Re: [PVE-User] proxmox hyperconverged pg calculations in pve 7.2, ceph pacific

[Alwin Antreich]

On December 1, 2022 8:37:36 AM GMT+01:00, Rainer Krienke <krienke at uni-koblenz.de> wrote:
>Hello,
>
>I run a a hyperconverged pve cluster (V7.2) with 11 nodes. Each node has 8 4TB disks. pve and ceph are installed an running.

What's the intended use for these? And what disks are they!

>
>Next I wanted to create some ceph-pools with each 512 pgs. Since I want to use erasure coding (5+3) when creating a pool one rbd pool for metadata and the data pool are created. I used pveceph pool this command:
>
>pveceph pool create px-e --erasure-coding k=5,m=3 --pg_autoscale_mode off --pg_num 512 --pg_num_min 128
>

It's __512 * 8 / num_osds__ to get the rough amount of PGs a OSD will be associated with.

And use 4+3, as erasure profiles with a power of two perform better.

Also the m is the amount off independent OSDs you can loose before loosing data. Is 3 your intent? 

And last but not least 5+3 will involve always 8 OSDs for a read/write. Plus objects are split, the size of the actual chunk matters much when HDDs are used.

>I was able to create two pools in this way but the third pveceph call threw this error:
>
>"got unexpected control message: TASK ERROR: error with 'osd pool create': mon_command failed -  pg_num 512 size 8 would mean 22148 total pgs, which exceeds max 22000 (mon_max_pg_per_osd 250 * num_in_osds 88)"
>
>What I do not understand now are the calculations behind the scenes for the calculated total pg number of 22148. But how is this total number "22148"  calculated?
>
>I already reduced the number of pgs for the metadata pool of each ec-pool and so I was able to create 4 pools in this way. But just for fun I now tried to create ec-pool number 5 and I see the message from above.
>
>Here are the pools created by now (scraped from ceph osd pool autoscale-status):
>Pool:                Size:   Bias:  PG_NUM:
>rbd                  4599    1.0      32
>px-a-data          528.2G    1.0     512
>px-a-metadata      838.1k    1.0     128
>px-b-data              0     1.0     512
>px-b-metadata         19     1.0     128
>px-c-data              0     1.0     512
>px-c-metadata         19     1.0     128
>px-d-data              0     1.0     512
>px-d-metadata          0     1.0     128
>
>So the total number of pgs for all pools is currently 2592 which is far from 22148 pgs?
>
>Any ideas?
>Thanks Rainer

Cheers,
Alwin
Hi Rainer,