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FAQ: Reallocation for Provisioned Storage

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Applies to

  • Data ONTAP

Answer

  • Reallocation optimizes sequential read performance by optimizing the layout of data on the disks.
  • It works on volumes, LUNs, individual files, and in a special case, aggregates.
  • The reallocate process can be started, quiesced, stopped, and scheduled from the command line interface (CLI) by using the reallocate command.
  • The workload that most benefits from the use of reallocate is sequential reads after random writes; however, other workloads might also show some improvement depending on the workload characteristics.
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  • Best Practices
    • Sufficient space on the target volume is required.  
      • In order for a reallocate process to run, there must be a minimum of 10% free space available (excluding snap reserve) on the target volume to allow for block movement and relocation.  
    • Run the corresponding 'reallocate measure -o' command for the version of ONTAP being used to determine if a reallocate will provide optimal results that will provide the desired benefit for data access to the target volume. 
    • Reallocate will not operate on blocks that are held in a snapshot.  
      • Scheduled snapshots should be suspended prior to running reallocate to prevent any affected blocks from being placed into the snap reserve, which could cause the snap reserve utilization to exceed its allocation and begin using the active file system, reducing the amount of available free space and impacting the overall result of the operation.
    • Remove as many snapshots as can be allowed prior to the reallocation as possible to allow for optimal results.  
    • SnapMirror and SnapVault snapshots will need to be left in place to avoid a re-initialization of the respective relationships. 
    • For SnapMirror volumes, only run the reallocate operation on the source volume.
    • Volume guarantee (thick provisioning) should be enabled.  
      • This will prevent the reallocation operation from inducing an over-subscription for the volume's allocated space.
    • Do not change the setting for wafl scan speed in order to attempt to increase the speed of the reallocate process.  
      • Doing so will impact the overall performance of the system.
    • If the volume or volumes are prone to operations that induce a dynamic rate of change and/or growth then it may be desirable to schedule future reallocate operations. 
    • As a reallocate can be resource-intensive, only run the operation during a low utilization time if possible to avoid unnecessary repeat operations due to write requests to the target volume.  
      • Too many concurrent write requests during a reallocation can cause excessive space usage.

Possible Issues

  • If a volume-wide full reallocation fails because of space issues, try a reallocate on a per-file basis, if it is feasible.
  • Reallocation can not be performed on any SnapLock volumes as the associated data is marked as write once read many (WORM) and can not be altered until the retention date is reached.  

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