We've seen an issue wherein a tablet server crashed because of disk space issues. The thing is, the tablet server itself had space, but there were a number of disks that were full.
Note that the error message is a red herring: the failure really came from selecting a directory to place a container, not from selecting a directory to the directory group.
There were 4 full disks; presumably the tablet had a default directory group size of 3, and all of its directories were full.
It would be nice for directory groups to be dynamically resized as needed. If getting a directory for block placement yields an ENOSPC, we should consider adding a directory to the directory group based on available space or based on the number of replicas in the remaining directories.