Add 'blockdev-snapshot' command

One of the limitations of the 'blockdev-snapshot-sync' command is that it does not allow passing BlockdevOptions to the newly created snapshots, so they are always opened using the default values.

Extending the command to allow passing options is not a practical solution because there is overlap between those options and some of the existing parameters of the command.

This patch introduces a new 'blockdev-snapshot' command with a simpler
interface: it just takes two references to existing block devices that will be used as the source and target for the snapshot.

Since the main difference between the two commands is that one of them creates and opens the target image, while the other uses an already opened one, the bulk of the implementation is shared.

Signed-off-by Alberto Garcia <[email protected]>

Cc Eric Blake <[email protected]>

Cc Max Reitz <[email protected]>

---
 blockdev.c           | 163 ++++++++++++++++++++++++++++++++-------------------
 qapi-schema.json     |   2 +
 qapi/block-core.json |  28 +++++++++
 qmp-commands.hx      |  38 ++++++++++++
 4 files changed, 171 insertions(+), 60 deletions(-)

qmp-commands.hx +28 -0

+    },
+
+SQMP
+blockdev-snapshot
+-----------------
+Since 2.5
+
+Create a snapshot, by installing 'node' as the backing image of
+'overlay'. Additionally, if 'node' is associated with a block
+device, the block device changes to using 'overlay' as its new active
+image.
+
+Arguments:
+
+- "node": device that will have a snapshot created (json-string)
+- "overlay": device that will have 'node' as its backing image (json-string)
+
+Example:
+
+-> { "execute": "blockdev-add",
+                "arguments": { "options": { "driver": "qcow2",
+                                            "node-name": "node1534",
+                                            "file": { "driver": "file",
+                                                      "filename": "hd1.qcow2" },
+                                            "backing": "" } } }
+
+<- { "return": {} }
+

Signed-off-by Alberto Garcia <[email protected]>

Cc Max Reitz <[email protected]>

---
 tests/qemu-iotests/085     | 102 ++++++++++++++++++++++++++++++++++++++++++---
 tests/qemu-iotests/085.out |  34 ++++++++++++++-
 2 files changed, 128 insertions(+), 8 deletions(-)

qmp-commands.hx Example: · line 1459

Quoted 3 lines

qmp-commands.hx +3 -0

+        .name       = "blockdev-snapshot",
+        .args_type  = "node:s,overlay:s",
+        .mhandler.cmd_new = qmp_marshal_input_blockdev_snapshot,

As of 7fad30f06eb6aa57aaa8f3d264288f24ae7646f0, this needs to be qmp_marshal_blockdev_snapshot.

One of the limitations of the 'blockdev-snapshot-sync' command is that it does not allow passing BlockdevOptions to the newly created snapshots, so they are always opened using the default values.

Extending the command to allow passing options is not a practical solution because there is overlap between those options and some of the existing parameters of the command.

This patch introduces a new 'blockdev-snapshot' command with a simpler
interface: it just takes two references to existing block devices that will be used as the source and target for the snapshot.

Since the main difference between the two commands is that one of them creates and opens the target image, while the other uses an already opened one, the bulk of the implementation is shared.

Signed-off-by Alberto Garcia <[email protected]>

Cc Eric Blake <[email protected]>

Cc Max Reitz <[email protected]>

---
 blockdev.c           | 163 ++++++++++++++++++++++++++++++++-------------------
 qapi-schema.json     |   2 +
 qapi/block-core.json |  28 +++++++++
 qmp-commands.hx      |  38 ++++++++++++
 4 files changed, 171 insertions(+), 60 deletions(-)

blockdev.c +51 -49

index 1a5b889..daf72f3 100644
@@ -1183,6 +1183,18 @@ void qmp_blockdev_snapshot_sync(bool has_device, const char *device,
                        &snapshot, errp);
 }
 
+void qmp_blockdev_snapshot(const char *node, const char *overlay,
+                           Error **errp)
+{
+    BlockdevSnapshot snapshot_data = {
+        .node = (char *) node,
+        .overlay = (char *) overlay
+    };
+
+    blockdev_do_action(TRANSACTION_ACTION_KIND_BLOCKDEV_SNAPSHOT,
+                       &snapshot_data, errp);
+}
+
 void qmp_blockdev_snapshot_internal_sync(const char *device,
                                          const char *name,
                                          Error **errp)

