Re: [Qemu-devel] [PATCH 1/3] FVD: Added support for 'qemu-img update'

[Chunqiang Tang]

> It should be possible to change prefetching and copy-on-read while the
> VM is running.  For example, having to shut down a VM in order to
> pause the prefetching is not workable.  In the QED image streaming
> tree there are monitor commands for this:
> 
> http://repo.or.cz/w/qemu/stefanha.git/shortlog/refs/heads/stream-command

I took a quick look at the code. Using a monitor command to dynamically 
control copy-on-read and prefetching is a good idea. This should be 
adopted in FVD as well. 

On another note, I saw that the code does not support (copy_on_read=off && 
stream=on). My previous benchmarking shows that copy_on_read does slow 
down other normal reads and writes, because it needs to save data to disk. 
For example, numbers in my papers show that, on ext3, FVD with 
copy_on_read=on actually boots a VM slower than QCOW2 does, even if FVD's 
copy-on-read is already heavily optimized and is not on the  critical path 
of read (i.e., callback is invoked and data is returned to the VM first, 
and then save copy-on-read data asynchronously in the background). 
Therefore, it might be possible that a user does not want to enable 
copy-on-read and only wants to do prefetching when resources are idle.