Professional File System

Commercially sold filesystem software for the Amiga.

Technically very interesting filesystem. It tends to perform very well, which is mainly because of the simplicity of design, almost to a point of elegance.

The device is split into two main areas. At the beginning of the device is the metadata section, which consists of a root block, and a generic array of blocks that can be allocated to store metadata. The rest of the device is another contiguous generic array of blocks that can be allocated to store data. The metadata section usually uses a few percent of the device, depending on the size of the device. I believe that when either section becomes full it can allocate into the other section, but I'm not sure.

The metadata is stored as a tree of single blocks in the metadata section. The entire directory structure is recorded in the metadata, so the data section purely contains data from files. The metadata describes the location of data in files with extents of blocks, which makes the metadata quite compact.

When a metadata update occurs, the system looks at the block containing the metadata to be changed, and copies it to a newly-allocated block from the metadata section, with the change made, then it recursively changes the metadata in the block that points to that block in the same way. This way, eventually the root block needs to be changed, which causes the atomic metadata update.

The filesystem is reasonably good at keeping files unfragmented, although there is a defragmentation tool available which will work on an online filesystem.

As is common with Amiga filesystems, the filesystem does not need to be unmounted in order to be "cleanly" mounted afterwards.