Hopefully someone can sanity check me here, but I'm getting the feeling that 
the MAX AVAIL in ceph df isn't reporting the correct value in 14.2.8 
(mon/mgr/mds are .8, most OSDs are .7)

> RAW STORAGE:
>     CLASS     SIZE        AVAIL       USED        RAW USED     %RAW USED
>     hdd       530 TiB     163 TiB     366 TiB      367 TiB         69.19
>     ssd       107 TiB      37 TiB      70 TiB       70 TiB         65.33
>     TOTAL     637 TiB     201 TiB     436 TiB      437 TiB         68.54
> 
> POOLS:
>     POOL                      ID     STORED      OBJECTS     USED        
> %USED     MAX AVAIL
>     fs-metadata               16      44 GiB       4.16M      44 GiB      
> 0.25       5.6 TiB
>     cephfs-hdd-3x             17      46 TiB     109.54M     144 TiB     
> 61.81        30 TiB
>     objects-hybrid            20      46 TiB     537.08M     187 TiB     
> 91.71       5.6 TiB
>     objects-hdd               24     224 GiB      50.81k     676 GiB      
> 0.74        30 TiB
>     rbd-hybrid                29     3.8 TiB       1.19M      11 TiB     
> 40.38       5.6 TiB
>     device_health_metrics     33     270 MiB         327     270 MiB         
> 0        30 TiB
>     rbd-ssd                   34     4.2 TiB       1.19M      12 TiB     
> 41.55       5.6 TiB
>     cephfs-hdd-ec73           37      42 TiB      30.35M      72 TiB     
> 44.78        62 TiB

I have a few pools that don't look like they are calculating the available 
storage for that pool correctly.

Specifically, any of my hybrid pools (20,29) or all-SSD pools (16,34).

For my hybrid pools, I have a crush rule of take 1 of host in the ssd root, 
take -1 chassis in the hdd root.
For my ssd pools, I have a crush rule of take 0 of host in the ssd root.

Now I have 60 ssd osds 1.92T each, and sadly distribution is imperfect (leaving 
those issues out of this), and I have plenty of underfull and overfull osds, 
which I am trying to manually reweighs to get my most full's down to free up 
space:
> $ ceph osd df class ssd | sort -k17
> ID  CLASS WEIGHT  REWEIGHT SIZE    RAW USE DATA     OMAP    META    AVAIL   
> %USE  VAR  PGS STATUS
> MIN/MAX VAR: 0.80/1.21  STDDEV: 5.56
>                      TOTAL 107 TiB  70 TiB   68 TiB 163 GiB 431 GiB  37 TiB 
> 65.33
>  28   ssd 1.77879  1.00000 1.8 TiB 951 GiB  916 GiB 7.1 GiB 5.8 GiB 871 GiB 
> 52.20 0.80  68     up
>  33   ssd 1.77879  1.00000 1.8 TiB 1.0 TiB 1010 GiB 6.0 MiB 5.9 GiB 777 GiB 
> 57.33 0.88  74     up
>  47   ssd 1.77879  1.00000 1.8 TiB 1.0 TiB 1011 GiB 6.7 GiB 6.4 GiB 776 GiB 
> 57.38 0.88  75     up
> [SNIP]
>  57   ssd 1.77879  0.98000 1.8 TiB 1.4 TiB  1.3 TiB 6.2 GiB 8.6 GiB 417 GiB 
> 77.08 1.18 102     up
> 107   ssd 1.80429  1.00000 1.8 TiB 1.4 TiB  1.3 TiB 7.0 GiB 8.7 GiB 422 GiB 
> 77.15 1.18 102     up
>  50   ssd 1.77879  1.00000 1.8 TiB 1.4 TiB  1.4 TiB 5.5 MiB 8.6 GiB 381 GiB 
> 79.10 1.21 105     up
>  60   ssd 1.77879  0.92000 1.8 TiB 1.4 TiB  1.4 TiB 6.2 MiB 9.0 GiB 379 GiB 
> 79.17 1.21 105     up

That said, as a straw man argument, ~380GiB free, times 60 OSDs, should be 
~22.8TiB free, if all OSD's grew evenly, which they won't, which is still far 
short of 37TiB raw free, as expected.
However, what doesn't track is the 5.6TiB available at the pools level, even 
for a 3x replicated pool (5.6*3=16.8TiB, which is about 34% less than my napkin 
math, which would be 22.8/3=7.6TiB.
But what tracks even less is the hybrid pools, which use 1/3 of what the 
3x-replicated data consumes.
Meaning if my napkin math is right, should show ~22.8TiB free.

Am I grossly mis-understanding how this is calculated?
Maybe this is fixed in Octopus?

Just trying to get a grasp on what I'm seeing not matching expectations.

Thanks,

Reed

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