Hi. I'm reading the thesis, and wondering what the thinking is behind
how Ceph uses the placement groups (PGs).
It seems that CRUSH is used for a deterministic, pseudorandom mapping,
object_id --> pg --> osds. I'm wondering why the extra level of PGs
was felt desirable, why that isn't just object_id --> osds.
Colin explained on IRC that the primary OSD for a PG is responsible
for some managerial duties, but that just tells me *how* PGs are used;
not *why*. Surely you could organize these responsibilities
differently, e.g. manage replication of an object on an
object-by-object basis, by the primary OSD for that object.
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