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. Paolo > > Correct? > > Peter >
