMs can fail without shutting down the system. For a pool that contains 24 drives, up to 4 DDMs can fail, but not all at the same time.

Reserved (spare) space is allocated across all the drives within a dynamic disk pool. The amount of reserved (spare) space in a pool is equal to about two disks worth of space per drawer.

Low Maintenance

No dedicated physical spare drives.

No RAID groups.

No dedicated parity drives.