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Re: [Qemu-devel] [PATCH 2/3] block: adjust qmp_transaction to be extenda


From: Wenchao Xia
Subject: Re: [Qemu-devel] [PATCH 2/3] block: adjust qmp_transaction to be extendable
Date: Wed, 03 Apr 2013 17:02:27 +0800
User-agent: Mozilla/5.0 (Windows NT 5.1; rv:17.0) Gecko/20130307 Thunderbird/17.0.4


No, if bdrv_snapshot_delete() can fail, you need to split it in two
parts: one that can fail, and one that cannot.  If you cannot, then
there are two possibilities:

- if the failures are minor and could be repaired with "qemu-img check -r"
(e.g. lost clusters), then this is not optimal but can still be done;

- otherwise, the operation simply cannot be made transactionable.

In the case of qcow2_snapshot_delete, everything except

     ret = bdrv_pwrite_sync(bs->file, offsetof(QCowHeader, nb_snapshots),
                            &header_data, sizeof(header_data));
     if (ret < 0) {
         goto fail;
     }

must be in the prepare phase.  Everything after "fail" (which right now
is nothing, but it should at least undo the qcow2_alloc_clusters operation)
must be in the rollback phase.  Everything in the middle is the commit
phase.

Paolo

  Sorry I haven't state it clearly. What about bdrv_snapshot_create()
operation? If it need to be rolled back, I think bdrv_snapshot_delete()
will get called and it may fail. But in most case if
bdrv_snapshot_create() succeed before, the bdrv_snapshot_delete should
succeed also, so if fail there may be unexpected error below, could
assert be used for this?


@@ -806,125 +950,37 @@ void qmp_transaction(BlockdevActionList *dev_list,
Error **errp)
       /* We don't do anything in this loop that commits us to the snapshot
       */
       while (NULL != dev_entry) {
           BlockdevAction *dev_info = NULL;
-        BlockDriver *proto_drv;
-        BlockDriver *drv;
-        int flags;
-        enum NewImageMode mode;
-        const char *new_image_file;
-        const char *device;
-        const char *format = "qcow2";
-
           dev_info = dev_entry->value;
           dev_entry = dev_entry->next;

           states = g_malloc0(sizeof(BlkTransactionStates));
           QSIMPLEQ_INSERT_TAIL(&snap_bdrv_states, states, entry);

+        states->action = dev_info;
           switch (dev_info->kind) {
           case BLOCKDEV_ACTION_KIND_BLOCKDEV_SNAPSHOT_SYNC:
-            device = dev_info->blockdev_snapshot_sync->device;
-            if (!dev_info->blockdev_snapshot_sync->has_mode) {
-                dev_info->blockdev_snapshot_sync->mode =
NEW_IMAGE_MODE_ABSOLUTE_PATHS;
-            }
-            new_image_file =
dev_info->blockdev_snapshot_sync->snapshot_file;
-            if (dev_info->blockdev_snapshot_sync->has_format) {
-                format = dev_info->blockdev_snapshot_sync->format;
-            }
-            mode = dev_info->blockdev_snapshot_sync->mode;
+            states->ops = &external_snapshot_ops;
               break;
           default:
               abort();
           }

-        drv = bdrv_find_format(format);
-        if (!drv) {
-            error_set(errp, QERR_INVALID_BLOCK_FORMAT, format);
-            goto delete_and_fail;
-        }
-
-        states->old_bs = bdrv_find(device);
-        if (!states->old_bs) {
-            error_set(errp, QERR_DEVICE_NOT_FOUND, device);
-            goto delete_and_fail;
-        }
-
-        if (!bdrv_is_inserted(states->old_bs)) {
-            error_set(errp, QERR_DEVICE_HAS_NO_MEDIUM, device);
-            goto delete_and_fail;
-        }
-
-        if (bdrv_in_use(states->old_bs)) {
-            error_set(errp, QERR_DEVICE_IN_USE, device);
-            goto delete_and_fail;
-        }
-
-        if (!bdrv_is_read_only(states->old_bs)) {
-            if (bdrv_flush(states->old_bs)) {
-                error_set(errp, QERR_IO_ERROR);
-                goto delete_and_fail;
-            }
-        }
-
-        flags = states->old_bs->open_flags;
-
-        proto_drv = bdrv_find_protocol(new_image_file);
-        if (!proto_drv) {
-            error_set(errp, QERR_INVALID_BLOCK_FORMAT, format);
-            goto delete_and_fail;
-        }
-
-        /* create new image w/backing file */
-        if (mode != NEW_IMAGE_MODE_EXISTING) {
-            bdrv_img_create(new_image_file, format,
-                            states->old_bs->filename,
-                            states->old_bs->drv->format_name,
-                            NULL, -1, flags, &local_err, false);
-            if (error_is_set(&local_err)) {
-                error_propagate(errp, local_err);
-                goto delete_and_fail;
-            }
-        }
-
-        /* We will manually add the backing_hd field to the bs later */
-        states->new_bs = bdrv_new("");
-        /* TODO Inherit bs->options or only take explicit options with an
-         * extended QMP command? */
-        ret = bdrv_open(states->new_bs, new_image_file, NULL,
-                        flags | BDRV_O_NO_BACKING, drv);
-        if (ret != 0) {
-            error_set(errp, QERR_OPEN_FILE_FAILED, new_image_file);
+        if (states->ops->commit(states->action, &states->opaque, errp)) {
               goto delete_and_fail;
           }
       }

The following block is the commit:

-
-    /* Now we are going to do the actual pivot.  Everything up to this
point
-     * is reversible, but we are committed at this point */
-    QSIMPLEQ_FOREACH(states, &snap_bdrv_states, entry) {
-        /* This removes our old bs from the bdrv_states, and adds the new
bs */
-        bdrv_append(states->new_bs, states->old_bs);
-        /* We don't need (or want) to use the transactional
-         * bdrv_reopen_multiple() across all the entries at once, because
we
-         * don't want to abort all of them if one of them fails the
reopen */
-        bdrv_reopen(states->new_bs, states->new_bs->open_flags &
~BDRV_O_RDWR,
-                    NULL);
-    }
-
       /* success */
       goto exit;

And this is rollback:

   delete_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->new_bs) {
-             bdrv_delete(states->new_bs);
-        }
+        states->ops->rollback(states->action, states->opaque);
       }

Kevin



--
Best Regards

Wenchao Xia





--
Best Regards

Wenchao Xia




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