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H A D | qcow2-cluster.c | 57538c86 Fri Jul 10 16:12:47 GMT 2020 Alberto Garcia <berto@igalia.com> qcow2: Process QCOW2_CLUSTER_ZERO_ALLOC clusters in handle_copied()
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 <berto@igalia.com> Reviewed-by: Eric Blake <eblake@redhat.com> Reviewed-by: Max Reitz <mreitz@redhat.com> Message-Id: <eb17fc938f6be7be2e8d8ff42763d2c19241f866.1594396418.git.berto@igalia.com> Signed-off-by: Max Reitz <mreitz@redhat.com>
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