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Re: [Qemu-devel] [RFC] block-trace Low Level Command Supporting Disk Int

From: Kevin Wolf
Subject: Re: [Qemu-devel] [RFC] block-trace Low Level Command Supporting Disk Introspection
Date: Tue, 14 May 2013 10:50:46 +0200
User-agent: Mutt/1.5.21 (2010-09-15)

Am 13.05.2013 um 23:21 hat Wolfgang Richter geschrieben:
> 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.

Or, to translate it into our existing terminology, drive-mirror
implements a passive mirror, you're proposing an active one (which we
do want to have).

With an active mirror, we'll want to have another choice: The mirror can
be synchronous (guest writes only complete after the mirrored write has
completed) or asynchronous (completion is based only on the original
image). It should be easy enough to support both once an active mirror

> 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.

You're leaving out the most interesting section: How should block-trace
be implemented?

The first question is what the API should look like, on the QMP level. I
think originally the idea was to use drive-mirror for all kinds of
mirrors, but maybe it makes more sense indeed to keep the active mirror
separate. I don't particularly like the name block-trace for a separate
command, but let's save the bikeshedding for later.

The other question is how to implement it internally. I don't think
adding specific code for each new block job into bdrv_co_do_writev() is
acceptable. We really need a generic way to intercept I/O operations.
The keyword from earlier discussions is "block filters". Essentially the
idea is that the block job temporarily adds a BlockDriverState on top of
the format driver and becomes able to implement all callbacks it likes
to intercept. The bad news is that the infrastructure isn't there yet
to actually make this happen in a sane way.


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