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Reclaiming Space using TRIM UNMAP in Windows

Most legacy operating systems integrally do not have the capability to reclaim the space when a host performs large delete operations from a thin provisioned volume without rewriting new data into the deleted space. This deletion may perform by an application or guest operating system.

Dead Space Reclamation process enables to reclaim the previously allocated blocks of thin-provisioned LUNs that are no longer in use. Those blocks that contain deleted data are unallocated and the storage array is updated.

This is performed by one of two techniques: TRIM and UNMAP, to use the storage space more efficiently.

What is TRIM & UNMAP?

TRIM is the specification for this functionality that handles all standards for ATA interfaces and UNMAP is the full equivalent of TRIM but for SCSI disks.

What is SSD TRIM?

First TRIM is not a command that forces the SSD to immediately erase data. Basically, TRIM is a command that notifies the SSD which Logical Block Addresses (LBAs) are no longer needed. The SSD takes those addresses and updates its own internal record in order to mark them as invalid. And the SSD will no longer move that records marked internally as invalid blocks during garbage collection, which eliminates the time wasted in order to rewrite invalid data to new flash pages. Trim requires physical disks directly attached to the server and trim-compatible hardware.


  • Less data is rewritten, and more free space is available during the garbage collection
  • Less data to move during garbage collection and higher throughput
  • Writing less to the flash by not rewriting invalid data during garbage collection achieves improved flash endurance
  • TRIM prevents the SSD drive from making unnecessary writes and being overused


UNMAP is the full equivalent of TRIM specifically for SCSI disks.

UNMAP is a SCSI command that a host can issue to a storage array to free Logical Block Addresses (LBAs) that no longer need to be allocated. UNMAP is used to remove physical blocks from the storage allocation in thinly provisioned Storage Array


  • Reclaimed blocks will be put back into the unused storage pool
  • Avoid out of space condition for thinly provisioned pools of storage
  • No manual operation required on the host
  • No array tools required to perform thin reclamation in the backend

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