doc: Preallocation does not require writing zeroes
When preallocating an encrypted qcow2 image, it just lets the protocol
driver write data and then does not mark the clusters as zero.
Therefore, reading this image will yield effectively random data.
As such, we have not fulfilled the promise of always writing zeroes when
preallocating an image in a while. It seems that nobody has really
cared, so change the documentation to conform to qemu's actual behavior.
Signed-off-by: Max Reitz <mreitz@redhat.com>
Message-id: 20190711132935.13070-1-mreitz@redhat.com
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>
diff --git a/qapi/block-core.json b/qapi/block-core.json
index a5ab38d..e6edd64 100644
--- a/qapi/block-core.json
+++ b/qapi/block-core.json
@@ -5178,10 +5178,11 @@
# @off: no preallocation
# @metadata: preallocate only for metadata
# @falloc: like @full preallocation but allocate disk space by
-# posix_fallocate() rather than writing zeros.
-# @full: preallocate all data by writing zeros to device to ensure disk
-# space is really available. @full preallocation also sets up
-# metadata correctly.
+# posix_fallocate() rather than writing data.
+# @full: preallocate all data by writing it to the device to ensure
+# disk space is really available. This data may or may not be
+# zero, depending on the image format and storage.
+# @full preallocation also sets up metadata correctly.
#
# Since: 2.2
##
