Re: [Qemu-devel] [PATCH 1/5] RFC: Efficient VM backup for qemu (v1)

[Dietmar Maurer]

> > The only solution I came up with is to add before/after hooks in the
> > block job.  I agree with the criticism, but I think it's general
> > enough and at the same time easy enough to implement.
> >
> > > IMHO, the current implementation is quite simple and easy to maintain.
> >
> > No, "if (bs->backup_info)" simply doesn't belong in bdrv_co_writev.
> 
> I do not really understand that argument, because the current
> COPY_ON_READ implementation also works that way:
> 
>     if (bs->copy_on_read) {
>         flags |= BDRV_REQ_COPY_ON_READ;
>     }
>     if (flags & BDRV_REQ_COPY_ON_READ) {
>         bs->copy_on_read_in_flight++;
>     }
> 
>     if (bs->copy_on_read_in_flight) {
>         wait_for_overlapping_requests(bs, sector_num, nb_sectors);
>     }
> 
>     tracked_request_begin(&req, bs, sector_num, nb_sectors, false);
> 
>     if (flags & BDRV_REQ_COPY_ON_READ) { ...
> 
> Or do you also want to move that to block job hooks?

Just tried to move that code, but copy on read feature is unrelated to block 
jobs,
i.e. one can open a bdrv with BDRV_O_COPY_ON_READ, and that does not create
a job.

I already suggested to add those hooks to BDS instead - don't you think that 
would work?