Greetings,
I am having issues in the way PGs are calculated in https://ceph.com/pgcalc/ [Ceph PGs per Pool Calculator ] and the formulae mentioned in the site. Below are my findings The formula to calculate PGs as mentioned in the https://ceph.com/pgcalc/ : 1. Need to pick the highest value from either of the formulas *(( Target PGs per OSD ) x ( OSD # ) x ( %Data ))/(size)* Or *( OSD# ) / ( Size )* 2. The output value is then rounded to the nearest power of 2 1. If the nearest power of 2 is more than 25% below the original value, the next higher power of 2 is used. Based on the above procedure, we calculated PGs for 25, 32 and 64 OSDs *Our Dataset:* *%Data:* 0.10 *Target PGs per OSD:* 100 *OSDs* 25, 32 and 64 *For 25 OSDs* (100*25* (0.10/100))/(3) = 0.833 ( 25 ) / ( 3 ) = 8.33 1. Raw pg num 8.33 ( Since we need to pick the highest of (0.833, 8.33)) 2. max pg 16 ( For, 8.33 the nearest power of 2 is 16) 3. 16 > 2.08 ( 25 % of 8.33 is 2.08 which is more than 25% the power of 2) So 16 PGs ü GUI Calculator gives the same value and matches with Formula. *For 32 OSD* (100*32*(0.10/100))/3 = 1.066 ( 32 ) / ( 3 ) = 10.66 1. Raw pg num 10.66 ( Since we need to pick the highest of (1.066, 10.66)) 2. max pg 16 ( For, 10.66 the nearest power of 2 is 16) 3. 16 > 2.655 ( 25 % of 10.66 is 2.655 which is more than 25% the power of 2) So 16 PGs û GUI Calculator gives different value (32 PGs) which doesn’t match with Formula. *For 64 OSD* (100 * 64 * (0.10/100))/3 = 2.133 ( 64 ) / ( 3 ) 21.33 1. Raw pg num 21.33 ( Since we need to pick the highest of (2.133, 21.33)) 2. max pg 32 ( For, 21.33 the nearest power of 2 is 32) 3. 32 > 5.3325 ( 25 % of 21.33 is 5.3325 which is more than 25% the power of 2) So 32 PGs û GUI Calculator gives different value (64 PGs) which doesn’t match with Formula. We checked the PG calculator logic from [ https://ceph.com/pgcalc_assets/pgcalc.js ] which is not matching from above formulae. Can someone Guide/reference us to correct formulae to calculate PGs. Thanks in advance. Regards, Krishna Venkata
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