[Qemu-devel] vxhs caching behaviour (was: [PATCH v4 RFC] block/vxhs: Initial commit to add) Veritas HyperScale VxHS block device support

[Kevin Wolf]

Am 08.09.2016 um 16:00 hat Jeff Cody geschrieben:
> > >> +/*
> > >> + * This is called by QEMU when a flush gets triggered from within
> > >> + * a guest at the block layer, either for IDE or SCSI disks.
> > >> + */
> > >> +int vxhs_co_flush(BlockDriverState *bs)
> > >> +{
> > >> +    BDRVVXHSState *s = bs->opaque;
> > >> +    uint64_t size = 0;
> > >> +    int ret = 0;
> > >> +
> > >> +    ret = qemu_iio_ioctl(s->qnio_ctx,
> > >> +            s->vdisk_hostinfo[s->vdisk_cur_host_idx].vdisk_rfd,
> > >> +            VDISK_AIO_FLUSH, &size, NULL, IIO_FLAG_SYNC);
> > >> +
> > >> +    if (ret < 0) {
> > >> +        /*
> > >> +         * Currently not handling the flush ioctl
> > >> +         * failure because of network connection
> > >> +         * disconnect. Since all the writes are
> > >> +         * commited into persistent storage hence
> > >> +         * this flush call is noop and we can safely
> > >> +         * return success status to the caller.
> > >
> > > I'm not sure I understand here.  Are you saying the qemu_iio_ioctl() call
> > > above is a noop?
> > >
> > 
> > Yes, qemu_iio_ioctl(VDISK_AIO_FLUSH) is only a place-holder at present
> > in case we later want to add some functionality to it. I have now
> > added a comment to this affect to avoid any confusion.
> > 
> 
> The problem is you don't know which version of the qnio library any given
> QEMU binary will be using, since it is a shared library.  Future versions
> may implement the flush ioctl as expressed above, in which case we may hide
> a valid error.
> 
> Am I correct in assuming that this call suppresses errors because an error
> is returned for an unknown ioctl operation of VDISK_AIO_FLUSH?  If so, and
> you want a placeholder here for flushing, you should go all the way and stub
> out the underlying ioctl call to return success.  Then QEMU can at least
> rely on the error return from the flush operation.

So what's the story behind the missing flush command?

Does the server always use something like O_SYNC, i.e. all potential
write caches in the stack operate in a writethrough mode? So each write
request is only completed successfully if it is ensured that the data is
safe on disk rather than in a volatile writeback cache?

As soon as any writeback cache can be involved (e.g. the kernel page
cache or a volatile disk cache) and there is no flush command (a real
one, not just stubbed), the driver is not operating correctly and
therefore not ready for inclusion.

So Ashish, can you tell us something about caching behaviour across the
storage stack when vxhs is involved?

Kevin