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Load balancing

The multi-volume support feature provides DST the ability to define allocation policies with a balance allocation order and chunk size to files or a file system, known as load balancing. The chunk size is the maximum size of any extent that files or a file system with this assigned policy can have. The chunk size can only be specified for allocation policies with a balance allocation order.

A load balancing policy specifies the balance allocation order and a non-zero chunk size. The balance allocation order distributes allocations randomly across the volumes specified in the policy and limits each allocation to a maximum size equal to the specified chunk size.

Load balancing extends the behavior of policy enforcement by rebalancing extent allocations such that each volume in the policy is mostly equally used. The policy enforcement handles the following cases:

• New volumes are added to the policy, and the extents associated with a file need rebalancing across all volumes, including the new ones.
• Volumes are removed from the volume set or from the policy, and the extents for a file residing on a removed volume need to be moved to other volumes in the policy.
• A load balancing policy is assigned to a file and its extents have to be reorganized to meet the chunk size requirements defined in the policy.

The load balancing policy is intended for balancing data extents belonging to files across volumes defined in the policy. However, there is no restriction imposed in assigning load balancing policy for metadata.

  Note   If the fixed extent size is less than the chunk size, then the extent size will be limited to the largest multiple of the fixed extent size that is less than the chunk size. If the fixed extent size is greater than the chunk size, then the extent size will be the fixed extent size.