Re: [Qemu-devel] [PATCH] add file: migration support (r2)

[Anthony Liguori]

Paul Brook wrote:
> On Thursday 13 November 2008, Charles Duffy wrote:
>   
> > This patch adds support for migration to and from file: targets, moves
> > common helpers between exec: and file: use cases from migration-exec.c
> > to migration.c, and adds a qemu_fdopen() helper parallel to qemu_fopen().
> >
> > Compared to the previous version submitted, it removes some cruft which
> > was accidentally included, and documents the limitations of O_NONBLOCK
> > on Linux with local filesystem access.
> >     
>
> I don't see any documentation.
>
> The documentation should make it clear that this is not intended as a 
> substitute for savvm/loadvm. A "live" migration doesn't make sense if you're 
> writing to an actual file (rather than a FIFO) as you'll end up with 
> unecessarily large files.
>   

Forget the large file, it's not "live" which makes it not useful 
compared with savevm/loadvm.  Using rate limiting as a crutch to avoid 
this is wrong because it requires the user to know how much bandwidth 
QEMU can use before blocking.

To do live migration to a file, you need to tie into something like 
posix-aio.  You could actually use a bdrv_raw and expand it like we do 
for a qcow file although that's sort of an abuse of the block driver API.

Regards,

Anthony Liguori

> Paul
>
>
>