Re: [Qemu-devel] [PATCHv3] block: optimize zero writes with bdrv_write_zeroes

[Kevin Wolf <kwolf@redhat.com>]

Am 07.05.2014 um 17:26 hat Peter Lieven geschrieben:
> On 07.05.2014 17:19, Kevin Wolf wrote:
> >Am 07.05.2014 um 16:26 hat Peter Lieven geschrieben:
> >>On 07.05.2014 04:52, Eric Blake wrote:
> >>>On 05/06/2014 06:23 PM, 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>
> >>>>---
> >>>>v2->v3: - moved parameter parsing to blockdev_init
> >>>>         - added per device detect_zeroes status to
> >>>>           hmp (info block -v) and qmp (query-block) [Eric]
> >>>>         - added support to enable detect-zeroes also
> >>>>           for hot added devices [Eric]
> >>>>         - added missing entry to qemu_common_drive_opts
> >>>>         - fixed description of qemu_iovec_is_zero [Fam]
> >>>>
> >>>>+static BdrvDetectZeroes parse_detect_zeroes(const char *buf, Error **errp)
> >>>>+{
> >>>>+    if (!buf || !strcmp(buf, "off")) {
> >>>>+        return BDRV_DETECT_ZEROES_OFF;
> >>>>+    } else if (!strcmp(buf, "on")) {
> >>>>+        return BDRV_DETECT_ZEROES_ON;
> >>>>+    } else if (!strcmp(buf, "unmap")) {
> >>>>+        return BDRV_DETECT_ZEROES_UNMAP;
> >>>>+    } else {
> >>>>+        error_setg(errp, "invalid value for detect-zeroes: %s",
> >>>>+                   buf);
> >>>>+    }
> >>>>+    return BDRV_DETECT_ZEROES_OFF;
> >>>>+}
> >>>Isn't there QAPI generated code that you can use instead of open-coding
> >>>this conversion between string and enum values?
> >>Actually I have no idea. As you pointed out in the qapi patch I sent
> >>it was quite hard for me to crawl through the whole stuff as one who is not
> >>familiar with it. Can somebody advise here? Anyhow, I wonder
> >>how this would work since qapi doesn't know the C Macros.
> >QAPI does generate C enums, so you should take whatever identifier it
> >uses instead of defining your own macros. You may need to include
> >qapi-types.h for this. It also creates a *_lookup array that maps enum
> >IDs to strings.
> 
> Ah, cool stuff, thank you. I found the enum and the lookup array,
> but is there also a function that maps a string to an enum ID?

I don't think so, but if you need it, you're probably doing something
wrong because QAPI already calls you with an enum parameter and not a
char* one.

Kevin