Hi, Thank you, everyone, for the help. I absolutely was mixing up the two, which is why I was asking for guidance. The example made it clear. The question I was trying to answer was: what would the capacity of the cluster be, for actual data, based on the raw disk space + server/drive count + erasure coding profile. It sounds like the 'usable' calculation (66% in this case) is the accurate number, assuming I were to fill the cluster to 100%, which I realize is not ideal with Ceph. Respectfully, David Orman On Wed, Jul 29, 2020 at 2:27 AM Janne Johansson <icepic.dz@xxxxxxxxx> wrote: > > > 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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