Time Machine in the "Cleaning Up..." state forever

My Time Machine is in the cleanup state for a long time (several days). The backup is to an external drive and the disk on the mac laptop is less than 500 GB. It usaually takes much less time to clean up.

Is there a way to inspect what the Time Machine actually is doing during the cleanup?

The following log is not helpful.

$ log show --debug --predicate 'subsystem == "com.apple.TimeMachine"' --info --last 4h
Filtering the log data using "subsystem == "com.apple.TimeMachine""
Timestamp                       Thread     Type        Activity             PID    TTL  
2020-02-21 13:36:41.329729-0600 0xb4acfa   Info        0x0                  98     0    backupd-helper: (TimeMachine) [com.apple.TimeMachine:TMLogInfo] Not starting scheduled Time Machine backup: Backup already running
2020-02-21 14:36:55.424274-0600 0xb60241   Info        0x0                  98     0    backupd-helper: (TimeMachine) [com.apple.TimeMachine:TMLogInfo] Not starting scheduled Time Machine backup: Backup already running
2020-02-21 15:42:20.848345-0600 0xb64cb9   Info        0x0                  98     0    backupd-helper: (TimeMachine) [com.apple.TimeMachine:TMLogInfo] Not starting scheduled Time Machine backup: Backup already running
2020-02-21 16:36:27.614466-0600 0xb8060d   Info        0x0                  98     0    backupd-helper: (TimeMachine) [com.apple.TimeMachine:TMLogInfo] Not starting scheduled Time Machine backup: Backup already running
--------------------------------------------------------------------------------------------------------------------
Log      - Default:          0, Info:                4, Debug:             0, Error:          0, Fault:          0
Activity - Create:           0, Transition:          0, Actions:           0

Good to see you @Jason Campbell! Mine is backing up to a Synology (aptly named "Hades") and there is no "*.inProgress" anywhere in the SMB-mounted Synology folder. But there is one in my local TM snapshot folder. That, however is empty and hasn't been touched in an hour:

tkunze@zork ~ % ls -la /Volumes/Time\ Machine\ Backups/Backups.backupdb/.RecoverySets/
total 0
drwxr-xr-x  4 root  wheel  136 Aug  1 19:29 ./
drwxr-xr-x@ 6 root  wheel  204 Dec 24  2019 ../
drwxr-xr-x  3 root  wheel  102 Oct 15  2020 0/
drwxr-xr-x  2 root  wheel   68 Aug  1 19:29 1.F9D6D42C-90FB-4BFD-A7E6-44E2D39CAB88.inProgress/
tkunze@zork ~ % ls -la /Volumes/Time\ Machine\ Backups/Backups.backupdb/.RecoverySets/1.F9D6D42C-90FB-4BFD-A7E6-44E2D39CAB88.inProgress
total 0
drwxr-xr-x  2 root  wheel   68 Aug  1 19:29 ./
drwxr-xr-x  4 root  wheel  136 Aug  1 19:29 ../

Meanwhile, the TM preference panel still says "Cleaning up…". It seems backupd is still reading and writing to disk, so I upped its IO priority by running

tkunze@zork ~ % sudo sysctl debug.lowpri_throttle_enabled=0
debug.lowpri_throttle_enabled: 1 -> 0

and that made a difference. And now, as I just typed that sentence, the cleanup is done. Don't forget to revert the sudo…

I noticed that cleaning up old backups can be very slow. In my case, I can hear my external backup drive rattling along, so I know it's doing something.

You can check on progress by opening the latest backup log. Look for the directory with a name ending in .inProgress on your backup drive. Within it, open the log file with the most recent timestamp:

$ sudo less +F "/Volumes/MacBackup/Backups.backupdb/MacBook Pro/2020-08-05-163227.inProgress/.Backup.618330747.626060.log"

Substitute MacBackup, MacBook Pro and the backup/log path. If you're not familiar with less, press Ctrl-C, followed by q to exit.

The log file will tell you how much space needs to be cleared and which of the backups it's (very slowly) deleting. Looking at the amount of free space on your backup disk should give you an idea of how far along it is.