Darren J Moffat wrote:
It is a mechanism for part of the storage system above the "disk" (eg
ZFS) to inform the "disk" that it is no longer using a given set of blocks.

This is useful when using an SSD - see Saso's excellent response on that.

However it can also be very useful when your "disk" is an iSCSI LUN.  It
allows the filesystem layer (eg ZFS or NTFS, etc) when on iSCSI LUN that
advertises SCSI UNMAP to tell the target there are blocks in that LUN it
isn't using any more (eg it just deleted some blocks).

That is something I have been waiting a long time for! I have to run a periodic "fill the pool with zeros" cycle on a couple of iSCSI backed pools to reclaim free space.

I guess the big question is do oracle storage appliances advertise SCSI UNMAP?

--
Ian.

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