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

From: Zhi Yong Wu
Subject: Re: [Qemu-devel] [RFC] Generic image streaming
Date: Mon, 26 Sep 2011 17:11:00 +0800

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.
>> 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?
> issued a read request, this should never result in writing to the image
> file.
> Although legitimate guests do not do this, a buggy guest could corrupt
> its disk in this way!
> Stefan


Zhi Yong Wu

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