Comment 0 for bug 1986747

Ubuntu packaging is configuring `osd_class_dir` with a relative path `CMAKE_INSTALL_LIBDIR` instead of the required absolute path `CMAKE_INSTALL_FULL_LIBDIR` [0].

The default value for `osd_class_dir` changed in Quincy, starting with v17.1.0 [1].

The ceph-osd service relies on the `osd_class_dir` path to find and load class libraries that extend RADOS [2]. When this is set incorrectly, RADOS clients fail with repeated "Operation not supported" errors:
```
2022-08-16T17:42:15.044+0000 7fe375685e40 0 rgw main: ERROR: failed reading data (obj=default.rgw.log:bucket.sync-target-hints.), r=-95
2022-08-16T17:42:15.048+0000 7fe375685e40 0 rgw main: ERROR: failed to read targets index for bucket=:[]) r=-95
2022-08-16T17:42:15.048+0000 7fe375685e40 0 rgw main: ERROR: failed to initialize bucket sync policy handler: get_bucket_sync_hints() on bucket=-- returned r=-95
2022-08-16T17:42:15.048+0000 7fe375685e40 -1 rgw main: ERROR: could not initialize zone policy handler for zone=default
2022-08-16T17:42:15.048+0000 7fe375685e40 0 rgw main: ERROR: failed to start notify service ((95) Operation not supported
2022-08-16T17:42:15.048+0000 7fe375685e40 0 rgw main: ERROR: failed to init services (ret=(95) Operation not supported)
```

The ceph-osd service will also report `_load_class` errors:
```
2022-08-16T19:05:55.562+0000 7f4770ff9700 0 _load_class could not stat class lib/x86_64-linux-gnu/rados-classes/libcls_rbd.so: (2) No such file or directory
```

Admins can resolve this issue by manually setting `osd_class_dir` to the correct value. Run the following command on a ceph-mon:
```
sudo ceph config set global osd_class_dir /usr/lib/x86_64-linux-gnu/rados-classes
```

Then restart all ceph-osd services to pick up the new `osd_class_dir` location.

[0] https://cmake.org/cmake/help/v3.24/module/GNUInstallDirs.html#result-variables
[1] https://github.com/ceph/ceph/commit/3bee4b02611459b9ae949cebf5967e4d83ef55de
[2] https://docs.ceph.com/en/latest/dev/osd-class-path/