diff --git a/docs/qemu-block-drivers.texi b/docs/qemu-block-drivers.texi
index 91ab0eceae..c02547e28c 100644
--- 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 a5ab38db99..e6edd641f1 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
 ##
diff --git a/qemu-img.texi b/qemu-img.texi
index c8e9bba515..b5156d6316 100644
--- 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