Hi All,
I have few questions regarding the Primary affinity. In the original
blueprint
(https://wiki.ceph.com/Planning/Blueprints/Firefly/osdmap%3A_primary_role_affinity
), one example has been given.
For PG x, CRUSH returns [a, b, c]
If a has primary_affinity of .5, b and c have 1 , with 50% probability, we will
choose b or c instead of a. (25% for b, 25% for c)
A) I was browsing through the code, but I could not find this logic of
splitting the rest of configured primary affinity value between other osds. How
is this handled?
1. if (a < CEPH_OSD_MAX_PRIMARY_AFFINITY &&
2. (crush_hash32_2(CRUSH_HASH_RJENKINS1,
3. seed, o) >> 16) >= a) {
4. // we chose not to use this primary. note it anyway as a
5. // fallback in case we don't pick anyone else, but keep looking.
6. if (pos < 0)
7. pos = i;
8. } else {
9. pos = i;
10. break;
11. }
12. }
B) Since, primary affinity value is configured independently, there can be a
situation with [0.1,0.1,0.1] with total value that don’t add to 1. How is
this taken care of?
C) Slightly confused. What happens for a situation with [1,0.5,1] ? Is osd.0
always returned?
D) After calculating primary based on the affinity values, I see a shift of
osds so that primary comes to the front. Why is this needed?. I thought,
primary affinity value affects only reads and hence, osd ordering need not be
changed.
Thanks,
Johnu
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