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[Qemu-block] [PULL 11/12] block/backup: avoid copying less than full tar
From: |
Jeff Cody |
Subject: |
[Qemu-block] [PULL 11/12] block/backup: avoid copying less than full target clusters |
Date: |
Mon, 29 Feb 2016 15:08:51 -0500 |
From: John Snow <address@hidden>
During incremental backups, if the target has a cluster size that is
larger than the backup cluster size and we are backing up to a target
that cannot (for whichever reason) pull clusters up from a backing image,
we may inadvertantly create unusable incremental backup images.
For example:
If the bitmap tracks changes at a 64KB granularity and we transmit 64KB
of data at a time but the target uses a 128KB cluster size, it is
possible that only half of a target cluster will be recognized as dirty
by the backup block job. When the cluster is allocated on the target
image but only half populated with data, we lose the ability to
distinguish between zero padding and uninitialized data.
This does not happen if the target image has a backing file that points
to the last known good backup.
Even if we have a backing file, though, it's likely going to be faster
to just buffer the redundant data ourselves from the live image than
fetching it from the backing file, so let's just always round up to the
target granularity.
The same logic applies to backup modes top, none, and full. Copying
fractional clusters without the guarantee of COW is dangerous, but even
if we can rely on COW, it's likely better to just re-copy the data.
Reported-by: Fam Zheng <address@hidden>
Signed-off-by: John Snow <address@hidden>
Reviewed-by: Fam Zheng <address@hidden>
Message-id: address@hidden
Signed-off-by: Jeff Cody <address@hidden>
---
block/backup.c | 25 ++++++++++++++++++++++---
1 file changed, 22 insertions(+), 3 deletions(-)
diff --git a/block/backup.c b/block/backup.c
index 76addef..0f1b1bc 100644
--- a/block/backup.c
+++ b/block/backup.c
@@ -501,6 +501,8 @@ void backup_start(BlockDriverState *bs, BlockDriverState
*target,
BlockJobTxn *txn, Error **errp)
{
int64_t len;
+ BlockDriverInfo bdi;
+ int ret;
assert(bs);
assert(target);
@@ -570,15 +572,32 @@ void backup_start(BlockDriverState *bs, BlockDriverState
*target,
goto error;
}
- bdrv_op_block_all(target, job->common.blocker);
-
job->on_source_error = on_source_error;
job->on_target_error = on_target_error;
job->target = target;
job->sync_mode = sync_mode;
job->sync_bitmap = sync_mode == MIRROR_SYNC_MODE_INCREMENTAL ?
sync_bitmap : NULL;
- job->cluster_size = BACKUP_CLUSTER_SIZE_DEFAULT;
+
+ /* If there is no backing file on the target, we cannot rely on COW if our
+ * backup cluster size is smaller than the target cluster size. Even for
+ * targets with a backing file, try to avoid COW if possible. */
+ ret = bdrv_get_info(job->target, &bdi);
+ if (ret < 0 && !target->backing) {
+ error_setg_errno(errp, -ret,
+ "Couldn't determine the cluster size of the target image, "
+ "which has no backing file");
+ error_append_hint(errp,
+ "Aborting, since this may create an unusable destination image\n");
+ goto error;
+ } else if (ret < 0 && target->backing) {
+ /* Not fatal; just trudge on ahead. */
+ job->cluster_size = BACKUP_CLUSTER_SIZE_DEFAULT;
+ } else {
+ job->cluster_size = MAX(BACKUP_CLUSTER_SIZE_DEFAULT, bdi.cluster_size);
+ }
+
+ bdrv_op_block_all(target, job->common.blocker);
job->common.len = len;
job->common.co = qemu_coroutine_create(backup_run);
block_job_txn_add_job(txn, &job->common);
--
1.9.3
- [Qemu-block] [PULL 02/12] sheepdog: allow to delete snapshot, (continued)
- [Qemu-block] [PULL 02/12] sheepdog: allow to delete snapshot, Jeff Cody, 2016/02/29
- [Qemu-block] [PULL 04/12] curl: add support for HTTP authentication parameters, Jeff Cody, 2016/02/29
- [Qemu-block] [PULL 03/12] rbd: add support for getting password from QCryptoSecret object, Jeff Cody, 2016/02/29
- [Qemu-block] [PULL 05/12] iscsi: add support for getting CHAP password via QCryptoSecret API, Jeff Cody, 2016/02/29
- [Qemu-block] [PULL 06/12] vhdx: DIV_ROUND_UP() in vhdx_calc_bat_entries(), Jeff Cody, 2016/02/29
- [Qemu-block] [PULL 07/12] vhdx: Simplify vhdx_set_shift_bits(), Jeff Cody, 2016/02/29
- [Qemu-block] [PULL 09/12] mirror: Add mirror_wait_for_io, Jeff Cody, 2016/02/29
- [Qemu-block] [PULL 08/12] mirror: Rewrite mirror_iteration, Jeff Cody, 2016/02/29
- [Qemu-block] [PULL 10/12] block/backup: make backup cluster size configurable, Jeff Cody, 2016/02/29
- [Qemu-block] [PULL 12/12] iotests/124: Add cluster_size mismatch test, Jeff Cody, 2016/02/29
- [Qemu-block] [PULL 11/12] block/backup: avoid copying less than full target clusters,
Jeff Cody <=