Den ons 29 juli 2020 kl 03:17 skrev David Orman <ormandj@xxxxxxxxxxxx>: > That's what the formula on the ceph link arrives at, a 2/3 or 66.66% > overhead. But if a 4 byte object is split into 4x1 byte chunks data (4 > bytes total) + 2x 1 byte chunks parity (2 bytes total), you arrive at 6 > bytes, which is 50% more than 4 bytes. So 50% overhead, vs. 33.33% overhead > as the other formula arrives at. I'm curious what I'm missing. > > Are you sure you are not just mixing up overhead with usable %? 50% overhead means you write 4 bytes, get 2 bytes "extra" for a total of 6. In this case 4 out of 6 is 66.67% usable space, i.e. two thirds. So if the formula says you will get 66% usable it means you get two-thirds usable out of your drives with EC4+2, and it can also be said that the data is 100%, and the overhead is 50% of that, but you need to know which of the figures you want to calculate. Either "how large is the growth of the data I put in" OR "How much of the stored data is my original bytes and how much in percent is the checksums". For 4+2, the growth is 50%, since you add two (50% of four) to 4 original bytes, and for a six-drive setting, two drives go to checksums so you only get 66% usable if you fill that cluster up. The space allocated to checksums (33%) is "50% of 66%" so the overhead is still 50% no matter how you calculate i -- May the most significant bit of your life be positive. _______________________________________________ ceph-users mailing list -- ceph-users@xxxxxxx To unsubscribe send an email to ceph-users-leave@xxxxxxx
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