BSD Fast File System
Until the creation of the Berkeley BSD Fast File System for BSD 4.1, all prior Unixes had used basically the file system designed for the early PDP-11 versions of Unix. While very simple (in on-disk structure, and thus implementation), it had some performance, functionality and robustness issues. The FFS fixed these, so successfully that the FFS and its descendants are still the file system of most Unix descendants today.
Issues
The performance issue were the most critical, but there were lesser functionality and robustness issue as well. The FFS fixed most of these issues (although without duplication of most meta-data, and all data, it was still susceptible to hardware failures such as head crashes.
Performance
The original Unix file system was too simple to be able to get good performance out of the disks of the era. One factor was the small block size (which was to some extent ameliorated in systems prior to the FFS by using larger block sizes); the other was the extreme non-locality of data placement, which was an endemic (albeit un-intended) feature of the earlier system.
Once the system had been running for a while, free blocks were scattered fairly randomly across the disk, and the blocks comprising a new file were thus widely scattered; reading the file resulted in a great deal of head motion (seeks). There was no cure for this state of affairs, other than dumping and re-building the disk.
Functionality
Although the original file system did include so-called hard links, those were non-optimal in a lot of ways; among other issue, they were restricted to a single logical volume.
Another problematic limitation was the length of file names; although all ASCII characters could be use, the maximum length of any element (level) of a file name was only 14 characters.
Robustness
The old file system had only one copy of all the meta-data of a volume, including the super block. While some of the meta-data (such as the free list could be re-constructed from other meta-data (by scanning all the inodes), most could not.
Further reading
Marshall K. McKusick, William N. Joy, Samuel J. Leffler, Robert S. Fabry, A Fast File System for UNIX
