On Mon, 2017-05-01 at 20:45 +0000, Bart Van Assche wrote:
> On Wed, 2017-04-05 at 19:21 +0200, Christoph Hellwig wrote:
> > Now that we use the proper REQ_OP_WRITE_ZEROES operation everywhere we can
> > kill this hack.
> >
> > Signed-off-by: Christoph Hellwig <hch@xxxxxx>
> > Reviewed-by: Martin K. Petersen <martin.petersen@xxxxxxxxxx>
> > Reviewed-by: Hannes Reinecke <hare@xxxxxxxx>
> > [ ... ]
> > diff --git a/drivers/target/target_core_device.c b/drivers/target/target_core_device.c
> > index c754ae33bf7b..d2f089cfa9ae 100644
> > --- a/drivers/target/target_core_device.c
> > +++ b/drivers/target/target_core_device.c
> > @@ -851,7 +851,7 @@ bool target_configure_unmap_from_queue(struct se_dev_attrib *attrib,
> > attrib->unmap_granularity = q->limits.discard_granularity / block_size;
> > attrib->unmap_granularity_alignment = q->limits.discard_alignment /
> > block_size;
> > - attrib->unmap_zeroes_data = q->limits.discard_zeroes_data;
> > + attrib->unmap_zeroes_data = 0;
> > return true;
> > }
> > EXPORT_SYMBOL(target_configure_unmap_from_queue);
>
> Hello Christoph,
>
> Sorry that I hadn't noticed this before but I think that this patch
> introduces a significant performance regressions for LIO users. Before
> this patch the LBPRZ flag was reported correctly to initiator systems
> through the thin provisioning VPD. With this patch applied that flag
> will always be reported as zero, forcing initiators to submit WRITE
> commands with zeroed data buffers instead of submitting the SCSI UNMAP
> command to block devices for which discard_zeroes_data was set. From
> target_core_spc.c:
>
> /* Thin Provisioning VPD */
> static sense_reason_t spc_emulate_evpd_b2(struct se_cmd *cmd, unsigned char *buf)
> {
> [ ... ]
> /*
> * The unmap_zeroes_data set means that the underlying device supports
> * REQ_DISCARD and has the discard_zeroes_data bit set. This satisfies
> * the SBC requirements for LBPRZ, meaning that a subsequent read
> * will return zeroes after an UNMAP or WRITE SAME (16) to an LBA
> * See sbc4r36 6.6.4.
> */
> if (((dev->dev_attrib.emulate_tpu != 0) ||
> (dev->dev_attrib.emulate_tpws != 0)) &&
> (dev->dev_attrib.unmap_zeroes_data != 0))
> buf[5] |= 0x04;
> [ ... ]
> }
>
According to sd_config_discard(), it's SD_LBP_WS16, SD_LBP_WS10 and
SD_LBP_ZERO that where ever setting unmap_zeros_data = 1 to begin with.
For UNMAP, q->limits.discard_zeroes_data was never set.
That said, it's pretty much implied that supporting DISCARD means
subsequent READs return zeros, so target_configure_unmap_from_queue()
should be setting attrib->unmap_zeroes_data = 1, or just dropping it
all-together.
Post -rc1, I'll push a patch to do the latter.