@@ -1521,57 +1533,48 @@ typedef struct ExternalSnapshotState {
 static void external_snapshot_prepare(BlkTransactionState *common,
                                       Error **errp)
 {
-    int flags, ret;
-    QDict *options;
+    int flags = 0, ret;
+    QDict *options = NULL;
     Error *local_err = NULL;
-    bool has_device = false;
+    /* Device and node name of the image to generate the snapshot from */
     const char *device;
-    bool has_node_name = false;
     const char *node_name;
-    bool has_snapshot_node_name = false;
-    const char *snapshot_node_name;
+    /* Reference to the new image (for 'blockdev-snapshot') */
+    const char *snapshot_ref;
+    /* File name of the new image (for 'blockdev-snapshot-sync') */
     const char *new_image_file;
-    const char *format = "qcow2";
-    enum NewImageMode mode = NEW_IMAGE_MODE_ABSOLUTE_PATHS;
     ExternalSnapshotState *state =
                              DO_UPCAST(ExternalSnapshotState, common, common);
     TransactionAction *action = common->action;
 
-    /* get parameters */
-    g_assert(action->kind == TRANSACTION_ACTION_KIND_BLOCKDEV_SNAPSHOT_SYNC);
-
-    has_device = action->blockdev_snapshot_sync->has_device;
-    device = action->blockdev_snapshot_sync->device;
-    has_node_name = action->blockdev_snapshot_sync->has_node_name;
-    node_name = action->blockdev_snapshot_sync->node_name;
-    has_snapshot_node_name =
-        action->blockdev_snapshot_sync->has_snapshot_node_name;
-    snapshot_node_name = action->blockdev_snapshot_sync->snapshot_node_name;
-
-    new_image_file = action->blockdev_snapshot_sync->snapshot_file;
-    if (action->blockdev_snapshot_sync->has_format) {
-        format = action->blockdev_snapshot_sync->format;
-    }
-    if (action->blockdev_snapshot_sync->has_mode) {
-        mode = action->blockdev_snapshot_sync->mode;
+    /* 'blockdev-snapshot' and 'blockdev-snapshot-sync' have similar
+     * purpose but a different set of parameters */
+    switch (action->kind) {
+    case TRANSACTION_ACTION_KIND_BLOCKDEV_SNAPSHOT:
+        {
+            BlockdevSnapshot *s = action->blockdev_snapshot;
+            device = s->node;
+            node_name = s->node;
+            new_image_file = NULL;
+            snapshot_ref = s->overlay;
+        }
+        break;
+    case TRANSACTION_ACTION_KIND_BLOCKDEV_SNAPSHOT_SYNC:
+        {
+            BlockdevSnapshotSync *s = action->blockdev_snapshot_sync;
+            device = s->has_device ? s->device : NULL;
+            node_name = s->has_node_name ? s->node_name : NULL;
+            new_image_file = s->snapshot_file;
+            snapshot_ref = NULL;
+        }
+        break;
+    default:
+        g_assert_not_reached();
     }
 
     /* start processing */
-    state->old_bs = bdrv_lookup_bs(has_device ? device : NULL,
-                                   has_node_name ? node_name : NULL,
-                                   &local_err);
-    if (local_err) {
-        error_propagate(errp, local_err);
-        return;
-    }
-
-    if (has_node_name && !has_snapshot_node_name) {
-        error_setg(errp, "New snapshot node name missing");
-        return;
-    }
-
-    if (has_snapshot_node_name && bdrv_find_node(snapshot_node_name)) {
-        error_setg(errp, "New snapshot node name already existing");
+    state->old_bs = bdrv_lookup_bs(device, node_name, errp);
+    if (!state->old_bs) {
         return;
     }
 

@@ -1601,35 +1604,69 @@ static void external_snapshot_prepare(BlkTransactionState *common,
         return;
     }
 
-    flags = state->old_bs->open_flags;
+    if (action->kind == TRANSACTION_ACTION_KIND_BLOCKDEV_SNAPSHOT_SYNC) {
+        BlockdevSnapshotSync *s = action->blockdev_snapshot_sync;
+        const char *format = s->has_format ? s->format : "qcow2";
+        enum NewImageMode mode;
+        const char *snapshot_node_name =
+            s->has_snapshot_node_name ? s->snapshot_node_name : NULL;
 
-    /* create new image w/backing file */
-    if (mode != NEW_IMAGE_MODE_EXISTING) {
-        bdrv_img_create(new_image_file, format,
-                        state->old_bs->filename,
-                        state->old_bs->drv->format_name,
-                        NULL, -1, flags, &local_err, false);
-        if (local_err) {
-            error_propagate(errp, local_err);
+        if (node_name && !snapshot_node_name) {
+            error_setg(errp, "New snapshot node name missing");
             return;
         }

blockdev.c +26 -5

+
+        flags = state->old_bs->open_flags;
+
+        /* create new image w/backing file */
+        mode = s->has_mode ? s->mode : NEW_IMAGE_MODE_ABSOLUTE_PATHS;
+        if (mode != NEW_IMAGE_MODE_EXISTING) {
+            bdrv_img_create(new_image_file, format,
+                            state->old_bs->filename,
+                            state->old_bs->drv->format_name,
+                            NULL, -1, flags, &local_err, false);
+            if (local_err) {
+                error_propagate(errp, local_err);
+                return;
+            }
+        }
+
+        options = qdict_new();
+        if (s->has_snapshot_node_name) {
+            qdict_put(options, "node-name",
+                      qstring_from_str(snapshot_node_name));
+        }
+        qdict_put(options, "driver", qstring_from_str(format));
+
+        flags |= BDRV_O_NO_BACKING;
     }
-    qdict_put(options, "driver", qstring_from_str(format));
 
