Commit 372c67ea authored by Vladimir Sementsov-Ogievskiy's avatar Vladimir Sementsov-Ogievskiy Committed by Max Reitz
Browse files

block/backup: improve comment about image fleecing 



Signed-off-by: default avatarVladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Reviewed-by: default avatarMax Reitz <mreitz@redhat.com>
Message-id: 20190920142056.12778-5-vsementsov@virtuozzo.com
Signed-off-by: default avatarMax Reitz <mreitz@redhat.com>
parent 0bd0c443

block/backup.c

+12 −3
Original line number Diff line number Diff line
@@ -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) |