Re: [PATCH] block: correctly fallback for zeroout

From: Martin K. Petersen
Date: Thu Jun 02 2016 - 22:56:30 EST

>>>>> "Sitsofe" == Sitsofe Wheeler <sitsofe@xxxxxxxxx> writes:

Sitsofe> The original SCSI WRITE SAME has overloaded semantics - not
Sitsofe> only does it mean "write this data multiple times" but it can
Sitsofe> also be used to mean "discard this range" too. If the kernel's
Sitsofe> command was modelled on the SCSI original perhaps this
Sitsofe> conflation clouded things?

REQ_WRITE_SAME in the context of the kernel explicitly means "write
payload to this block range".

A REQ_DISCARD command may be serviced using WRITE SAME(16) with the
UNMAP bit set in the SCSI disk driver but that's entirely orthogonal.

Martin K. Petersen Oracle Linux Engineering