Il 18/07/2013 15:29, Peter Lieven ha scritto:
If the driver would have a better method of writing zeroes than
discard it simply should not set bdi->write_zeroes_w_discard = 1.
If the driver had a better method of writing zeroes than discard, it
simply should ignore the BDRV_MAY_UNMAP (or BDRV_MAY_DISCARD) flag in
its bdrv_write_zeros implementation.
ok, but this would require an individual patch in every driver, wouldn't
it. i am ok with that.
Yes (making the drivers return the flag in the BDI would also require
per-driver patches).
BDRV_MAY_UNMAP is an advisory flag, that the driver is free to ignore
(just like bdrv_discard can be ignored altogether).
block.c probably should avoid passing down BDRV_MAY_UNMAP altogether if
the BDRV_O_UNMAP flag is not set.
the BDRV_MAY_DISCARD flag is at the end a hint if the driver can optimize
writing zeroes by a discard or if real zeroes should be written e.g. to
sanitize the device?
talking for iscsi:
bdrv->discard can remain to use UNMAP and silently fail if lbpu == 0.
bdrv->write_zeroes will use writesame16 and set the unmap flag only if
BDRV_MAY_DISCARD == 1 and BDRV_O_UNMAP == 1 and lbprz == 1.
in case lbpws == 0 it will return -ENOSUP.
Almost: for LBPWS == 0 it can use WRITE SAME(16) without the unmap flag.
If that doesn't work either it can return -ENOTSUP.