A k=3, m=3 scheme would be 3:3 = 50% , you get to use 4 bytes out of 6 bytes = 4:6 = 2:3 = 66.6%? From: David Orman <ormandj@xxxxxxxxxxxx> Sent: Wednesday, July 29, 2020 2:17 AM To: Alan Johnson (System) <alanj@xxxxxxxxxxxxxx> Cc: ceph-users <ceph-users@xxxxxxx> Subject: Re: Usable space vs. Overhead [CAUTION: External Mail] 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. On Tue, Jul 28, 2020 at 3:40 PM Alan Johnson <alanj@xxxxxxxxxxxxxx<mailto:alanj@xxxxxxxxxxxxxx>> wrote: It would be 4/(4+2) = 4/6 =2/3 or k/(k+m)? -----Original Message----- From: David Orman <ormandj@xxxxxxxxxxxx<mailto:ormandj@xxxxxxxxxxxx>> Sent: Tuesday, July 28, 2020 9:32 PM To: ceph-users <ceph-users@xxxxxxx<mailto:ceph-users@xxxxxxx>> Subject: Usable space vs. Overhead [CAUTION: External Mail] I'm having a hard time understanding the EC usable space vs. raw. https://urldefense.com/v3/__https://ceph.io/geen-categorie/ceph-erasure-coding-overhead-in-a-nutshell/__;!!B4Ndrdkg3tRaKVT9!79nn4ZG7ADJCY7JEhJwbPvHUn8dvmzAYz9_z-BUG_7Pe0uUETMW_AwDPmgiU4dc$<https://urldefense.com/v3/__https:/ceph.io/geen-categorie/ceph-erasure-coding-overhead-in-a-nutshell/__;!!B4Ndrdkg3tRaKVT9!79nn4ZG7ADJCY7JEhJwbPvHUn8dvmzAYz9_z-BUG_7Pe0uUETMW_AwDPmgiU4dc$> indicates "nOSD * k / (k+m) * OSD Size" is how you calculate usable space, but that's not lining up with what i'd expect just from k data chunks + m parity chunks. So, for example, k=4, m=2. you'd expect every 4 byte object written would consume 6 bytes, so 50% overhead. however, the prior formula in a 7 server cluster, using 4+2 encoding, would indicate 66.67% usable capacity vs. raw storage. What am I missing here? _______________________________________________ ceph-users mailing list -- ceph-users@xxxxxxx<mailto:ceph-users@xxxxxxx> To unsubscribe send an email to ceph-users-leave@xxxxxxx<mailto:ceph-users-leave@xxxxxxx> _______________________________________________ ceph-users mailing list -- ceph-users@xxxxxxx To unsubscribe send an email to ceph-users-leave@xxxxxxx
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