Re: [Qemu-devel] [PATCHv4] block: optimize zero writes with bdrv_write_zeroes

[Eric Blake <eblake@redhat.com>]

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On 05/08/2014 10:22 AM, Peter Lieven wrote:
> this patch tries to optimize zero write requests
> by automatically using bdrv_write_zeroes if it is
> supported by the format.
> 
> This significantly speeds up file system initialization and
> should speed zero write test used to test backend storage
> performance.
> 

> Signed-off-by: Peter Lieven <pl@kamp.de>
> ---
> v3->v4: - use QAPI generated enum and lookup table [Kevin]
>         - added more details about the options in the comments
>           of the qapi-schema [Eric]
>         - changed the type of detect_zeroes from str to
>           BlockdevDetectZeroesOptions. I left the name
>           as is because it is consistent with e.g.
>           BlockdevDiscardOptions or BlockdevAioOptions [Eric]
>         - changed the parse function in blockdev_init to
>           be generic usable for other enum parameters

If you wouldn't mind, I think the generic function is useful enough that
people might want to backport it independently from this optimization.
It would be better to split this into a two-patch series, one for the
new parse_enum_option, the other for bdrv_write_zeroes utilizing it.


> +        },{
> +            .name = "detect-zeroes",
> +            .type = QEMU_OPT_STRING,
> +            .help = "try to optimize zero writes",

Might be worth listing (off, on, unmap) in the text.

Everything else looked okay, but I'll wait for R-b until I see a
response about the idea of splitting the patch (even if that response is
justification for keeping it as one)

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


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