For replicated pools (w/o rounding to nearest power of two) overall
PGs number is calculated so:
Pools_PGs = 100 * (OSDs / Pool_Size),
where
100 -- target number of PGs per single OSD related to that pool,
Pool_Size -- factor showing how much raw storage would in fact be
used to store one logical data unit.
By analogy I can suppose that with EC pools corresponding Pool_Size
can be calculated so:
Raw_Storage_Use / Logical_Storage_Use
or, using EC semantics, (k + m) / k. And for EC (k=2, m=1) it gives:
Raw_Storage_Use = 3
Logical_Storage_Use = 2
-- Hence, Pool_Size should be 1.5.
OTOH, CEPH documentation says that about same EC pool (underline is mine):
"It is equivalent to a replicated pool of size __two__ but
requires 1.5TB instead of 2TB to store 1TB of data"
So how does CEPH calculate PGs distribution per OSD for it?
Using (k + m) / k? Or just k? Or differently at all?
--
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