PG-upmap offline optimization is not working as expected

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Hi,

I have tested the PG-upmap offline optimization with 1 of my pools: ssd

This pool is unbalanced; here's the ouput of ceph osd df tree before the
optimization:
root@ld3955:~# ceph osd df tree class ssd
ID  CLASS WEIGHT     REWEIGHT SIZE    RAW USE DATA    OMAP    META    
AVAIL   %USE VAR  PGS STATUS TYPE NAME
 -1       1532.88501        -  27 TiB  27 GiB 4.4 GiB 816 KiB   23 GiB 
27 TiB 0.10 1.00   -        root default
-46        353.82300        - 1.1 TiB 3.7 GiB 702 MiB 144 KiB  3.0 GiB
1.1 TiB 0.33 3.35   -            host ld4257
 20   ssd    0.37099  1.00000 371 GiB 1.2 GiB 234 MiB  44 KiB 1024 MiB
370 GiB 0.33 3.35  25     up         osd.20
 21   ssd    0.37099  1.00000 371 GiB 1.2 GiB 234 MiB  20 KiB 1024 MiB
370 GiB 0.33 3.35  27     up         osd.21
 22   ssd    0.37099  1.00000 371 GiB 1.2 GiB 234 MiB  80 KiB 1024 MiB
370 GiB 0.33 3.35  26     up         osd.22
-34        356.22299        - 4.6 TiB 4.9 GiB 936 MiB 228 KiB  4.0 GiB
4.6 TiB 0.10 1.06   -            host ld4464
 23   ssd    0.37099  1.00000 371 GiB 1.2 GiB 234 MiB  44 KiB 1024 MiB
370 GiB 0.33 3.35  29     up         osd.23
 24   ssd    0.37099  1.00000 371 GiB 1.2 GiB 234 MiB  40 KiB 1024 MiB
370 GiB 0.33 3.35  31     up         osd.24
 25   ssd    0.37099  1.00000 371 GiB 1.2 GiB 234 MiB  52 KiB 1024 MiB
370 GiB 0.33 3.35  31     up         osd.25
 26   ssd    3.48999  1.00000 3.5 TiB 1.2 GiB 234 MiB  92 KiB 1024 MiB
3.5 TiB 0.03 0.35 265     up         osd.26
-37        356.22299        - 4.6 TiB 4.9 GiB 936 MiB 152 KiB  4.0 GiB
4.6 TiB 0.10 1.06   -            host ld4465
 27   ssd    0.37099  1.00000 371 GiB 1.2 GiB 234 MiB  20 KiB 1024 MiB
370 GiB 0.33 3.35  24     up         osd.27
 28   ssd    0.37099  1.00000 371 GiB 1.2 GiB 234 MiB  20 KiB 1024 MiB
370 GiB 0.33 3.35  28     up         osd.28
 29   ssd    0.37099  1.00000 371 GiB 1.2 GiB 234 MiB  20 KiB 1024 MiB
370 GiB 0.33 3.35  22     up         osd.29
 30   ssd    3.48999  1.00000 3.5 TiB 1.2 GiB 234 MiB  92 KiB 1024 MiB
3.5 TiB 0.03 0.35 258     up         osd.30
 -3        116.65399        - 4.2 TiB 3.5 GiB 491 MiB  76 KiB  3.0 GiB
4.2 TiB 0.08 0.82   -            host ld5505
  8   ssd    3.48999  1.00000 3.5 TiB 1.2 GiB 164 MiB  20 KiB 1024 MiB
3.5 TiB 0.03 0.33 288     up         osd.8
  9   ssd    0.37199  1.00000 372 GiB 1.2 GiB 164 MiB  24 KiB 1024 MiB
371 GiB 0.31 3.16  28     up         osd.9
 10   ssd    0.37199  1.00000 372 GiB 1.2 GiB 164 MiB  32 KiB 1024 MiB
371 GiB 0.31 3.16  31     up         osd.10
 -7        116.65399        - 4.2 TiB 3.5 GiB 491 MiB  72 KiB  3.0 GiB
4.2 TiB 0.08 0.82   -            host ld5506
 11   ssd    0.37199  1.00000 372 GiB 1.2 GiB 164 MiB  24 KiB 1024 MiB
371 GiB 0.31 3.16  36     up         osd.11
 12   ssd    3.48999  1.00000 3.5 TiB 1.2 GiB 164 MiB  32 KiB 1024 MiB
3.5 TiB 0.03 0.33 260     up         osd.12
 13   ssd    0.37199  1.00000 372 GiB 1.2 GiB 164 MiB  16 KiB 1024 MiB
371 GiB 0.31 3.16  28     up         osd.13
-10        116.65399        - 4.2 TiB 3.5 GiB 491 MiB  80 KiB  3.0 GiB
4.2 TiB 0.08 0.82   -            host ld5507
 14   ssd    0.37199  1.00000 372 GiB 1.2 GiB 164 MiB  24 KiB 1024 MiB
371 GiB 0.31 3.16  24     up         osd.14
 15   ssd    0.37199  1.00000 372 GiB 1.2 GiB 164 MiB  32 KiB 1024 MiB
371 GiB 0.31 3.16  26     up         osd.15
 16   ssd    3.48999  1.00000 3.5 TiB 1.2 GiB 164 MiB  24 KiB 1024 MiB
3.5 TiB 0.03 0.33 259     up         osd.16
-13        116.65399        - 4.2 TiB 3.5 GiB 490 MiB  64 KiB  3.0 GiB
4.2 TiB 0.08 0.82   -            host ld5508
 17   ssd    0.37199  1.00000 372 GiB 1.2 GiB 164 MiB  28 KiB 1024 MiB
371 GiB 0.31 3.16  19     up         osd.17
 18   ssd    0.37199  1.00000 372 GiB 1.2 GiB 163 MiB   8 KiB 1024 MiB
371 GiB 0.31 3.16  24     up         osd.18
 19   ssd    3.48999  1.00000 3.5 TiB 1.2 GiB 164 MiB  28 KiB 1024 MiB
3.5 TiB 0.03 0.33 259     up         osd.19
                        TOTAL  27 TiB  27 GiB 4.4 GiB 823 KiB   23 GiB 
27 TiB 0.10
MIN/MAX VAR: 0.33/3.35  STDDEV: 0.20

