Re: [Qemu-devel] [Qemu-block] [PATCH v2] block/rbd: implement .bdrv_get_allocated_file_size callback

[Jason Dillaman]

On Thu, Jun 27, 2019 at 1:24 PM John Snow <address@hidden> wrote:
>
>
>
> On 6/27/19 4:48 AM, Stefano Garzarella wrote:
> > On Wed, Jun 26, 2019 at 05:04:25PM -0400, John Snow wrote:
> >> It looks like this has hit a 30 day expiration without any reviews or
> >> being merged; do we still want this? If so, can you please resend?
> >
> > Yes, I think we still want :)
> >
> > Is it okay if I send a v3 following your comments?
> >
>
> Yes, but I don't know who is responsible for final approval; I guess
> that's Josh Durgin?

I'm the new (for the past several years) upstream PTL for RBD, so feel
free to tag me.

> >>
> >> On 5/10/19 11:33 AM, Stefano Garzarella wrote:
> >>> This patch allows 'qemu-img info' to show the 'disk size' for
> >>> the RBD images that have the fast-diff feature enabled.
> >>>
> >>> If this feature is enabled, we use the rbd_diff_iterate2() API
> >>> to calculate the allocated size for the image.
> >>>
> >>> Signed-off-by: Stefano Garzarella <address@hidden>
> >>> ---
> >>> v2:
> >>>   - calculate the actual usage only if the fast-diff feature is
> >>>     enabled [Jason]
> >>> ---
> >>>  block/rbd.c | 54 +++++++++++++++++++++++++++++++++++++++++++++++++++++
> >>>  1 file changed, 54 insertions(+)
> >>>
> >>> diff --git a/block/rbd.c b/block/rbd.c
> >>> index 0c549c9935..f1bc76ab80 100644
> >>> --- a/block/rbd.c
> >>> +++ b/block/rbd.c
> >>> @@ -1046,6 +1046,59 @@ static int64_t qemu_rbd_getlength(BlockDriverState 
> >>> *bs)
> >>>      return info.size;
> >>>  }
> >>>
> >>> +static int rbd_allocated_size_cb(uint64_t offset, size_t len, int exists,
> >>> +                                 void *arg)
> >>> +{
> >>> +    int64_t *alloc_size = (int64_t *) arg;
> >>> +
> >>> +    if (exists) {
> >>> +        (*alloc_size) += len;
> >>> +    }
> >>> +
> >>> +    return 0;
> >>> +}
> >>> +
> >>> +static int64_t qemu_rbd_get_allocated_file_size(BlockDriverState *bs)
> >>> +{
> >>> +    BDRVRBDState *s = bs->opaque;
> >>> +    uint64_t flags, features;
> >>> +    int64_t alloc_size = 0;
> >>> +    int r;
> >>> +
> >>> +    r = rbd_get_flags(s->image, &flags);
> >>> +    if (r < 0) {
> >>> +        return r;
> >>> +    }
> >>> +
> >>
> >> Do you know where rbd_get_flags is documented? I can't seem to quickly
> >> find a reference that tells me what to expect from calling it. It
> >> returns an int, I guess an error code, but how can I confirm this?
> >>
> >> *clones the ceph repository*
> >>
> >> src/librbd/internal.cc get_flags convinces me it probably works like I
> >> think, but is there not a reference here?
> >>
> >
> > Good question!
> > I didn't find any docs, but looking in the ceph tests test/librbd/fsx.cc,
> > they print an error message if the return value is less than 0.
> >
> > A 'get_flags' implemented in cls/rbd/cls_rbd.cc for example returns 0 at the
> > end and -EINVAL in a try/catch. It also uses 'read_key()' that in some cases
> > returns -EIO, so I hope that the error returned by rbd_get_flags() is one of
> > the errors defined in errno.h
> >
> >>> +    r = rbd_get_features(s->image, &features);
> >>> +    if (r < 0) {
> >>> +        return r;
> >>> +    }
> >>> +
> >>> +    /*
> >>> +     * We use rbd_diff_iterate2() only if the RBD image have fast-diff
> >>> +     * feature enabled. If it is disabled, rbd_diff_iterate2() could be
> >>> +     * very slow on a big image.
> >>> +     */
> >>> +    if (!(features & RBD_FEATURE_FAST_DIFF) ||
> >>> +        (flags & RBD_FLAG_FAST_DIFF_INVALID)) {
> >>> +        return -1;
> >>> +    }
> >>> +
> >>
> >> (Is there a reference for the list of flags to make sure there aren't
> >> other cases we might want to skip this?)
> >
> > Unfortunately no :(
> > As Jason suggested, I followed what libvirt did in the
> > volStorageBackendRBDUseFastDiff() [src/storage/storage_backend_rbd.c]

These are the only ones.

> >>
> >> It looks reasonable at a glance, but maybe let's return -ENOTSUP instead
> >> of -1, based on the idea that bdrv_get_allocated_file_size returns
> >> -ENOMEDIUM in a prominent error case -- let's match that error convention.
> >
> > Sure, -ENOTSUP is absolutely better!
> >
> >>
> >> (Well, I wonder what the librbd calls are returning and if THOSE mean
> >> anything.)
> >
> > I hope they return an errno.h errors, but I'm not sure if the meaning
> > make sense for us.
> >
> > Do you think is better to return -ENOTSUP or -EIO when librbd calls
> > fail?
> >
>
> I'll be honest, I have no idea because I don't know what failure of
> these calls means _at all_, so I don't know if it should be something
> severe like EIO or something more mundane.
>
> I guess just leave them alone in absence of better information, honestly.

It looks like QEMU treats any negative error code like the actual size
isn't available. However, for clarity I would vote for -ENOTSUPP since
that's what would be returned if the driver doesn't support it.

> >
> > Thanks for your comments,
> > Stefano
> >
>
> Thank you for trying to patch rbd :)


-- 
Jason