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

[ashish mittal]

Hi Kevin,

By design, our writeback cache is on non-volatile SSD device. We do
async writes to this cache and also maintain a persistent index map of
the data written. This gives us the capability to recover write-back
cache if needed.

Thanks,
Ashish

On Thu, Sep 8, 2016 at 7:20 AM, Kevin Wolf <address@hidden> wrote:
> 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