The output of osdmaptool implies many modifications affecting osd.11 and
osd.12, means the optimizer wants to shift PGs from 12 to 11.
root@ld3955:~# source out_ssd.txt
set 66.41 pg_upmap_items mapping to [12->13]
set 66.4e pg_upmap_items mapping to [22->20]
set 66.7c pg_upmap_items mapping to [28->29]
set 66.9f pg_upmap_items mapping to [12->11]
set 66.147 pg_upmap_items mapping to [12->11]
set 66.1b1 pg_upmap_items mapping to [12->11]
set 66.203 pg_upmap_items mapping to [12->11]
set 66.257 pg_upmap_items mapping to [28->30]
set 66.27d pg_upmap_items mapping to [28->30]
set 66.300 pg_upmap_items mapping to [12->11]
set 66.354 pg_upmap_items mapping to [28->29]
set 66.35b pg_upmap_items mapping to [28->30]
set 66.38a pg_upmap_items mapping to [12->11]
set 66.3d0 pg_upmap_items mapping to [28->30]


However this makes no sense as osd.11 has already more PGs than other OSDs.
In fact there's one OSD with least PGs: osd.17

Why is the optimizer not shifting PGs to osd.17?

Here's the output of ceph osd df tree after the optimization:
root@ld3955:~# ceph osd df tree class ssd
ID  CLASS WEIGHT     REWEIGHT SIZE    RAW USE DATA    OMAP    META    
AVAIL   %USE VAR  PGS STATUS TYPE NAME
 -1       1532.88501        -  27 TiB  27 GiB 3.2 GiB 816 KiB   23 GiB 
27 TiB 0.10 1.00   -        root default
-46        353.82300        - 1.1 TiB 3.6 GiB 502 MiB 144 KiB  3.0 GiB
1.1 TiB 0.33 3.40   -            host ld4257
 20   ssd    0.37099  1.00000 371 GiB 1.2 GiB 167 MiB  44 KiB 1024 MiB
370 GiB 0.33 3.49  25     up         osd.20
 21   ssd    0.37099  1.00000 371 GiB 1.2 GiB 167 MiB  20 KiB 1024 MiB
370 GiB 0.33 3.44  27     up         osd.21
 22   ssd    0.37099  1.00000 371 GiB 1.2 GiB 167 MiB  80 KiB 1024 MiB
370 GiB 0.31 3.28  26     up         osd.22
-34        356.22299        - 4.6 TiB 4.8 GiB 668 MiB 228 KiB  4.0 GiB
4.6 TiB 0.10 1.07   -            host ld4464
 23   ssd    0.37099  1.00000 371 GiB 1.2 GiB 167 MiB  44 KiB 1024 MiB
370 GiB 0.33 3.44  29     up         osd.23
 24   ssd    0.37099  1.00000 371 GiB 1.2 GiB 167 MiB  40 KiB 1024 MiB
370 GiB 0.31 3.28  31     up         osd.24
 25   ssd    0.37099  1.00000 371 GiB 1.2 GiB 167 MiB  52 KiB 1024 MiB
370 GiB 0.33 3.49  31     up         osd.25
 26   ssd    3.48999  1.00000 3.5 TiB 1.2 GiB 167 MiB  92 KiB 1024 MiB
3.5 TiB 0.03 0.34 265     up         osd.26
-37        356.22299        - 4.6 TiB 4.8 GiB 669 MiB 152 KiB  4.0 GiB
4.6 TiB 0.10 1.07   -            host ld4465
