Re: Forcing Ceph into mapping all objects to a single PG

[Daniel Hofmann <daniel@trvx.org>]

The main issue however is not the hash's strength, but the fact that
once pre-computed, I'm able to use preimages on **every Ceph cluster out
there**. (As the hash functions's output is a deterministic function of
the object's name only)

I agree in that the general issue is inherent in hash-placement systems.

But what I don't agree with is the following:

Why do I have to be able to calculate my object's placement for **every
Ceph cluster** out there?

Why does it not suffice for me to be able to calculate the placement
only for the cluster I'm currently accessing?

From a logical standpoint it seems reasonable. Why then, are we not able
to constrain the placement calculation in that regard?


If the placement is bound to a specific cluster it should suffice to
derive e.g. a key for SipHash based on cluster specifics.

Is this doable from an implementation point of view?


Note: I only did this as a proof-of-concept for the object store.
Think about the implications, if you're able to do this e.g. for every
RadosGW out there and servies using RadosGW.