Easy Tier automatic data placement requirements, recommendations, and limitations

Some limitations exist if you use the IBM® Easy Tier® function on your system.

  • The Easy Tier function supports the following tiered storage configurations:
    • Local (internal) Serial Attached SCSI (SAS) flash drives in a storage pool with local (internal) Serial Attached SCSI (SAS) hard disk drives (HDDs).
    • Local (internal) Serial Attached SCSI (SAS) flash drives in a storage pool with Fibre Channel-attached hard disk drives (HDDs).
    • External Fibre Channel-attached flash drives in a storage pool with local (internal) Serial Attached SCSI (SAS) hard disk drives (HDDs).
    • External Fibre Channel-attached flash drives in a storage pool with Fibre Channel-attached hard disk drives (HDDs).
  • To ensure optimal performance, all MDisks in a storage pool tier must have the same technology and performance characteristics.
  • Easy Tier automatic data placement is not supported on volume copies that are image mode or sequential. I/O monitoring for such volumes is supported, but you cannot migrate extents unless you convert the image or sequential volume copies to striped volumes.
  • Automatic data placement and extent I/O activity monitors are supported on each copy of a mirrored volume. The Easy Tier function works with each copy independently of the other copy. For example, you can enable or disable Easy Tier automatic data placement for each copy independently of the other copy.
  • The system creates new volumes or volume expansions by using extents from the enterprise tier, if possible. However, if necessary, the system uses extents from the tier 0 flash, tier 1 flash, or nearline tiers.
  • When a volume is migrated out of a storage pool that is managed with the Easy Tier function, Easy Tier automatic data placement mode is no longer active on that volume. Automatic data placement is also turned off while a volume is being migrated between two pools that have Easy Tier automatic data placement enabled. Automatic data placement for the volume is reenabled when the migration completes.

Limitations when removing an MDisk by using the force parameter

When an MDisk is deleted from a storage pool with the force parameter, extents in use are migrated to MDisks in the same tier as the MDisk being removed, if possible. If insufficient extents exist in that tier, extents from the other tier are used.

Limitations when migrating extents

When Easy Tier automatic data placement is enabled for a volume, the migrateexts command-line interface (CLI) command cannot be used on that volume.

Limitations when migrating a volume to another storage pool

When the system migrates a volume to a new storage pool, Easy Tier automatic data placement between the generic flash drive tier and the Flash, Enterprise, or Nearline tier is temporarily suspended. After the volume is migrated to its new storage pool, Easy Tier automatic data placement between the generic flash drive tier and the Flash, Enterprise, or Nearline tier resumes for the newly moved volume, if appropriate.

When the system migrates a volume from one storage pool to another, it attempts to migrate each extent to an extent in the new storage pool from the same tier as the original extent. In some cases, for example when the target tier is unavailable, the other tier is used. For example, the generic flash drive tier might be unavailable in the new storage pool.

If the automatic data placement is enabled in the new storage pool, pending Easy Tier status changes are assigned after the volume completes its move to the new storage pool. Although the status changes are based on volume use in the old storage pool, the new status is honored in the new storage pool.

Limitations when migrating a volume to image mode

Easy Tier automatic data placement does not support image mode. No automatic data placement occurs in this situation. When a volume with Easy Tier automatic data placement mode active is migrated to image mode, Easy Tier automatic data placement mode is no longer active on that volume.

The Easy Tier function does support evaluation mode for image mode volumes.

Overallocation requirements

If you use Easy Tier with IBM FlashCore Module devices as the top tier, consider setting an overallocation within this tier. Easy Tier migrates storage only at a slow rate, which might not keep up with changes to the compression ratio within the tier. As such, the user might specify the maximum overallocation ratio for FlashCore Module arrays. The default setting is to allow no overallocation on new pools. To set this value in the management GUI, select Pools > Pools. Right-click the pool and select Modify Overallocation Limit. In the command-line interface, you can use the -etfcmoverallocationmax parameter on mkmdiskgrp or chmdiskgrp to specify this ratio.