-    /* TODO Inherit bs->options or only take explicit options with an
-     * extended QMP command? */
     assert(state->new_bs == NULL);
-    ret = bdrv_open(&state->new_bs, new_image_file, NULL, options,
-                    flags | BDRV_O_NO_BACKING, &local_err);
+    ret = bdrv_open(&state->new_bs, new_image_file, snapshot_ref, options,
+                    flags, errp);
     /* We will manually add the backing_hd field to the bs later */

We will introduce the 'blockdev-snapshot' command that will require its own struct for the parameters, so we need to rename this one in order to avoid name clashes.

Signed-off-by Alberto Garcia <[email protected]>

Reviewed-by Eric Blake <[email protected]>

Reviewed-by Max Reitz <[email protected]>

Passing an empty string allows opening an image but not its backing file. This was already described in the API documentation, only the implementation was missing.

This is useful for creating snapshots using images opened with blockdev-add, since they are not supposed to have a backing image before the operation.

Signed-off-by Alberto Garcia <[email protected]>

Reviewed-by Max Reitz <[email protected]>

Reviewed-by Eric Blake <[email protected]>

blockdev.c static void external_snapshot_prepare(BlkTransactionState *common, · line 1622

Quoted 8 lines

blockdev.c +4 -4

-    options = qdict_new();
-    if (has_snapshot_node_name) {
-        qdict_put(options, "node-name",
-                  qstring_from_str(snapshot_node_name));
+        if (snapshot_node_name && bdrv_find_node(snapshot_node_name)) {
+            error_setg(errp, "New snapshot node name already exists");
+            return;
+        }

Preexisting, but shouldn't we use bdrv_lookup_bs() here (because devices and node names share a namespace)?

blockdev.c static void external_snapshot_prepare(BlkTransactionState *common, · line 1661

Quoted 5 lines

blockdev.c +5 -0

+    if (state->new_bs->blk != NULL) {
+        error_setg(errp, "The snapshot is already in use by %s",
+                   blk_name(state->new_bs->blk));
+        return;
+    }

Is it even possible yet to create a root BDS without a BB?

blockdev.c static void external_snapshot_prepare(BlkTransactionState *common, · line 1670

Quoted 8 lines

blockdev.c +7 -0

+    if (bdrv_op_is_blocked(state->new_bs, BLOCK_OP_TYPE_EXTERNAL_SNAPSHOT,
+                           errp)) {
+        return;
+    }
+
+    if (state->new_bs->backing_hd != NULL) {
+        error_setg(errp, "The snapshot already has a backing image");
     }

The error cases after bdrv_open() should probably bdrv_unref() the node.

Quoted 11 lines

blockdev.c static void external_snapshot_prepare(BlkTransactionState *common, · line 1622

Quoted 8 lines

blockdev.c +4 -4

-    options = qdict_new();
-    if (has_snapshot_node_name) {
-        qdict_put(options, "node-name",
-                  qstring_from_str(snapshot_node_name));
+        if (snapshot_node_name && bdrv_find_node(snapshot_node_name)) {
+            error_setg(errp, "New snapshot node name already exists");
+            return;
+        }

Preexisting, but shouldn't we use bdrv_lookup_bs() here (because devices and node names share a namespace)?

I think you're right, good catch!

Quoted 6 lines

Quoted 5 lines

blockdev.c +5 -0

+    if (state->new_bs->blk != NULL) {
+        error_setg(errp, "The snapshot is already in use by %s",
+                   blk_name(state->new_bs->blk));
+        return;
+    }

Quoted 1 line

Is it even possible yet to create a root BDS without a BB?

It is possible with Max's series, on which mine depends.

   http://patchwork.ozlabs.org/patch/519375/

Quoted 9 lines

Quoted 8 lines

blockdev.c +7 -0

+    if (bdrv_op_is_blocked(state->new_bs, BLOCK_OP_TYPE_EXTERNAL_SNAPSHOT,
+                           errp)) {
+        return;
+    }
+
+    if (state->new_bs->backing_hd != NULL) {
+        error_setg(errp, "The snapshot already has a backing image");
     }

Quoted 2 lines

The error cases after bdrv_open() should probably bdrv_unref() the node.

I don't think it's necessary, external_snapshot_abort() already takes care of that.