[nwlieb]

Related: is it possible to reliably maintain physical disk space quotas in Linux (similar to cgroups)?

Furthermore, is it possible to say how much "space" you would use if you were to create a file with a given size, accounting for block-size, fragmentation, and metadata? Matters such as block-size, inode usage, and metadata seem to make this very difficult even if you add special integration to the userspace application, for example by using stat or statfs. This could help prevent quota overruns for example.

These seem like hard problems unfortunately, and I suspect the best solution is to just create separate disk partitions for each quota group.

[chungy]

First question: Quotas have been supported on Linux for a very long time. All major (and native) file systems support them.

Secondly: disk usage accounting for metadata as well as regular file data may or may not be tricky. ZFS always tells you how much data+metadata is used by a file, helped by metadata itself being dynamically allocated on ZFS like everything else is. File systems like ext4 that have fixed metadata locations on disk don't report back metadata allocation with the file; it wouldn't really be useful to see this information since removing the file doesn't free any metadata in the ext4 case.

[ifcho]

Project quotas appear to be more similar to cgroups. They are available in xfs and ext4 https://lwn.net/Articles/623835/