Improving Filestructure of preview folder

[PichlAlex]

the Issue that i adress to
I am running a huge Nextcloud installation (not in filesize, but in filecount) in private use. Currently i have around 1.000.000 Files hosted on the nextcloud instance, across 4 Users. There are Issues accessing the previews (lots of disk IO when loading a single preview and viewing a listing page gets really slow) because of the foldercount in nextcloud/data/appdata_xxxx/preview/
Inside this folder i currently have ~1,3 Million folders with multiple previews in each -> 300 GB

I am using Nextcloud on ubuntu Server 16.04 on BTRFS and the Previews are stored on Raid10 7200rpm disks

Describe the solution you'd like
i would recommend to store all the folders through subfolders that in each single folder the number of Folders is limited to eg 1000 folders that still can be quickly accessed and does not cause a lot of issues when accessing.
eg - Variant 1
preview/0/0/0/0 stores images 1 to 999
preview/0/0/0/1 stores images 1000 to 1999
preview/0/0/1/0/ stores images 10000-10999
preview/0/0/5/6 stores images 56000 - 569999

eg Variant 2:
preview/1/ stores images 1 to 999
preview/1000 stores images 1000 to 1999
preview/56000 stores images 56000 to 56999
...

Describe alternatives you've considered
maybe another storing method should be applied (by hash of filelocking? (and use a kind of Variant 2)
https://help.nextcloud.com/t/info-how-the-oc-file-locks-table-actually-works/28727 )

[MorrisJobke]

Was fixed in 19 with #19214