Capacity saving and accelerated compression functions

Your storage system provides the following functions to make efficient use of user capacity:

  • Capacity saving: The capacity saving function includes data deduplication and data compression. Capacity saving enables you to reduce your bitcost for the stored data by deduplicating and compressing the data. These deduplication and compression functions are performed by the controllers of the storage system.
  • Accelerated compression: The accelerated compression function enables you to reduce your bitcost for the stored data by allowing you to take advantage of the compression function in the FMD drives. Accelerated compression allows you to assign FMD capacity to a pool that is larger than the physical capacity of the FMD parity groups. The data access performance of the storage system is maintained when the accelerated compression function is used, as the compression engine is offloaded to the FMD drives.

The following table lists combinations of using deduplication and compression together, controller-based compression only, or accelerated compression and describes the functionality and performance impacts for each combination.

Combination

Functionality

Using only accelerated compression

Data compression is performed by the FMD drives and not by the storage controller.

Because there is no overhead due to compression processing by the storage controller, I/O performance is not affected.

Using only controller-based compression

The storage controller compresses data and stores the compressed data in the pool.

Use controller-based compression for drives that do not support accelerated compression (for example, SAS, SSD) and for FMD drives for which accelerated compression is not enabled (for example, an encrypted parity group of FMD DC2 drives).

If both accelerated compression and controller-based compression are used on the same drives, compression is performed only in the FMD drives. Because of differences in storing and managing data by the storage controller, performance is lower than when only accelerated compression is used. Therefore, when accelerated compression can be enabled, you should use accelerated compression instead of controller-based compression. Use controller-based compression only when accelerated compression cannot be enabled.

Using accelerated compression and deduplication and compression

See Example 1 below

When multiple copies of identical data are stored in the pool, the storage controller keeps only one copy (deduplication). For compression, the storage controller detects that accelerated compression is enabled* and uses it instead of controller-based compression.

*Accelerated compression must be enabled for all parity groups in the pool.

Using deduplication and compression

See Example 2 below

The storage controller performs the compression and deduplication processing.

The storage controller has the largest overhead of the capacity saving processing.

Note For details about enabling both encryption and accelerated compression on a parity group, see the Encryption License Key User Guide.

The following figures illustrate effects achieved by the capacity saving and accelerated compression functions. In these figures, items enclosed by the square brackets are displayed by HDvM - SN, and items with an asterisk are displayed using CCI.

Example 1: Using accelerated compression and deduplication and compression

diagram illustrating use of both accelerated compression and deduplication and compression

Example 2: Using the capacity saving function (deduplication and compression)

diagram illustrating use of deduplication and compression

Note
  • Saving Effect: When the capacity saving function is used, the saving ratio is calculated to include user data, metadata and garbage data (generated by the system). When the data volume used capacity before the capacity saving processing is smaller than the pool used capacity, the saving ratio might be lower than expected and the saving capacity value might be invalid (a hyphen (-) is displayed).

    When data in DP-VOLs is deleted by performing the following tasks, the saving ratio might be lower than expected due to nonformatted data on FMDs:

    • Deleting DP-VOLs
    • Formatting DP-VOLs
    • Initializing duplicated data in a pool

    By default, the maximum displayable value (99999999999999.99) for the saving effect is displayed.

  • Software Saving: When the capacity saving function is used, the saving ratio is calculated with user data, metadata and garbage data (generated by the system). When the data volume used capacity before the capacity saving processing is smaller than the pool used capacity, the saving ratio might be lower than expected and the saving capacity value might be invalid (a hyphen (-) is displayed).

    When you initialize the duplicated data in a pool, the saving effect might be different than expected, at least until after the LDEV format processing has completed. Verify the saving effect only after the duplicated data in a pool has finished initializing.

  • FMD Saving: When data in DP-VOLs is deleted by performing the following tasks, the saving ratio might be lower than expected due to nonformatted data on FMDs:
    • Deleting DP-VOLs
    • Formatting DP-VOLs
    • Initializing duplicated data in a pool

For details about the formatted pool capacity or progress, see the item for Free + Reserved in the View Pool Management Status window.

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