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

From: Eric Blake
Subject: Re: [Qemu-devel] [PATCHv4] block: optimize zero writes with bdrv_write_zeroes
Date: Mon, 12 May 2014 14:28:44 -0600
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:24.0) Gecko/20100101 Thunderbird/24.5.0

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 <address@hidden>
> ---
> 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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