On 2018-12-06 23:09, Andrei Borzenkov wrote:
06.12.2018 16:04, Austin S. Hemmelgarn пишет:
* On SCSI devices, a discard operation translates to a SCSI UNMAP
command. As pointed out by Ronnie Sahlberg in his reply, this command
is purely advisory, may not result in any actual state change on the
target device, and is not guaranteed to wipe the data. To actually wipe
things, you have to explicitly write bogus data to the given regions
(using either regular writes, or a WRITESAME command with the desired
pattern), and _then_ call UNMAP on them.
WRITE SAME command has UNMAP bit and depending on device and kernel
version kernel may actually issue either UNMAP or WRITE SAME with UNMAP
bit set when doing discard.
Good to know. I've not looked at the SCSI code much, and actually
didn't know about the UNMAP bit for the WRITE SAME command, so I just
assumed that the kernel only used the UNMAP command.
