Re: [Qemu-devel] [PATCH] bdrv_aio_flush

[Ian Jackson]

Andrea Arcangeli writes ("Re: [Qemu-devel] [PATCH] bdrv_aio_flush"):
> In case you meant fsync is just fine, Linux will use the
> WIN_FLUSH_CACHE/WIN_FLUSH_CACHE_EXT see
> idedisk_prepare_flush:
> 
>                  if (barrier) {
>                       ordered = QUEUE_ORDERED_DRAIN_FLUSH;
>                       prep_fn = idedisk_prepare_flush;
>                  }

I'm not sure I follow but I'm not familiar with the relevant Linux
code.  Do you mean that Linux does this

    submit IO 1 to device
    submit flush to device
    collect IO 1 completion
    collect flush completion

and then expects IO 1 to be on disk ?

If this is a documented behaviour of the controller or device we're
emulating then I think the qemu emulation (hw/scsi-disk.c or whatever)
should do an aio_flush barrier before aio_fsync.

What spec should I be referring to ?

Perhaps I'm naive but I would expect the device interface to look more
like the kernel syscall interface, whose specification text (in SuSv3)
I analysed in April in
 http://lists.nongnu.org/archive/html/qemu-devel/2008-04/msg00046.html

Ian.