Re: [Qemu-devel] [PATCH v8 09/21] null: Switch to .bdrv_co_block_status()

[Kevin Wolf <kwolf@redhat.com>]

Am 01.03.2018 um 10:57 hat Vladimir Sementsov-Ogievskiy geschrieben:
> 01.03.2018 12:48, Kevin Wolf wrote:
> > Am 01.03.2018 um 08:25 hat Vladimir Sementsov-Ogievskiy geschrieben:
> > > 26.02.2018 17:05, Kevin Wolf wrote:
> > > > Essentially, assuming a simple backing chain 'base <- overlay', we got
> > > > these combinations to represent in NBD (with my suggestion of the flags
> > > > to use):
> > > > 
> > > > 1. Cluster allocated in overlay
> > > >      a. non-zero data                         0
> > > >      b. explicit zeroes                       0 or ZERO
> > > > 2. Cluster marked zero in overlay           HOLE | ZERO
> > > > 3. Cluster preallocated/zero in overlay     ZERO
> > > > 4. Cluster unallocated in overlay
> > > >      a. Cluster allocated in base (non-zero)  HOLE
> > > >      b. Cluster allocated in base (zero)      HOLE or HOLE | ZERO
> > > >      c. Cluster marked zero in base           HOLE | ZERO
> > > >      d. Cluster preallocated/zero in base     HOLE | ZERO
> > > >      e. Cluster unallocated in base           HOLE | ZERO
> > > > 
> > > > Instead of 'base' you can read 'anywhere in the backing chain' and the
> > > > flags should stay the same.
> > > I think only "anywhere in the backing chain" is valid here. Otherwise,
> > > semantics of bdrv_is_allocated would differ for NBD and for not-NBD.
> > This was meant as a mapping from cases to flags, not the other way
> > round, so really doesn't say anything about the cases where the block is
> > allocated further down the chain.
> > 
> > But yes, it shouldn't make a difference where in the backing chain a
> > block is allocated, so these cases are the same as 4.
> > 
> > > I think, if bdrv_is_allocated returns false, it means that we can skip
> > > this region in copying process, am I right?
> > -ENOCONTEXT? Which copying process?
> > 
> > There are cases where you want to copy such regions, and other cases
> > where you want to skip them. It depends on the use case. For example,
> > 'qemu-img convert' skips them with -B (because the backing file is
> > reused), but not without -B (which creates a full copy).
> > 
> > Kevin
> 
> Hm, I thought that bdrv_is_allocated loops through backings, but it doesn't,
> sorry.

That would be bdrv_is_allocated_above() with a NULL base.

Kevin