Re: [Qemu-block] [PATCH v8 8/9] qcow2: skip writing zero buffers to empty COW areas

[Anton Nefedov]

On 16/3/2018 4:58 PM, Alberto Garcia wrote:
> On Mon 12 Mar 2018 11:16:57 AM CET, Anton Nefedov wrote:
> > --- a/qapi/block-core.json
> > +++ b/qapi/block-core.json
> > @@ -2584,6 +2584,8 @@
> >   #
> >   # @cor_write: a write due to copy-on-read (since 2.11)
> >   #
> > +# @cluster_alloc_space: an allocation of a cluster file space (since 2.12)

now it's 2.13 I believe

> That doesn't sound like correct English to me.

how about
  "an allocation of file space for a cluster"

> > +static bool is_unallocated(BlockDriverState *bs, int64_t offset, int64_t bytes)
> > +{
> > +    int64_t nr;
> > +    return !bytes ||
> > +        (!bdrv_is_allocated_above(bs, NULL, offset, bytes, &nr) && nr == 
> > bytes);
> > +}
> > +
>
> Is this really more efficient than the previous is_zero() call ?
>
> It seems that in both cases the code ends up calling
> bdrv_common_block_status_above().

the difference is that is_allocated passes 'want_zero = 0'.
It hints drivers to skip thorough seeks for zeroes (e.g. file-posix
returns (BDRV_BLOCK_DATA | BDRV_BLOCK_OFFSET_VALID) right away).
Moreover, bdrv_co_block_status() even skips the check in local_file
unless the protocol driver returned BDRV_BLOCK_RAW.

> > +static bool is_zero_cow(BlockDriverState *bs, QCowL2Meta *m)
> > +{
> > +    /* content with false negatives, giving is_allocated() is faster than
> > +     * a proper zero detection with possible actual image seeks, which is
> > +     * performed by is_zero() */
>
> It took me a bit to understand this sentence, maybe some native English
> speaker can suggest an alternate wording?
>
>    "is_allocated() is not as accurate as is_zero() and can give us some
>     false negatives, but it is much more efficient so let's use it here"

wording is the hardest part of it all :)
maybe:

  /* This check is designed for optimization shortcut so it must be
   * efficient.
   * Instead of is_zero(), use is_unallocated() as it is faster (but not
   * as accurate and can result in false negatives). */

> > +    return is_unallocated(bs, m->offset + m->cow_start.offset,
> > +                          m->cow_start.nb_bytes) &&
> > +           is_unallocated(bs, m->offset + m->cow_end.offset,
> > +                          m->cow_end.nb_bytes);
> > +}
>
> Berto