 27   ssd    0.37099  1.00000 371 GiB 1.2 GiB 167 MiB  20 KiB 1024 MiB
370 GiB 0.31 3.28  24     up         osd.27
 28   ssd    0.37099  1.00000 371 GiB 1.2 GiB 167 MiB  20 KiB 1024 MiB
370 GiB 0.33 3.49  28     up         osd.28
 29   ssd    0.37099  1.00000 371 GiB 1.2 GiB 167 MiB  20 KiB 1024 MiB
370 GiB 0.31 3.28  23     up         osd.29
 30   ssd    3.48999  1.00000 3.5 TiB 1.2 GiB 167 MiB  92 KiB 1024 MiB
3.5 TiB 0.03 0.36 257     up         osd.30
 -3        116.65399        - 4.2 TiB 3.3 GiB 350 MiB  76 KiB  3.0 GiB
4.2 TiB 0.08 0.81   -            host ld5505
  8   ssd    3.48999  1.00000 3.5 TiB 1.1 GiB 117 MiB  20 KiB 1024 MiB
3.5 TiB 0.03 0.33 288     up         osd.8
  9   ssd    0.37199  1.00000 372 GiB 1.1 GiB 117 MiB  24 KiB 1024 MiB
371 GiB 0.30 3.13  28     up         osd.9
 10   ssd    0.37199  1.00000 372 GiB 1.1 GiB 117 MiB  32 KiB 1024 MiB
371 GiB 0.30 3.13  31     up         osd.10
 -7        116.65399        - 4.2 TiB 3.3 GiB 350 MiB  72 KiB  3.0 GiB
4.2 TiB 0.08 0.81   -            host ld5506
 11   ssd    0.37199  1.00000 372 GiB 1.1 GiB 117 MiB  24 KiB 1024 MiB
371 GiB 0.30 3.13  41     up         osd.11
 12   ssd    3.48999  1.00000 3.5 TiB 1.1 GiB 117 MiB  32 KiB 1024 MiB
3.5 TiB 0.03 0.33 254     up         osd.12
 13   ssd    0.37199  1.00000 372 GiB 1.1 GiB 117 MiB  16 KiB 1024 MiB
371 GiB 0.30 3.13  29     up         osd.13
-10        116.65399        - 4.2 TiB 3.3 GiB 350 MiB  80 KiB  3.0 GiB
4.2 TiB 0.08 0.81   -            host ld5507
 14   ssd    0.37199  1.00000 372 GiB 1.1 GiB 117 MiB  24 KiB 1024 MiB
371 GiB 0.30 3.13  24     up         osd.14
 15   ssd    0.37199  1.00000 372 GiB 1.1 GiB 117 MiB  32 KiB 1024 MiB
371 GiB 0.30 3.13  26     up         osd.15
 16   ssd    3.48999  1.00000 3.5 TiB 1.1 GiB 117 MiB  24 KiB 1024 MiB
3.5 TiB 0.03 0.33 259     up         osd.16
-13        116.65399        - 4.2 TiB 3.3 GiB 350 MiB  64 KiB  3.0 GiB
4.2 TiB 0.08 0.81   -            host ld5508
 17   ssd    0.37199  1.00000 372 GiB 1.1 GiB 117 MiB  28 KiB 1024 MiB
371 GiB 0.30 3.13  19     up         osd.17
 18   ssd    0.37199  1.00000 372 GiB 1.1 GiB 117 MiB   8 KiB 1024 MiB
371 GiB 0.30 3.13  24     up         osd.18
 19   ssd    3.48999  1.00000 3.5 TiB 1.1 GiB 117 MiB  28 KiB 1024 MiB
3.5 TiB 0.03 0.33 259     up         osd.19
                        TOTAL  27 TiB  27 GiB 3.2 GiB 823 KiB   23 GiB 
27 TiB 0.10
MIN/MAX VAR: 0.33/3.49  STDDEV: 0.19

Regards
Thomas
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