Re: [Qemu-devel] [PULL 18/43] block: Switch discard length bounds to byte-based

[Eric Blake]

On 07/05/2016 09:50 AM, Kevin Wolf wrote:
> From: Eric Blake <address@hidden>
> 
> Sector-based limits are awkward to think about; in our on-going
> quest to move to byte-based interfaces, convert max_discard and
> discard_alignment.  Rename them, using 'pdiscard' as an aid to
> track which remaining discard interfaces need conversion, and so
> that the compiler will help us catch the change in semantics
> across any rebased code.  The BlockLimits type is now completely
> byte-based; and in iscsi.c, sector_limits_lun2qemu() is no
> longer needed.
> 

> +++ b/include/block/block_int.h
> @@ -324,11 +324,17 @@ struct BlockDriver {
>  };
>  
>  typedef struct BlockLimits {
> -    /* maximum number of sectors that can be discarded at once */
> -    int max_discard;
> -
> -    /* optimal alignment for discard requests in sectors */
> -    int64_t discard_alignment;
> +    /* maximum number of bytes that can be discarded at once (since it
> +     * is signed, it must be < 2G, if set), should be multiple of
> +     * pdiscard_alignment, but need not be power of 2. May be 0 if no
> +     * inherent 32-bit limit */
> +    int32_t max_pdiscard;
> +
> +    /* optimal alignment for discard requests in bytes, must be power
> +     * of 2, less than max_pdiscard if that is set, and multiple of
> +     * bs->request_alignment. May be 0 if bs->request_alignment is
> +     * good enough */
> +    uint32_t pdiscard_alignment;

Given the recent thread on an iscsi device with 15M optimum alignment
for zero and discards, I guess I have some followup patches to write if
we don't want to stall this pull request.

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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