Re: [Qemu-devel] vm disk blockio and fileio

[Stefan Hajnoczi]

On Mon, Mar 31, 2014 at 10:09:13PM +0800, longguang.yue wrote:
> from the aspect of qemu, there are file and block  disk type,
> what is the final difference between the two types ?
> if its type is block and source is volume or physical disk, who finally 
> read/write the disk?
> i thought block is raw format, so raw driver will do the actual r/w, am i 
> right?
> if so qemu r/w using fileio  no matter what disk type it is ?
>  
> what is the call flows when disk type is  block when vm r/w?

QEMU is a host userspace application and it performs disk I/O on behalf
of the guest.  The difference between raw image files and host block
devices (e.g. LVM logical volumes or host partitions) is very small -
the code for both is shared in block/raw-posix.c.

If you are using the default -drive aio=threads option then a thread
pool will perform pread()/pwrite()/preadv()/pwritev()/fdatasync().

If you are using -drive aio=native then Linux AIO APIs are used
(io_submit() and io_getevents()).

Stefan