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Re: [Qemu-block] [PATCH v3] block/rbd: implement .bdrv_get_allocated_fil


From: Stefano Garzarella
Subject: Re: [Qemu-block] [PATCH v3] block/rbd: implement .bdrv_get_allocated_file_size callback
Date: Fri, 5 Jul 2019 12:43:18 +0200
User-agent: NeoMutt/20180716

On Fri, Jul 05, 2019 at 11:58:43AM +0200, Max Reitz wrote:
> On 05.07.19 11:32, 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>
> > ---
> > v3:
> >   - return -ENOTSUP instead of -1 when fast-diff is not available
> >     [John, Jason]
> > v2:
> >   - calculate the actual usage only if the fast-diff feature is
> >     enabled [Jason]
> > ---
> >  block/rbd.c | 54 +++++++++++++++++++++++++++++++++++++++++++++++++++++
> >  1 file changed, 54 insertions(+)
> 
> Well, the librbd documentation is non-existing as always, but while
> googling, I at least found that libvirt has exactly the same code.  So I
> suppose it must be quite correct, then.
> 

While I wrote this code I took a look at libvirt implementation and also
at the "rbd" tool in the ceph repository: compute_image_disk_usage() in
src/tools/rbd/action/DiskUsage.cc

> > diff --git a/block/rbd.c b/block/rbd.c
> > index 59757b3120..b6bed683e5 100644
> > --- a/block/rbd.c
> > +++ b/block/rbd.c
> > @@ -1084,6 +1084,59 @@ 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;
> > +    }
> > +
> > +    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 -ENOTSUP;
> > +    }
> > +
> > +    /*
> > +     * rbd_diff_iterate2(), if the source snapshot name is NULL, invokes
> > +     * the callback on all allocated regions of the image.
> > +     */
> > +    r = rbd_diff_iterate2(s->image, NULL, 0,
> > +                          bs->total_sectors * BDRV_SECTOR_SIZE, 0, 1,
> > +                          &rbd_allocated_size_cb, &alloc_size);
> 
> But I have a question.  This is basically block_status, right?  So it
> gives us information on which areas are allocated and which are not.
> The result thus gives us a lower bound on the allocation size, but is it
> really exactly the allocation size?
> 
> There are two things I’m concerned about:
> 
> 1. What about metadata?

Good question, I don't think it includes the size used by metadata and I
don't know if there is a way to know it. I'll check better.

> 
> 2. If you have multiple snapshots, this will only report the overall
> allocation information, right?  So say there is something like this:
> 
> (“A” means an allocated MB, “-” is an unallocated MB)
> 
> Snapshot 1: AAAA---
> Snapshot 2: --AAAAA
> Snapshot 3: -AAAA--
> 
> I think the allocated data size is the number of As in total (13 MB).
> But I suppose this API will just return 7 MB, because it looks on
> everything an it sees the whole image range (7 MB) to be allocated.  It
> doesn’t report in how many snapshots some region is allocated.

Looking at the documentation of rbd_diff_iterate2() [1] they says:

 *                        If the source snapshot name is NULL, we
 * interpret that as the beginning of time and return all allocated
 * regions of the image.

But I don't know the answer of your question (maybe Jason can help
here).
I should check better the implementation to understand if I can cycle
on all snapshot to get the exact allocated data size.

https://github.com/ceph/ceph/blob/master/src/include/rbd/librbd.h#L925

I'll back when I have more details on the rbd implementation to better
answer your questions.

Thanks,
Stefano



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