On 07/11/2014 06:08, Martin K. Petersen wrote:
> The T10 SBC UNMAP command does not provide any hard guarantees that
> blocks will return zeroes on a subsequent READ. This is due to the fact
> that the device server is free to silently ignore all or parts of the
> request.
>
> The only way to ensure that a block consistently returns zeroes after
> being unmapped is to use WRITE SAME with the UNMAP bit set. Should the
> device be unable to unmap one or more blocks described by the command it
> is required to manually write zeroes to them.
>
> Until now we have preferred UNMAP over the WRITE SAME variants to
> accommodate thinly provisioned devices that predated the final SBC-3
> spec. This patch changes the heuristic so that we favor WRITE SAME(16)
> or (10) over UNMAP if these commands are marked as supported in the
> Logical Block Provisioning VPD page.
>
> The patch also disables discard_zeroes_data for devices operating in
> UNMAP mode.
>
> Signed-off-by: Martin K. Petersen <martin.petersen@xxxxxxxxxx>
> ---
> drivers/scsi/sd.c | 10 ++++++----
> 1 file changed, 6 insertions(+), 4 deletions(-)
>
> diff --git a/drivers/scsi/sd.c b/drivers/scsi/sd.c
> index b041eca8955d..95bfb7bfbb9d 100644
> --- a/drivers/scsi/sd.c
> +++ b/drivers/scsi/sd.c
> @@ -656,7 +656,7 @@ static void sd_config_discard(struct scsi_disk *sdkp, unsigned int mode)
> unsigned int logical_block_size = sdkp->device->sector_size;
> unsigned int max_blocks = 0;
>
> - q->limits.discard_zeroes_data = sdkp->lbprz;
> + q->limits.discard_zeroes_data = 0;
> q->limits.discard_alignment = sdkp->unmap_alignment *
> logical_block_size;
> q->limits.discard_granularity =
> @@ -680,11 +680,13 @@ static void sd_config_discard(struct scsi_disk *sdkp, unsigned int mode)
> case SD_LBP_WS16:
> max_blocks = min_not_zero(sdkp->max_ws_blocks,
> (u32)SD_MAX_WS16_BLOCKS);
> + q->limits.discard_zeroes_data = sdkp->lbprz;
> break;
>
> case SD_LBP_WS10:
> max_blocks = min_not_zero(sdkp->max_ws_blocks,
> (u32)SD_MAX_WS10_BLOCKS);
> + q->limits.discard_zeroes_data = sdkp->lbprz;
> break;
>
> case SD_LBP_ZERO:
> @@ -2622,12 +2624,12 @@ static void sd_read_block_limits(struct scsi_disk *sdkp)
>
> } else { /* LBP VPD page tells us what to use */
>
> - if (sdkp->lbpu && sdkp->max_unmap_blocks)
> - sd_config_discard(sdkp, SD_LBP_UNMAP);
> - else if (sdkp->lbpws)
> + if (sdkp->lbpws)
> sd_config_discard(sdkp, SD_LBP_WS16);
> else if (sdkp->lbpws10)
> sd_config_discard(sdkp, SD_LBP_WS10);
> + else if (sdkp->lbpu && sdkp->max_unmap_blocks)
> + sd_config_discard(sdkp, SD_LBP_UNMAP);
> else
> sd_config_discard(sdkp, SD_LBP_DISABLE);
> }
>
Reviewed-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
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