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?