My experience has been similar. I have a large music library - over 375,000 songs, and I have them kept on a RAID volume rather than copied into my library. I am a DJ and I import new music Monthly, usually on the order of thousands of songs at a time. Occasionally, I have to move folders around. iTunes and the Music app have no means of allowing me to move the physical file location from within the application, so this sometimes forces me to delete a bunch of stuff from the library and re-add it (usually tens of thousands of songs at a time). This has been quite a test against iTunes - which struggled to manage it. I understand that's not a common use case, and I chalked the performance issues up to old hardware.
When I upgraded hardware, I was appalled to find that the Music application in Catalina, keeps losing a large number of my files. I import songs, everything works until I close the Music app, and then most of them are gone the next time I open the application. I've tried this on multiple machines, on an iTunes->Music upgrade and also on a clean install. The application does not appear to be able to manage large libraries properly. I searched these discussion boards, and found that many people have had the same experience - most of them just gave up.
Things I've observed:
* Opening the application while holding option, browsing to the original ITL file is slightly better. But the application still loses songs and playlists when I exit.
* Library.musicdb doesn't change immediately when I add music. The application appears to keep a working copy in memory somewhere, and another process or agent actually commits the changes to the database. This is where everything goes wrong.
* If you add a LOT of songs at once, it will stop updating the Library.musicdb, and any changes you make after that point will disappear the next time you open Music.
* When this abandonment of committing the library occurs, the console is usually filled with log messages about "track not created because it is not new and does not exist"
* Importing from the iTunes XML (rather than opening the ITL) or importing an XML with over 150,000 tracks in it, will make the symptoms appear almost immediately
I've been able to kind of skirt around the problem by doing the following. You lose your import dates and play stats, but it prevents the Music app from losing stuff when you close it:
* Open Music, and spend a long time putting everything into it again.
* Create a new playlist that contains all your songs (or split them up into multiple playlists if you want - I did mine by Genre) -- just ensure you all of your songs are in one or more playlists
* Export the library to a new XML file
* Export each of your smart playlists to separate XML files
* Close and re-open Music. Observe that all your hard work is gone again.
* Close Music. Open music while holding the Option key.
* Choose a location for your new library.
* Use the pieterderycke/itunes-export tool on GitHub will turn your iTunes XML into m3u playlists.
* Open each m3u in a text editor like TextMate or Atom, and ensure the path to the files is correct. I had to do a search/replace, because the paths the tool output started with '../'.
* Break any playlist that's bigger than 3MB into multiple smaller files. You can do it in a text editor, but I'm old school andI used terminal:
$ cat BigList.m3u | wc -l
$ cat BigList.m3u | head -n25000 > BigList_1.m3u
$ cat BigList.m3u | head -n50000 | tail -n +25001 > BigList_2.m3u
$ cat BigList.m3u | head -n75000 | tail -n +50001 > BigList_3.m3u
$ cat BigList.m3u | head -n100000 | tail -n +75001 > BigList_4.m3u
* Import each m3u file, one at a time. Wait for music to finish processing each file, and doing gapless playback analysis.
* Watch the Library.musicdb to ensure that it grows after each playlist you import. This took several minutes, sometimes, after I imported an m3u. If you import an m3u and the library stops growing, the playlist is too large - stop importing, close Music, split that m3u into smaller chunks, and reopen Music before trying again.
* Import your smart playlists from the XMLs. If any are dependent on other playlists, you will likely need to update them.
* Backup your Library.musicdb file
* Most importantly -- write an angry letter to Apple about this. Many people have libraries unsuitable for storage in the cloud. They give us literally one way to sync music to our phones and then make the application so brittle that it crumbles under even the slightest amount of load. We can't be quiet about it.
Hope this helps someone.
