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Re: [Qemu-devel] [PATCH v4 2/3] block: Add blockdev-backup to transactio
From: |
Max Reitz |
Subject: |
Re: [Qemu-devel] [PATCH v4 2/3] block: Add blockdev-backup to transaction |
Date: |
Thu, 04 Dec 2014 14:59:29 +0100 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.2.0 |
On 2014-12-04 at 03:29, Fam Zheng wrote:
Also add version info for other transaction types.
Signed-off-by: Fam Zheng <address@hidden>
---
blockdev.c | 81 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++
qapi-schema.json | 7 +++++
2 files changed, 88 insertions(+)
diff --git a/blockdev.c b/blockdev.c
index f44441a..a98a4f8 100644
--- a/blockdev.c
+++ b/blockdev.c
@@ -1559,6 +1559,81 @@ static void drive_backup_clean(BlkTransactionState
*common)
}
}
+typedef struct BlockdevBackupState {
+ BlkTransactionState common;
+ BlockDriverState *bs;
+ BlockJob *job;
+ AioContext *aio_context;
+} BlockdevBackupState;
+
+static void blockdev_backup_prepare(BlkTransactionState *common, Error **errp)
+{
+ BlockdevBackupState *state = DO_UPCAST(BlockdevBackupState, common,
common);
+ BlockdevBackup *backup;
+ BlockDriverState *bs, *target;
+ Error *local_err = NULL;
+
+ assert(common->action->kind == TRANSACTION_ACTION_KIND_BLOCKDEV_BACKUP);
+ backup = common->action->blockdev_backup;
+
+ bs = bdrv_find(backup->device);
+ if (!bs) {
+ error_set(errp, QERR_DEVICE_NOT_FOUND, backup->device);
+ return;
+ }
+
+ target = bdrv_find(backup->target);
+ if (!target) {
+ error_set(errp, QERR_DEVICE_NOT_FOUND, backup->target);
+ return;
+ }
+
+ /* AioContext is released in .clean() */
+ state->aio_context = bdrv_get_aio_context(bs);
+ if (state->aio_context != bdrv_get_aio_context(target)) {
+ state->aio_context = NULL;
+ error_setg(errp, "Backup between two IO threads are not implemented");
Either *Backups ore s/are/is/.
+ return;
+ }
+ aio_context_acquire(state->aio_context);
+
+ qmp_blockdev_backup(backup->device, backup->target,
+ backup->sync,
+ backup->has_speed, backup->speed,
+ backup->has_on_source_error, backup->on_source_error,
+ backup->has_on_target_error, backup->on_target_error,
+ &local_err);
+ if (local_err) {
+ error_propagate(errp, local_err);
+ state->bs = NULL;
+ state->job = NULL;
No need for these assignments, state is 0-initialized. I wouldn't point
that out if Stefan wouldn't just have sent a patch which removed such
assignments in some other transaction preparation.
+ return;
+ }
+
+ state->bs = bdrv_find(backup->device);
+ state->job = state->bs->job;
+}
+
+static void blockdev_backup_abort(BlkTransactionState *common)
+{
+ BlockdevBackupState *state = DO_UPCAST(BlockdevBackupState, common,
common);
+ BlockDriverState *bs = state->bs;
+
+ /* Only cancel if it's the job we started */
+ if (bs && bs->job && bs->job == state->job) {
+ block_job_cancel_sync(bs->job);
+ }
+}
+
+static void blockdev_backup_clean(BlkTransactionState *common)
+{
+ BlockdevBackupState *state = DO_UPCAST(BlockdevBackupState, common,
common);
+
+ if (state->aio_context) {
+ aio_context_release(state->aio_context);
+ }
+}
+
static void abort_prepare(BlkTransactionState *common, Error **errp)
{
error_setg(errp, "Transaction aborted using Abort action");
@@ -1582,6 +1657,12 @@ static const BdrvActionOps actions[] = {
.abort = drive_backup_abort,
.clean = drive_backup_clean,
},
+ [TRANSACTION_ACTION_KIND_BLOCKDEV_BACKUP] = {
+ .instance_size = sizeof(BlockdevBackupState),
+ .prepare = blockdev_backup_prepare,
+ .abort = blockdev_backup_abort,
+ .clean = blockdev_backup_clean,
+ },
[TRANSACTION_ACTION_KIND_ABORT] = {
.instance_size = sizeof(BlkTransactionState),
.prepare = abort_prepare,
diff --git a/qapi-schema.json b/qapi-schema.json
index 9ffdcf8..411d287 100644
--- a/qapi-schema.json
+++ b/qapi-schema.json
@@ -1254,11 +1254,18 @@
#
# A discriminated record of operations that can be performed with
# @transaction.
+#
+# Since 1.1
+# drive-backup since 1.6
+# abort since 1.6
+# blockdev-snapshot-internal-sync since 1.7
+# blockdev-backup since 2.3
This seems a bit hard to read... Maybe an empty line after the "Since
1.1" would help, but I'm not sure...
##
{ 'union': 'TransactionAction',
'data': {
'blockdev-snapshot-sync': 'BlockdevSnapshot',
'drive-backup': 'DriveBackup',
+ 'blockdev-backup': 'BlockdevBackup',
'abort': 'Abort',
'blockdev-snapshot-internal-sync': 'BlockdevSnapshotInternal'
} }
So, about this patch in general: I know drive-backup works nearly the
same way. It starts block job in prepare(), which is aborted in abort().
But it seems a bit like cheating to me. For me, a transaction is
something which you can start and if any of the operations cannot be
executed (because its preparation failed), all are aborted (that is, not
even started). The commit() part will really do the operation, and that
will never fail because prepare() has made sure it will not.
This isn't the case when starting block jobs in prepare(). If some other
operation's prepare() fails, some data may have been copied already, so
you can't really roll back the operation. It isn't so bad for
drive-backup, because you're normally writing to a new image, so having
overwritten that image by a bit isn't so bad. But with blockdev-backup,
you're overwriting a block device inside qemu, so overwriting it
partially may actually be bad (or even overwriting it completely; if a
transaction fails, I'd expect the operations to have done nothing at all).
I understand that drive-backup does the same thing already (although I
don't deem the impact there a bit less), so it may just be that my
notion of transactions is wrong.
However, in this state, where is the advantage of making this an
operation usable in a transaction? I can just start a number of
blockdev-backup block jobs manually and cancel them if anything goes
wrong. There's no difference, so I don't see the benefit of making it a
transaction operation if a transaction does not actually mean "Do not do
anything if we don't know for sure that all operations will complete
successfully."
Max