block: vhdx - change .vhdx_create default block state to ZERO

The VHDX spec specifies that the default new block state is
PAYLOAD_BLOCK_NOT_PRESENT for a dynamic VHDX image, and
PAYLOAD_BLOCK_FULLY_PRESENT for a fixed VHDX image.

However, in order to create space-efficient VHDX images with qemu-img
convert, it is desirable to be able to set has_zero_init to true for
VHDX.

There is currently an option when creating VHDX images, to use block
state ZERO for new blocks.  However, this currently defaults to 'off'.
In order to be able to eventually set has_zero_init to true for VHDX,
this needs to default to 'on'.

This patch changes the default to 'on', and provides some help
information to warn against setting it to 'off' when using qemu-img
convert.

[Max Reitz pointed out that a full stop was missing at the end of the
VHDX_BLOCK_OPT_ZERO option help text.  I have added it.
--Stefan]
Signed-off-by: NJeff Cody <jcody@redhat.com>
Reviewed-by: NMax Reitz <mreitz@redhat.com>
Message-id: 85164899eacc86e150c3ceba793cf93b398dedd7.1418018421.git.jcody@redhat.com
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>

block/vhdx.c

@@ -1774,7 +1774,7 @@ static int vhdx_create(const char *filename, QemuOpts *opts, Error **errp)
     log_size = qemu_opt_get_size_del(opts, VHDX_BLOCK_OPT_LOG_SIZE, 0);
     block_size = qemu_opt_get_size_del(opts, VHDX_BLOCK_OPT_BLOCK_SIZE, 0);
     type = qemu_opt_get_del(opts, BLOCK_OPT_SUBFMT);
-    use_zero_blocks = qemu_opt_get_bool_del(opts, VHDX_BLOCK_OPT_ZERO, false);
+    use_zero_blocks = qemu_opt_get_bool_del(opts, VHDX_BLOCK_OPT_ZERO, true);
 
     if (image_size > VHDX_MAX_IMAGE_SIZE) {
         error_setg_errno(errp, EINVAL, "Image size too large; max of 64TB");
@@ -1936,7 +1936,9 @@ static QemuOptsList vhdx_create_opts = {
        {
            .name = VHDX_BLOCK_OPT_ZERO,
            .type = QEMU_OPT_BOOL,
-           .help = "Force use of payload blocks of type 'ZERO'.  Non-standard."
+           .help = "Force use of payload blocks of type 'ZERO'. "\
+                   "Non-standard, but default.  Do not set to 'off' when "\
+                   "using 'qemu-img convert' with subformat=dynamic."
        },
        { NULL }
     }

qemu-doc.texi

@@ -698,7 +698,11 @@ Supported options:
 Specifies which VHDX subformat to use. Valid options are
 @code{dynamic} (default) and @code{fixed}.
 @item block_state_zero
-Force use of payload blocks of type 'ZERO'.
+Force use of payload blocks of type 'ZERO'.  Can be set to @code{on} (default)
+or @code{off}.  When set to @code{off}, new blocks will be created as
+@code{PAYLOAD_BLOCK_NOT_PRESENT}, which means parsers are free to return
+arbitrary data for those blocks.  Do not set to @code{off} when using
+@code{qemu-img convert} with @code{subformat=dynamic}.
 @item block_size
 Block size; min 1 MB, max 256 MB.  0 means auto-calculate based on image size.
 @item log_size