Re: [Qemu-devel] [PATCH 1/6] libqblock APIs

[Wenchao Xia <xiawenc@linux.vnet.ibm.com>]

于 2012-9-3 22:05, Paolo Bonzini 写道:
> Il 03/09/2012 15:56, Eric Blake ha scritto:
>> Exactly how does the *pnum argument work?  This interface looks like it
>> isn't fully thought out yet.  Either I want to know if a chunk of
>> sectors is allocated (I supply start and length of sectors to check),
>> regardless of how many sectors beyond that point are also allocated
>> (pnum makes no sense);
>
> pnum makes sense if the [start, start+length) range includes both
> allocated and unallocated sectors.
>
>> or I want to know how many sectors are allocated
>> from a given point (I supply start, and the function returns length, so
>> nb_sectors makes no sense).
>
   About using byte offset instead of sector, I think sector is better,
because the allocation status is based on sector, all bytes data in a
sector would have the same status.

> This operation could be O(number of blocks in disk) worst case, so it
> makes sense to provide nb_sectors as an upper bound.  nb_sectors is
> typically dictated by the size of your buffer.
>
> That said, QEMU's internal bdrv_is_allocated function does have one not
> entirely appealing property: the block at start + *pnum might have the
> same state as the block at start + *pnum - 1, even if *pnum < length.
> We may want to work around this in libqblock, but we could also simply
> document it.
>
> Paolo
>
int bdrv_is_allocated(BlockDriverState *bs, int64_t sector_num,
                       int nb_sectors,int *pnum)
will the issue happen when nb_sectors > *pnum? if so it seems a bug,
because caller is asking a range of sectors's allocation status, and
*pnum did not reflect the real status.

>> Either way, I think you are supplying too
>> many parameters for how I envision checking for allocated sectors.
>
   yes, it is a bit confusing, how about:

int qb_check_allocate_status(struct QBroker *broker,
                              struct QBlockState *qbs,
                              offset sector_start,
                              size_t sector_number,
                              size_t *pnum,
                              int *status)
user input sector_start and sector_number to ask check it in this range,
following parameter receive the status, return indicate exception.

>


-- 
Best Regards

Wenchao Xia