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/docs/qemu-block-drivers.texi b/docs/qemu-block-drivers.texi
index 91ab0eceae2e076aad82b683992fef30c065c3df..c02547e28c62d91f8b082ef831a590b890bc6dba 100644 (file)
--- a/docs/qemu-block-drivers.texi
+++ b/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

diff --git a/qapi/block-core.json b/qapi/block-core.json
index a5ab38db99bc9dc701378506d8587ea1eae48f05..e6edd641f18ad0a39a3ba56cd2d1da7997a5e340 100644 (file)
--- 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
 ##

diff --git a/qemu-img.texi b/qemu-img.texi
index c8e9bba5157896dc7948837c66747129c0e51e16..b5156d631681bbae75175eac8f137b253e891507 100644 (file)
--- a/qemu-img.texi
+++ b/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