[Qemu-devel] [PATCH v2 1/2] qapi: Introduce blockdev-group-snapshot-sync command

[Jeff Cody]

This is a QAPI/QMP only command to take a snapshot of a group of
devices. This is similar to the blockdev-snapshot-sync command, except
blockdev-group-snapshot-sync accepts a list devices, filenames, and
formats.

It is attempted to keep the snapshot of the group atomic; if the
creation or open of any of the new snapshots fails, then all of
the new snapshots are abandoned, and the name of the snapshot image
that failed is returned.  The failure case should not interrupt
any operations.

Rather than use bdrv_close() along with a subsequent bdrv_open() to
perform the pivot, the original image is never closed and the new
image is placed 'in front' of the original image via manipulation
of the BlockDriverState fields.  Thus, once the new snapshot image
has been successfully created, there are no more failure points
before pivoting to the new snapshot.

This allows the group of disks to remain consistent with each other,
even across snapshot failures.

Signed-off-by: Jeff Cody <address@hidden>
---
 block.c          |   47 ++++++++++++++++++++
 block.h          |    1 +
 blockdev.c       |  128 ++++++++++++++++++++++++++++++++++++++++++++++++++++++
 qapi-schema.json |   38 ++++++++++++++++
 4 files changed, 214 insertions(+), 0 deletions(-)

diff --git a/block.c b/block.c
index 3621d11..0045ab1 100644
--- a/block.c
+++ b/block.c
@@ -880,6 +880,53 @@ void bdrv_make_anon(BlockDriverState *bs)
     bs->device_name[0] = '\0';
 }
 
+/*
+ * Add new bs contents at the top of an image chain while the chain is live,
+ * while keeping required fields on the top layer.
+ *
+ * It is assumed that bs_new already points to an existing image,
+ * with the correct backing filename of top->backing_file
+ */
+void bdrv_append(BlockDriverState *bs_new, BlockDriverState *bs_top)
+{
+    BlockDriverState tmp;
+
+    /* the new bs must not be in bdrv_states */
+    bdrv_make_anon(bs_new);
+
+    tmp = *bs_new;
+    tmp.backing_hd = bs_new;
+
+    /* there are some fields that need to stay on the top layer: */
+
+    /* dev info */
+    tmp.dev_ops          = bs_top->dev_ops;
+    tmp.dev_opaque       = bs_top->dev_opaque;
+    tmp.dev              = bs_top->dev;
+    tmp.buffer_alignment = bs_top->buffer_alignment;
+    tmp.copy_on_read     = bs_top->copy_on_read;
+
+    /* i/o timing parameters */
+    tmp.slice_time        = bs_top->slice_time;
+    tmp.slice_start       = bs_top->slice_start;
+    tmp.slice_end         = bs_top->slice_end;
+    tmp.io_limits         = bs_top->io_limits;
+    tmp.io_base           = bs_top->io_base;
+    tmp.throttled_reqs    = bs_top->throttled_reqs;
+    tmp.block_timer       = bs_top->block_timer;
+    tmp.io_limits_enabled = bs_top->io_limits_enabled;
+
+    /* keep the same entry in bdrv_states */
+    pstrcpy(tmp.device_name, sizeof(tmp.device_name), bs_top->device_name);
+    tmp.list = bs_top->list;
+
+    /* swap contents of the fixed new bs and the current top */
+    *bs_new = *bs_top;
+    *bs_top = tmp;
+
+    bdrv_detach_dev(bs_new, bs_new->dev);
+}
+
 void bdrv_delete(BlockDriverState *bs)
 {
     assert(!bs->dev);
diff --git a/block.h b/block.h
index cae289b..190a780 100644
--- a/block.h
+++ b/block.h
@@ -114,6 +114,7 @@ int bdrv_create(BlockDriver *drv, const char* filename,
 int bdrv_create_file(const char* filename, QEMUOptionParameter *options);
 BlockDriverState *bdrv_new(const char *device_name);
 void bdrv_make_anon(BlockDriverState *bs);
+void bdrv_append(BlockDriverState *bs_new, BlockDriverState *bs_top);
 void bdrv_delete(BlockDriverState *bs);
 int bdrv_parse_cache_flags(const char *mode, int *flags);
 int bdrv_file_open(BlockDriverState **pbs, const char *filename, int flags);
diff --git a/blockdev.c b/blockdev.c
index 05e7c5e..560f7e8 100644
--- a/blockdev.c
+++ b/blockdev.c
@@ -714,6 +714,134 @@ void qmp_blockdev_snapshot_sync(const char *device, const 
char *snapshot_file,
     }
 }
 
