[Qemu-devel] [RFC 1/4] block: add bdrv_max_size() API

[Stefan Hajnoczi <stefanha@redhat.com>]

bdrv_max_size() 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>
---
 include/block/block.h     |  2 ++
 include/block/block_int.h |  2 ++
 block.c                   | 37 +++++++++++++++++++++++++++++++++++++
 3 files changed, 41 insertions(+)

diff --git a/include/block/block.h b/include/block/block.h
index c7c4a3a..12e1532 100644
--- a/include/block/block.h
+++ b/include/block/block.h
@@ -298,6 +298,8 @@ 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);
+uint64_t bdrv_max_size(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);
diff --git a/include/block/block_int.h b/include/block/block_int.h
index a57c0bf..0354e22 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);
+    uint64_t (*bdrv_max_size)(QemuOpts *opts, BlockDriverState *in_bs,
+                              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 f293ccb..4a228f0 100644
--- a/block.c
+++ b/block.c
@@ -3188,6 +3188,43 @@ int64_t bdrv_get_allocated_file_size(BlockDriverState *bs)
     return -ENOTSUP;
 }
 
+/*
+ * bdrv_max_size:
+ * @drv: Format driver
+ * @opts: Creation options
+ * @in_bs: Existing image containing data for new image (may be NULL)
+ * @errp: Error object
+ *
+ * Calculate maximum file size required by a new image.  The value is
+ * conservative so actual creation of the image never requires more space than
+ * what is returned by this function.
+ *
+ * Note that this value is the file size including sparse regions, not the
+ * allocated file size.  A new 5 GB raw file therefore has a maximum size of 5
+ * GB, not 0!
+ *
+ * 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.
+ *
+ * Returns the maximum size for the new image or 0 on error.
+ */
+uint64_t bdrv_max_size(BlockDriver *drv, QemuOpts *opts,
+                       BlockDriverState *in_bs, Error **errp)
+{
+    if (!drv->bdrv_max_size) {
+        error_setg(errp, "Block driver '%s' does not support max size "
+                   "calculation", drv->format_name);
+        return 0;
+    }
+
+    return drv->bdrv_max_size(opts, in_bs, errp);
+}
+
 /**
  * Return number of sectors on success, -errno on error.
  */
-- 
2.9.3