[Qemu-devel] [RFC] block-trace Low Level Command Supporting Disk Introspection

[Wolfgang Richter <wolf@cs.cmu.edu>]

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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?
--------------------------------------------------------

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?
----------------------------

(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

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