+
+/* New and old BlockDriverState structs for group snapshots */
+typedef struct BlkGroupSnapshotStates {
+    BlockDriverState *old_bs;
+    BlockDriverState *new_bs;
+    bool is_open;
+    QSIMPLEQ_ENTRY(BlkGroupSnapshotStates) entry;
+} BlkGroupSnapshotStates;
+
+/*
+ * 'Atomic' group snapshots.  The snapshots are taken as a set, and if any fail
+ *  then we do not pivot any of the devices in the group, and abandon the
+ *  snapshots
+ */
+void qmp_blockdev_group_snapshot_sync(SnapshotDevList *dev_list,
+                                                   Error **errp)
+{
+    int ret = 0;
+    SnapshotDevList *dev_entry = dev_list;
+    SnapshotDev *dev_info = NULL;
+    BlkGroupSnapshotStates *states;
+    BlockDriver *proto_drv;
+    BlockDriver *drv;
+    int flags;
+    const char *format;
+    const char *snapshot_file;
+
+    QSIMPLEQ_HEAD(snap_bdrv_states, BlkGroupSnapshotStates) snap_bdrv_states;
+    QSIMPLEQ_INIT(&snap_bdrv_states);
+
+    /* We don't do anything in this loop that commits us to the snapshot */
+    while (NULL != dev_entry) {
+        dev_info = dev_entry->value;
+        dev_entry = dev_entry->next;
+
+        states = g_malloc0(sizeof(BlkGroupSnapshotStates));
+        states->is_open = false;
+
+        states->old_bs = bdrv_find(dev_info->device);
+
+        if (!states->old_bs) {
+            error_set(errp, QERR_DEVICE_NOT_FOUND, dev_info->device);
+            goto fail;
+        }
+
+        if (bdrv_in_use(states->old_bs)) {
+            error_set(errp, QERR_DEVICE_IN_USE, dev_info->device);
+            goto fail;
+        }
+
+        snapshot_file = dev_info->snapshot_file;
+
+        flags = states->old_bs->open_flags;
+
+        if (!dev_info->has_format) {
+            format = "qcow2";
+        } else {
+            format = dev_info->format;
+        }
+
+        drv = bdrv_find_format(format);
+        if (!drv) {
+            error_set(errp, QERR_INVALID_BLOCK_FORMAT, format);
+            goto fail;
+        }
+
+        proto_drv = bdrv_find_protocol(snapshot_file);
+        if (!proto_drv) {
+            error_set(errp, QERR_INVALID_BLOCK_FORMAT, format);
+            goto fail;
+        }
+
+        /* create new image w/backing file */
+        ret = bdrv_img_create(snapshot_file, format,
+                              states->old_bs->filename,
+                              drv->format_name, NULL, -1, flags);
+        if (ret) {
+            error_set(errp, QERR_OPEN_FILE_FAILED, snapshot_file);
+            goto fail;
+        }
+
+        /* We will manually add the backing_hd field to the bs later */
+        states->new_bs = bdrv_new("");
+        ret = bdrv_open(states->new_bs, snapshot_file,
+                        flags | BDRV_O_NO_BACKING, drv);
+        states->is_open = true;
+
+        if (ret != 0) {
+            error_set(errp, QERR_OPEN_FILE_FAILED, snapshot_file);
+            goto close_and_fail;
+        }
+        QSIMPLEQ_INSERT_TAIL(&snap_bdrv_states, states, entry);
+    }
+
+    /*
+     * Now we will drain, flush, & pivot everything - we are committed at this
+     * point.
+     */
+    bdrv_drain_all();
+    QSIMPLEQ_FOREACH(states, &snap_bdrv_states, entry) {
+        bdrv_flush(states->old_bs);
+
+        /* This removes our old bs from the bdrv_states, and adds the new bs */
+        bdrv_append(states->new_bs, states->old_bs);
+    }
+
+    /* success */
+    goto exit;
+
+close_and_fail:
+    /*
+    * failure, and it is all-or-none; abandon each new bs, and keep using
+    * the original bs for all images
+    */
+    QSIMPLEQ_FOREACH(states, &snap_bdrv_states, entry) {
+        if (states->is_open) {
+             bdrv_close(states->new_bs);
+        }
+    }
+fail:
+exit:
+    QSIMPLEQ_FOREACH(states, &snap_bdrv_states, entry) {
+        g_free(states);
+    }
+    return;
+}
+
+
 static void eject_device(BlockDriverState *bs, int force, Error **errp)
 {
     if (bdrv_in_use(bs)) {
diff --git a/qapi-schema.json b/qapi-schema.json
index 80debe6..a0cab8f 100644
--- a/qapi-schema.json
+++ b/qapi-schema.json
@@ -1107,6 +1107,44 @@
 { 'command': 'block_resize', 'data': { 'device': 'str', 'size': 'int' }}
 
 ##
+# @SnapshotDev
+#
+# @device:  the name of the device to generate the snapshot from.
+#
+# @snapshot-file: the target of the new image. A new file will be created.
+#
+# @format: #optional the format of the snapshot image, default is 'qcow2'.
+##
+{ 'type': 'SnapshotDev',
+  'data': {'device': 'str', 'snapshot-file': 'str', '*format': 'str' } }
+
+##
+# @blockdev-group-snapshot-sync
+#
+# Generates a synchronous snapshot of a group of one or more block devices,
+# as atomically as possible.  If the snapshot of any device in the group
+# fails, then the entire group snapshot will be abandoned and the
+# appropriate error returned.
+#
+#  List of:
+#  @SnapshotDev: information needed for the device snapshot
+#
+# Returns: nothing on success
+#          If @device is not a valid block device, DeviceNotFound
+#          If @device is busy, DeviceInUse will be returned
+#          If @snapshot-file can't be created, OpenFileFailed
+#          If @snapshot-file can't be opened, OpenFileFailed
+#          If @format is invalid, InvalidBlockFormat
+#
+# Note: The group snapshot attempt returns failure on the first snapshot
+# device failure.  Therefore, there will be only one device or snapshot file
+# returned in an error condition, and subsequent devices will not have been
+# attempted.
+##
+{ 'command': 'blockdev-group-snapshot-sync',
+  'data': { 'devlist': [ 'SnapshotDev' ] } }
+
+##
 # @blockdev-snapshot-sync
 #
 # Generates a synchronous snapshot of a block device.
-- 
1.7.9.rc2.1.g69204