Re: [Qemu-devel] Question about drive mirror

[Stefan Hajnoczi <stefanha@gmail.com>]

On Tue, Jan 14, 2014 at 09:41:41AM +0800, Fam Zheng wrote:
> On Mon, 01/13 23:44, rudyfly@163.com wrote:
> > I tested the capability of drive mirror, I found the IO is low. Then I read the code, 
> > The code mirror_run() will call mirror_iteration() to read the size of buffer data  
> > from source storage, when the read callback ,and then in mirror_read_complete () 
> > write the data to the target storage, It is serial. 

And it also uses throttling to avoid impacting the guest too much.  It's
considered a background job, i.e. not trying to saturate storage
bandwidth.

That said, improving performance while keeping I/O isolated from higher
priority guest I/O can be good.

> > Now, I hope when it is writing the data to target storage ,we can send the request 
> > of reading data from source storage. Because of  using  coroutine to do it ,there is 
> > some troubles to achieve it. why not use Multi-thread? 
> > Some one can give me some idea?

It can be parallelized by splitting the code into two separate loops:

A reader coroutine that reads data from the source device and places
buffers in a queue.

A writer coroutine that takes buffers from the queue and writes data to
the target device.

(In fact there could be multiple readers and writers.)

Stefan