Re: [Qemu-discuss] [Qemu-devel] About the IO queue inside qemu

[Stefan Hajnoczi]

On Wed, Oct 02, 2013 at 01:28:11AM -0500, Yaodong Yang wrote:
> I'm wondering is there any io queue inside qemu? Is there any IO scheduling 
> inside it? I notice that there is an IO track feature inside block.c, but I 
> do not know the purpose of it. Could someone explain it for me? Thanks a lot!

There are two queues:

1. Throttled requests.  Used by the I/O throttling feature (-drive
   iops=5000).  Requests need to be suspended until there is enough
   budget for them.

2. Tracked requests.  This is a generic queue that is used by the
   copy-on-read feature to prevent corruption when the guest writes to a
   sector that is currently being copied.

There is also an API to batch writes so they can be merged.  virtio-blk
uses this and may therefore change the I/O pattern.

Stefan