diff options
author | Alberto Garcia <berto@igalia.com> | 2020-07-10 18:12:47 +0200 |
---|---|---|
committer | Max Reitz <mreitz@redhat.com> | 2020-08-25 08:33:20 +0200 |
commit | 57538c864f3ad67d515892fd54b11ff59981b84d (patch) | |
tree | 07164004a537c774dc423a9b2fcead100a791948 /version.texi.in | |
parent | c1587d877e223c4c3a8db6fa620b71384550c492 (diff) | |
download | qemu-57538c864f3ad67d515892fd54b11ff59981b84d.zip qemu-57538c864f3ad67d515892fd54b11ff59981b84d.tar.gz qemu-57538c864f3ad67d515892fd54b11ff59981b84d.tar.bz2 |
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>
Diffstat (limited to 'version.texi.in')
0 files changed, 0 insertions, 0 deletions