[Qemu-devel] [RFC v2 1/8] block: add bdrv_measure() API

[Stefan Hajnoczi <stefanha@redhat.com>]

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>
---
 qapi/block-core.json      | 19 +++++++++++++++++++
 include/block/block.h     |  4 ++++
 include/block/block_int.h |  2 ++
 block.c                   | 33 +++++++++++++++++++++++++++++++++
 4 files changed, 58 insertions(+)

diff --git a/qapi/block-core.json b/qapi/block-core.json
index 786b39e..673569d 100644
--- a/qapi/block-core.json
+++ b/qapi/block-core.json
@@ -463,6 +463,25 @@
            '*dirty-bitmaps': ['BlockDirtyInfo'] } }
 
 ##
+# @BlockMeasureInfo:
+#
+# Image size calculation information.  This structure describes the size
+# requirements for creating a new image.
+#
+# @required-bytes: Amount of space required for image creation.  This value is
+#                  the host file size including sparse file regions.  A new 5
+#                  GB raw file therefore has a required size of 5 GB, not 0
+#                  bytes.
+#
+# @fully-allocated-bytes: Space required once data has been written to all
+#                         sectors
+#
+# Since: 2.10
+##
+{ 'struct': 'BlockMeasureInfo',
+  'data': {'required-bytes': 'int', 'fully-allocated-bytes': 'int'} }
+
+##
 # @query-block:
 #
 # Get a list of BlockInfo for all virtual block devices.
diff --git a/include/block/block.h b/include/block/block.h
index 5149260..43c789f 100644
--- a/include/block/block.h
+++ b/include/block/block.h
@@ -298,6 +298,10 @@ int bdrv_truncate(BdrvChild *child, int64_t offset);
 int64_t bdrv_nb_sectors(BlockDriverState *bs);
 int64_t bdrv_getlength(BlockDriverState *bs);
 int64_t bdrv_get_allocated_file_size(BlockDriverState *bs);
+void bdrv_measure(BlockDriver *drv, QemuOpts *opts,
+                  BlockDriverState *in_bs,
+                  BlockMeasureInfo *info,
+                  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);
diff --git a/include/block/block_int.h b/include/block/block_int.h
index 6c699ac..45a7fbe 100644
--- a/include/block/block_int.h
+++ b/include/block/block_int.h
@@ -201,6 +201,8 @@ struct BlockDriver {
     int64_t (*bdrv_getlength)(BlockDriverState *bs);
     bool has_variable_length;
     int64_t (*bdrv_get_allocated_file_size)(BlockDriverState *bs);
+    void (*bdrv_measure)(QemuOpts *opts, BlockDriverState *in_bs,
+                         BlockMeasureInfo *info, Error **errp);
 
     int coroutine_fn (*bdrv_co_pwritev_compressed)(BlockDriverState *bs,
         uint64_t offset, uint64_t bytes, QEMUIOVector *qiov);
diff --git a/block.c b/block.c
index cb57370..532a4d1 100644
--- a/block.c
+++ b/block.c
@@ -3260,6 +3260,39 @@ int64_t bdrv_get_allocated_file_size(BlockDriverState *bs)
     return -ENOTSUP;
 }
 
+/*
+ * bdrv_measure:
+ * @drv: Format driver
+ * @opts: Creation options
+ * @in_bs: Existing image containing data for new image (may be NULL)
+ * @info: Result object
+ * @errp: Error object
+ *
+ * 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 are 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.
+ */
+void bdrv_measure(BlockDriver *drv, QemuOpts *opts,
+                  BlockDriverState *in_bs, BlockMeasureInfo *info,
+                  Error **errp)
+{
+    if (!drv->bdrv_measure) {
+        error_setg(errp, "Block driver '%s' does not support size measurement",
+                   drv->format_name);
+        return;
+    }
+
+    drv->bdrv_measure(opts, in_bs, info, errp);
+}
+
 /**
  * Return number of sectors on success, -errno on error.
  */
-- 
2.9.3