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?
Cheers, On 12 February 2018 at 21:01, Ilya Dryomov <idryo...@gmail.com> wrote: > On Mon, Feb 12, 2018 at 6:25 AM, Blair Bethwaite > <blair.bethwa...@gmail.com> 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 > -- Cheers, ~Blairo
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