On Mon, Feb 12, 2018 at 6:25 AM, Blair Bethwaite
> 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.
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