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block: Add bitmap successors

Enterprise / Virtualization / QEMU - John Snow [redhat.com] - 28 April 2015 08:36 UTC

A bitmap successor is an anonymous BdrvDirtyBitmap that is intended to be created just prior to a sensitive operation (e.g. Incremental Backup) that can either succeed or fail, but during the course of which we still want a bitmap tracking writes.

On creating a successor, we "freeze" the parent bitmap which prevents its deletion, enabling, anonymization, or creating a bitmap with the same name.

On success, the parent bitmap can "abdicate" responsibility to the successor, which will inherit its name. The successor will have been tracking writes during the course of the backup operation. The parent will be safely deleted.

On failure, we can "reclaim" the successor from the parent, unifying them such that the resulting bitmap describes all writes occurring since the last successful backup, for instance. Reclamation will thaw the parent, but not explicitly re-enable it.

BdrvDirtyBitmap operations that target a single bitmap are protected by assertions that the bitmap is not frozen and/or disabled.

BdrvDirtyBitmap operations that target a group of bitmaps, such as bdrv_{set,reset}_dirty will ignore frozen/disabled drives with a conditional instead.

Internal functions that enable/disable dirty bitmaps have assertions added to them to prevent modifying frozen bitmaps.

9bd2b08 block: Add bitmap successors
block.c | 104 ++++++++++++++++++++++++++++++++++++++++++++++++-
blockdev.c | 7 ++++
include/block/block.h | 10 +++++
qapi/block-core.json | 1 +
4 files changed, 121 insertions(+), 1 deletion(-)

Upstream: git.qemu.org