Thanks Ilya,

We can probably handle ~6.2MB for a 100TB volume. Is it reasonable to
expect a librbd client such as QEMU to only hold one object-map per guest?


On 12 February 2018 at 21:01, Ilya Dryomov <> wrote:

> On Mon, Feb 12, 2018 at 6:25 AM, Blair Bethwaite
> <> wrote:
> > Hi all,
> >
> > Wondering if anyone can clarify whether there are any significant
> overheads
> > from rbd features like object-map, fast-diff, etc. I'm interested in both
> > performance overheads from a latency and space perspective, e.g., can
> > object-map be sanely deployed on a 100TB volume or does the client try to
> > read the whole thing into memory...?
> Yes, it does.  Enabling object-map on images larger than 1PB isn't
> allowed for exactly that reason.  The memory overhead is 2 bits per
> object, i.e. 64K per 1TB assuming the default object size.
> object-map also depends on exclusive-lock, which is bad for use cases
> where sharing the same image between multiple clients is a requirement.
> Once object-map is enabled, fast-diff is virtually no overhead.
> Thanks,
>                 Ilya

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