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 <stefa...@gmail.com> 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 > >> <stefa...@linux.vnet.ibm.com> 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
