Re: [Qemu-devel] [RFC PATCHv2] block: optimize zero writes with bdrv_write_zeroes

[Eric Blake <eblake@redhat.com>]

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On 03/21/2014 08:36 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 should significantly speed up file system initialization and
> should speed zero write test used to test backend storage performance.
> 
> the difference can simply be tested by e.g.
> 
> dd if=/dev/zero of=/dev/vdX bs=1M
> 
> Signed-off-by: Peter Lieven <pl@kamp.de>
> ---
> v1->v2: - add detect-zeroes=off|on|unmap knob to drive cmdline parameter
>         - call zero detection only for format (bs->file != NULL)
> 

> +static int bdrv_set_detect_zeroes(BlockDriverState *bs,
> +                                  const char *detect_zeroes,
> +                                  Error **errp)
> +{
> +    if (!detect_zeroes || !strncmp(detect_zeroes, "off", 3)) {
> +        bs->detect_zeroes = BDRV_DETECT_ZEROES_OFF;

This parses "offer"

> +    } else if (!strncmp(detect_zeroes, "on", 2)) {
> +        bs->detect_zeroes = BDRV_DETECT_ZEROES_ON;

and this parses "onto".

> +    } else if (!strncmp(detect_zeroes, "unmap", 5)) {
> +        bs->detect_zeroes = BDRV_DETECT_ZEROES_UNMAP;

In all three cases, shouldn't you be using strcmp() instead of strncmp()?

> +    } else {
> +        error_setg(errp, "invalid value for detect-zeroes: %s",
> +                   detect_zeroes);

Especially since you warn about other unknown spellings, it feels weird
to not warn about the spellings where the prefix matches but the overall
spelling is unknown.
>  file sectors into the image file.
> +@item detect-zeroes=@var{detect-zeroes}
> +@var{detect-zeroes} is "off", "on" or "unmap" and enables the automatic
> +conversion of plain zero writes by the OS to driver specific optimized
> +zero write commands. If "unmap" is choosen and @var{discard} is "on"

s/choosen/chosen/

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


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