I do not know the technical limitations, but doubt you'll hit them even
with the numbers you specify.

On the other hand, are you planning for the situation where an OSD node
dies? Motherboard, power supply, etc.? When that happens, the system
will need to rebalance your thousands of OSDs' content, and while that is
fine, it implies you have several of these really big systems running
happily, and able to take the TBs or PBs of data right then to rebalance.

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Good luck.

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