block: add bdrv_measure() API

bdrv_measure() provides a conservative maximum for the size of a new
image.  This information is handy if storage needs to be allocated (e.g.
a SAN or an LVM volume) ahead of time.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Alberto Garcia <berto@igalia.com>
Message-id: 20170705125738.8777-2-stefanha@redhat.com
Signed-off-by: Max Reitz <mreitz@redhat.com>

block.c

@@ -3463,6 +3463,41 @@ int64_t bdrv_get_allocated_file_size(BlockDriverState *bs)
     return -ENOTSUP;
 }
 
+/*
+ * bdrv_measure:
+ * @drv: Format driver
+ * @opts: Creation options for new image
+ * @in_bs: Existing image containing data for new image (may be NULL)
+ * @errp: Error object
+ * Returns: A #BlockMeasureInfo (free using qapi_free_BlockMeasureInfo())
+ *          or NULL on error
+ *
+ * Calculate file size required to create a new image.
+ *
+ * If @in_bs is given then space for allocated clusters and zero clusters
+ * from that image are included in the calculation.  If @opts contains a
+ * backing file that is shared by @in_bs then backing clusters may be omitted
+ * from the calculation.
+ *
+ * If @in_bs is NULL then the calculation includes no allocated clusters
+ * unless a preallocation option is given in @opts.
+ *
+ * Note that @in_bs may use a different BlockDriver from @drv.
+ *
+ * If an error occurs the @errp pointer is set.
+ */
+BlockMeasureInfo *bdrv_measure(BlockDriver *drv, QemuOpts *opts,
+                               BlockDriverState *in_bs, Error **errp)
+{
+    if (!drv->bdrv_measure) {
+        error_setg(errp, "Block driver '%s' does not support size measurement",
+                   drv->format_name);
+        return NULL;
+    }
+
+    return drv->bdrv_measure(opts, in_bs, errp);
+}
+
 /**
  * Return number of sectors on success, -errno on error.
  */

include/block/block.h

@@ -306,6 +306,8 @@ int bdrv_truncate(BdrvChild *child, int64_t offset, Error **errp);
 int64_t bdrv_nb_sectors(BlockDriverState *bs);
 int64_t bdrv_getlength(BlockDriverState *bs);
 int64_t bdrv_get_allocated_file_size(BlockDriverState *bs);
+BlockMeasureInfo *bdrv_measure(BlockDriver *drv, QemuOpts *opts,
+                               BlockDriverState *in_bs, Error **errp);
 void bdrv_get_geometry(BlockDriverState *bs, uint64_t *nb_sectors_ptr);
 void bdrv_refresh_limits(BlockDriverState *bs, Error **errp);
 int bdrv_commit(BlockDriverState *bs);

include/block/block_int.h

@@ -209,6 +209,8 @@ struct BlockDriver {
     int64_t (*bdrv_getlength)(BlockDriverState *bs);
     bool has_variable_length;
     int64_t (*bdrv_get_allocated_file_size)(BlockDriverState *bs);
+    BlockMeasureInfo *(*bdrv_measure)(QemuOpts *opts, BlockDriverState *in_bs,
+                                      Error **errp);
 
     int coroutine_fn (*bdrv_co_pwritev_compressed)(BlockDriverState *bs,
         uint64_t offset, uint64_t bytes, QEMUIOVector *qiov);

qapi/block-core.json

@@ -487,6 +487,31 @@
            '*tray_open': 'bool', '*io-status': 'BlockDeviceIoStatus',
            '*dirty-bitmaps': ['BlockDirtyInfo'] } }
 
+##
+# @BlockMeasureInfo:
+#
+# Image file size calculation information.  This structure describes the size
+# requirements for creating a new image file.
+#
+# The size requirements depend on the new image file format.  File size always
+# equals virtual disk size for the 'raw' format, even for sparse POSIX files.
+# Compact formats such as 'qcow2' represent unallocated and zero regions
+# efficiently so file size may be smaller than virtual disk size.
+#
+# The values are upper bounds that are guaranteed to fit the new image file.
+# Subsequent modification, such as internal snapshot or bitmap creation, may
+# require additional space and is not covered here.
+#
+# @required: Size required for a new image file, in bytes.
+#
+# @fully-allocated: Image file size, in bytes, once data has been written
+#                   to all sectors.
+#
+# Since: 2.10
+##
+{ 'struct': 'BlockMeasureInfo',
+  'data': {'required': 'int', 'fully-allocated': 'int'} }
+
 ##
 # @query-block:
 #