Re: [Qemu-devel] [RFC] Generic image streaming

[Stefan Hajnoczi]

On Mon, Sep 26, 2011 at 05:11:00PM +0800, Zhi Yong Wu wrote:
> On Mon, Sep 26, 2011 at 3:55 PM, Stefan Hajnoczi <address@hidden> wrote:
> > On Mon, Sep 26, 2011 at 01:32:34PM +0800, Zhi Yong Wu wrote:
> >> On Fri, Sep 23, 2011 at 11:57 PM, Stefan Hajnoczi
> >> <address@hidden> wrote:
> >> > Here is my generic image streaming branch, which aims to provide a way
> >> > to copy the contents of a backing file into an image file of a running
> >> > guest without requiring specific support in the various block drivers
> >> > (e.g.  qcow2, qed, vmdk):
> >> >
> >> > http://repo.or.cz/w/qemu/stefanha.git/shortlog/refs/heads/image-streaming-api
> >> >
> >> > The tree does not provide full image streaming yet but I'd like to
> >> > discuss the approach taken in the code.  Here are the main points:
> >> >
> >> > The image streaming API is available through HMP and QMP commands.  When
> >> > streaming is started on a block device a coroutine is created to do the
> >> > background I/O work.  The coroutine can be cancelled.
> >> >
> >> > While the coroutine copies data from the backing file into the image
> >> > file, the guest may be performing I/O to the image file.  Guest reads do
> >> > not conflict with streaming but guest writes require special handling.
> >> > If the guest writes to a region of the image file that we are currently
> >> > copying, then there is the potential to clobber the guest write with old
> >> > data from the backing file.
> >> >
> >> > Previously I solved this in a QED-specific way by taking advantage of
> >> > the serialization of allocating write requests.  In order to do this
> >> > generically we need to track in-flight requests and have the ability to
> >> > queue I/O.  Guest writes that affect an in-flight streaming copy
> >> > operation must wait for that operation to complete before being issued.
> >> > Streaming copy operations must skip overlapping regions of guest writes.
> >> >
> >> > One big difference to the QED image streaming implementation is that
> >> > this generic implementation is not based on copy-on-read operations.
> >> > Instead we do a sequence of bdrv_is_allocated() to find regions for
> >> > streaming, followed by bdrv_co_read() and bdrv_co_write() in order to
> >> > populate the image file.
> >> >
> >> > It turns out that generic copy-on-read is not an attractive operation
> >> > because it requires using bounce buffers for every request.  Kevin
> >> bounce buffers == buffer ring?
> >
> > A bounce buffer is a temporary buffer that is used because the actual
> > data buffer is not addressable or cannot be directly accessed for some
> > other reason.  In this case it's because the guest should see read
> > semantics and not find that writes to its read data buffer result in
> > writes to disk.
> >
> >> > pointed out the case where a guest performs a read and pokes the data
> >> > buffer before the read completes, copy-on-read would write out the
> >> > modified memory into the image file unless we use a bounce buffer.
> Sorry, to be honest, i don't know which scenario will cause guest
> modified memory is written out into image file.

I showed the scenario in the steps posted below:

> >> Can you elaborate this?
> >
> > 1. Guest issues a read request.
> > 2. QEMU issues host read request as first step in copy-on-read.
> > 3. Host read request completes...
> > 4. Guest overwrites its data buffer before QEMU acknowledges request
> >   completion.
> > 5. ...QEMU issues host write request.
> > 6. Host completes write request and QEMU acknowledges guest read
> >   completion.
> Good, thanks.
> >
> > What happened is that we populated the image file with data from guest
> > memory that does not match what is in the backing file.  The guest
> How to find that the two data don't match?

Reread what I posted and think about the case where a QEMU read buffer
(the "bounce buffer") is used in step 2.  In that case the guest cannot
tamper with the data buffer while performing copy-on-read.

Stefan