I configured Ceph with the recommended values (using a formula from the docs). I have 3 OSDs, and my config (which I've put on the monitor node and all 3 OSDs) includes this:
osd pool default size = 2
osd pool default min size = 1
osd pool default pg num = 150
osd pool default pgp num = 150
When I run ceph status I get:
health HEALTH_WARN
too many PGs per OSD (1042 > max 300)
This is confusing for two reasons. First, because the recommended formula did not satisfy Ceph. Second, and most puzzling, is that it says I have 1042 PGs per OSD, when my configuration says 150.
What am I doing wrong?
Answer
Before setting PG count you need to know 3 things.
1. Number of OSD
ceph osd ls
Sample Output:
0
1
2
Here Total number of osd is three.
2. Number of Pools
ceph osd pool ls or rados lspools
Sample Output:
rbd
images
vms
volumes
backups
Here Total number of pool is five.
3. Replication Count
ceph osd dump | grep repli
Sample Output:
pool 0 'rbd' replicated size 2 min_size 2 crush_ruleset 0 object_hash rjenkins pg_num 64 pgp_num 64 last_change 38 flags hashpspool stripe_width 0
pool 1 'images' replicated size 2 min_size 2 crush_ruleset 1 object_hash rjenkins pg_num 30 pgp_num 30 last_change 40 flags hashpspool stripe_width 0
pool 2 'vms' replicated size 2 min_size 2 crush_ruleset 1 object_hash rjenkins pg_num 30 pgp_num 30 last_change 42 flags hashpspool stripe_width 0
pool 3 'volumes' replicated size 2 min_size 2 crush_ruleset 1 object_hash rjenkins pg_num 30 pgp_num 30 last_change 36 flags hashpspool stripe_width 0
pool 4 'backups' replicated size 2 min_size 2 crush_ruleset 1 object_hash rjenkins pg_num 30 pgp_num 30 last_change 44 flags hashpspool stripe_width 0
You can see each pool has replication count two.
Now Let get into calculation
Calculations:
Total PGs Calculation:
Total PGs = (Total_number_of_OSD * 100) / max_replication_count
This result must be rounded up to the nearest power of 2.
Example:
No of OSD: 3
No of Replication Count: 2
Total PGs = (3 * 100) / 2 = 150. Nearest Power of 150 to 2 is 256.
So Maximum Recommended PGs is 256
You can set PG for every Pool
Total PGs per pool Calculation:
Total PGs = ((Total_number_of_OSD * 100) / max_replication_count) / pool count
This result must be rounded up to the nearest power of 2.
Example:
No of OSD: 3
No of Replication Count: 2
No of pools: 5
Total PGs = ((3 * 100) / 2 ) / 5 = 150 / 5 = 30 . Nearest Power of 30 to 2 is 32.
So Total No of PGs per pool is 32.
Power of 2 Table:
2^0 1
2^1 2
2^2 4
2^3 8
2^4 16
2^5 32
2^6 64
2^7 128
2^8 256
2^9 512
2^10 1024
Useful Commands
ceph osd pool create <pool-name> <pg-number> <pgp-number> - To create a new pool
ceph osd pool get <pool-name> pg_num - To get number of PG in a pool
ceph osd pool get <pool-name> pgp_num - To get number of PGP in a pool
ceph osd pool set <pool-name> pg_num <number> - To increase number of PG in a pool
ceph osd pool set <pool-name> pgp_num <number> - To increase number of PGP in a pool
*usually pg and pgp number is same