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HPFS, short for High Performance File System, is a file system created specifically for the OS/2 operating system to improve upon the limitations of the FAT file system. It was written by Gordon Letwin and others at Microsoft and added to OS/2 version 1.2, at that time still a joint undertaking of Microsoft and IBM.
Among its improvements include support for mixed-case file names and long file names (256 characters as opposed to FAT's 11 characters), more efficient use of disk space (files are not stored using multiple-sector clusters but on a per-sector basis), an internal architecture that keeps related items close to each other on the disk volume, less fragmentation of data and a centrally-located root directory. It also can keep 64 KiB of metadata ("Extended attributes") per file.
IBM offers two kind of IFS drivers for this file system: the standard one with a cache limited to 2 MiB, and HPFS386 provided with the server versions of OS/2. HPFS386's cache is limited by the available memory. It's highly tunable by experienced administrators. Thus, HPFS386 is faster, but also IBM has to pay Microsoft for every copy sold.
There are also third-party drivers for DOS and Linux and official ones for Windows NT.