On 28.06.20 13:02, Alberto Garcia wrote: > When writing to a qcow2 file there are two functions that take a > virtual offset and return a host offset, possibly allocating new > clusters if necessary: > > - handle_copied() looks for normal data clusters that are already > allocated and have a reference count of 1. In those clusters we > can simply write the data and there is no need to perform any > copy-on-write. > > - handle_alloc() looks for clusters that do need copy-on-write, > either because they haven't been allocated yet, because their > reference count is != 1 or because they are ZERO_ALLOC clusters. > > The ZERO_ALLOC case is a bit special because those are clusters that > are already allocated and they could perfectly be dealt with in > handle_copied() (as long as copy-on-write is performed when required). > > In fact, there is extra code specifically for them in handle_alloc() > that tries to reuse the existing allocation if possible and frees them > otherwise. > > This patch changes the handling of ZERO_ALLOC clusters so the > semantics of these two functions are now like this: > > - handle_copied() looks for clusters that are already allocated and > which we can overwrite (NORMAL and ZERO_ALLOC clusters with a > reference count of 1). > > - handle_alloc() looks for clusters for which we need a new > allocation (all other cases). > > One important difference after this change is that clusters found > in handle_copied() may now require copy-on-write, but this will be > necessary anyway once we add support for subclusters. > > Signed-off-by: Alberto Garcia <be...@igalia.com> > Reviewed-by: Eric Blake <ebl...@redhat.com> > --- > block/qcow2-cluster.c | 256 +++++++++++++++++++++++------------------- > 1 file changed, 141 insertions(+), 115 deletions(-)
Reviewed-by: Max Reitz <mre...@redhat.com>
signature.asc
Description: OpenPGP digital signature
