Ran into an edge-case with a PBS virtual machine that had the chunks stored the host, backed by virtiofs. Garbage collection would consistently fail with "ENFILE: File table overflow"
Finding the limit wasn't the easiest since it isn't enforced...
I experienced the same issue with a Linux guest and a VirtIO-FS share containing about 1.3 million files.
In my case, the host filesystem is ext4 on LVM.
virtiofsd continuously increased its number of open file descriptors and hit its limit...