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 <[email protected]>
Message-id: [email protected]
Reviewed-by: Eric Blake <[email protected]>
Reviewed-by: Daniel P. Berrangé <[email protected]>
Reviewed-by: Maxim Levitsky <[email protected]>
Signed-off-by: Max Reitz <[email protected]>

Author: XanClic

docs/qemu-block-drivers.texi

@@ -31,8 +31,8 @@ Supported options:
 @item preallocation
 Preallocation mode (allowed values: @code{off}, @code{falloc}, @code{full}).
 @code{falloc} mode preallocates space for image by calling posix_fallocate().
-@code{full} mode preallocates space for image by writing zeros to underlying
-storage.
+@code{full} mode preallocates space for image by writing data to underlying
+storage.  This data may or may not be zero, depending on the storage location.
 @end table
 
 @item qcow2

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
 ##

qemu-img.texi

@@ -666,8 +666,8 @@ Supported options:
 @item preallocation
 Preallocation mode (allowed values: @code{off}, @code{falloc}, @code{full}).
 @code{falloc} mode preallocates space for image by calling posix_fallocate().
-@code{full} mode preallocates space for image by writing zeros to underlying
-storage.
+@code{full} mode preallocates space for image by writing data to underlying
+storage.  This data may or may not be zero, depending on the storage location.
 @end table
 
 @item qcow2