I'm working on a new patch series which will add a new QMP command,
block-trace, which turns on tracing of writes for a specified block device and
sends the stream unmodified to another block device. The 'trace' is meant to
be precise meaning that writes are not lost, which differentiates this command
from others. It can be turned on and off depending on when it is needed.
How is this different from block-backup or drive-mirror?
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block-backup is designed to create point-in-time snapshots and not clone the
entire write stream of a VM to a particular device. It implements
copy-on-write to create a snapshot. Thus whenever a write occurs, block-backup
is designed to send the original data and not the contents of the new write.
drive-mirror is designed to mirror a disk to another location. It operates by
periodically scanning a dirty bitmap and cloning blocks when dirtied. This is
efficient as it allows for batching of writes, but it does not maintain the
order in which guest writes occurred and it can miss intermediate writes when
they go to the same location on disk.
How can block-trace be used?
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(1) Disk introspection - systems which analyze the writes going to a disk for
introspection require a perfect clone of the write stream to an original disk
to stay in-sync with updates to guest file systems.
(2) Replicated block device - two block devices could be maintained as exact
copies of each other up to a point in the disk write stream that has
successfully been written to the destination block device.
--
Wolf