Re: [PATCH v8 3/5] hw/block/nvme: add dulbe support

[Keith Busch]

On Thu, Nov 12, 2020 at 08:59:43PM +0100, Klaus Jensen wrote:
> From: Klaus Jensen <k.jensen@samsung.com>
> 
> Add support for reporting the Deallocated or Unwritten Logical Block
> Error (DULBE).
> 
> Rely on the block status flags reported by the block layer and consider
> any block with the BDRV_BLOCK_ZERO flag to be deallocated.
> 
> Multiple factors affect when a Write Zeroes command result in
> deallocation of blocks.
> 
>   * the underlying file system block size
>   * the blockdev format
>   * the 'discard' and 'logical_block_size' parameters
> 
>      format | discard | wz (512B)  wz (4KiB)  wz (64KiB)
>     -----------------------------------------------------
>       qcow2    ignore   n          n          y
>       qcow2    unmap    n          n          y
>       raw      ignore   n          y          y
>       raw      unmap    n          y          y
> 
> So, this works best with an image in raw format and 4KiB LBAs, since
> holes can then be punched on a per-block basis (this assumes a file
> system with a 4kb block size, YMMV). A qcow2 image, uses a cluster size
> of 64KiB by default and blocks will only be marked deallocated if a full
> cluster is zeroed or discarded. However, this *is* consistent with the
> spec since Write Zeroes "should" deallocate the block if the Deallocate
> attribute is set and "may" deallocate if the Deallocate attribute is not
> set. Thus, we always try to deallocate (the BDRV_REQ_MAY_UNMAP flag is
> always set).

These all sound like reasonable constraints and consistent with the
spec.

Reviewed-by: Keith Busch <kbusch@kernel.org>