Most legacy operating systems integrally cannot 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 be performed by an application or guest operating system.
The dead Space Reclamation process enables to reclaim of 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. 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 internal record to mark them as invalid. And the SSD will no longer move the records marked internally as invalid blocks during garbage collection, which eliminates the time wasted to rewrite invalid data to new flash pages. Trim requires physical disks directly attached to the server and trim-compatible hardware.
Read this article to know more.
