In the QNX 4 filesystem, regular files and directory files are stored as a sequence of extents, contiguous sequences of blocks on a disk.

The directory entry for a file keeps track of the file's extents. If the filesystem needs more than one extent to hold a file, it uses a linked list of extent blocks to store information about the extents.

When a file needs more space, the filesystem tries to extend the file contiguously on the disk. If this isn't possible, the filesystem allocates a new extent, which may require allocating a new extent block as well. When it allocates or expands an extent, the filesystem may overallocate space, under the assumption that the process will continue to write and fill the extra space. When the file is closed, any extra space is returned.

This design ensures that when files—even several files at one time—are written, they're as contiguous as possible. Since most hard disk drives implement track caching, this not only ensures that files are read as quickly as possible from the disk hardware, but also serves to minimize the fragmentation of data on disk.

For more information about performance, see Fine-Tuning Your System.