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Re: [PATCH v3 1/2] hw/block/nvme: add dulbe support

From: Keith Busch
Subject: Re: [PATCH v3 1/2] hw/block/nvme: add dulbe support
Date: Wed, 21 Oct 2020 17:55:09 -0700

On Thu, Oct 22, 2020 at 12:17:35AM +0200, 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 (4kb)  wz (64kb)
>     ---------------------------------------------------
>       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 4kb 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 64kb 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).

This looks fine. The constraints sound reasonable to me.

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

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