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block/backup: improve comment about image fleecing 

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Message-id: 20190920142056.12778-5-vsementsov@virtuozzo.com
Signed-off-by: Max Reitz <mreitz@redhat.com>
This commit is contained in:
Vladimir Sementsov-Ogievskiy 2019-09-20 17:20:45 +03:00 committed by Max Reitz
parent 0bd0c44372
commit 372c67ea61
1 changed files with 12 additions and 3 deletions
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15
block/backup.c
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@ -747,9 +747,18 @@ BlockJob *backup_job_create(const char *job_id, BlockDriverState *bs,
job->bitmap_mode = bitmap_mode;
/*
* Set write flags:
* 1. Detect image-fleecing (and similar) schemes
* 2. Handle compression
* If source is in backing chain of target assume that target is going to be
* used for "image fleecing", i.e. it should represent a kind of snapshot of
* source at backup-start point in time. And target is going to be read by
* somebody (for example, used as NBD export) during backup job.
*
* In this case, we need to add BDRV_REQ_SERIALISING write flag to avoid
* intersection of backup writes and third party reads from target,
* otherwise reading from target we may occasionally read already updated by
* guest data.
*
* For more information see commit f8d59dfb40bb and test
* tests/qemu-iotests/222
*/
job->write_flags =
(bdrv_chain_contains(target, bs) ? BDRV_REQ_SERIALISING : 0) |