High Performance File System

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HPFS
Developer	Microsoft
Full name	High Performance File System
Introduced	November 1989 (OS/2 1.2)
Partition identifier	0x07 (MBR)
Structures
Directory contents	B+ tree
File allocation	B+ tree
Bad blocks	B+ tree
Limits
Max file size	2 GiB
Max number of files	Unlimited
Max filename length	256 characters
Max volume size	2 TiB
Allowed characters in filenames	Double-byte from 0x0020 to 0xFFFF
Features
Dates recorded	Access, Creation, Modified
Date range
Date resolution
Forks	Yes
Attributes	Read-only, hidden, system, archive
File system permissions	Yes (only in HPFS386)
Transparent compression	No
Transparent encryption	No
Supported operating systems	OS/2, Windows NT, Linux, FreeBSD

HPFS or 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 are:

support for mixed case file names, in different code pages
support for long file names (256 characters as opposed to FAT's 8+3 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
extent-based space allocation
separate datestamps for last modification, last access, and creation (as opposed to FAT's one last modification datestamp)
a B+ tree structure for directories
root directory located at the mid-point, rather than beginning of the disk, for faster average access

HPFS 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
HPFS386 provided with the server versions of OS/2

HPFS386's cache is limited by the available memory and was implemented in assembler. Thus, HPFS386 is faster, but IBM is required to pay Microsoft for each copy sold. It is highly tunable by experienced administrators.

IBM had still had right over HPFS , but not HPFS386.This was a result from the Microsoft and IBM collabaration that both IBM and Microsoft had right to use Windows and OS/2 technology. Microsoft used HPFS in Windows NT.

Because of the Microsoft dependence and the long disk check times after a crash, IBM ported the journaling file system JFS to OS/2 as a substitute.

DOS and Linux support HPFS via third-party drivers. Windows NT versions 3.51 and earlier had native support for HPFS.

[edit] Windows Native Support

Windows 95 and its successors Windows 98, Windows Me can read/write HPFS only when mapped via a network share, but cannot read it from a local disk. They listed the NTFS partitions of networked computers as "HPFS", because NTFS and HPFS share the same filesystem identification number in the partition table.

Windows NT 3.1 and 3.5 have native read/write support for local disks and can even be installed onto an HPFS partition. This is because NT was originally going to be a version of OS/2.

Windows NT 3.51 can also read and write from local HPFS formatted drives. However, Microsoft discouraged using HPFS in Windows NT 4 and in subsequent versions. Microsoft even removed the ability of NT 3.51 to format an HPFS file system. Starting with Windows NT 4 the filesystem driver pinball.sys enabling the read/write access is not included in a default installation anymore. Pinball.sys is included on the installation media for Windows 2000 and can be manually installed and used with some limitations.^{[citation needed]} Later Windows versions do not ship with this driver.

Microsoft decided to implement HPFS in Windows since IBM and Microsoft both have rights to Windows and OS/2 technology. The reason why Microsoft stopped supporting HPFS was because, when an Operating System like Windows or OS/2 crashes, the CHKDSK program would take a long time to check the errors on the file system.^{[citation needed]} Because of this, for example if Windows NT were to crash, and the server has 1TB volumes it would take a long time for error recovery. Newer file systems like NTFS are journaling and do not need to utilize CHKDSK.