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1994-12-08
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Differences in HPFS and FAT File Systems 7/20/94
File:OSDIHPFT.fax
TITLE: HPFS AND FAT FILE SYSTEMS
DESCRIPTION:
What are the differences between HPFS and FAT File Systems?
How do they interface with each other under OS/2?
SUMMARY:
The following is an excerpt from A Technical Guide to OS/2 2.0,
written by Martin McElroy, IBM United Kingdom.
DETAILS:
OS/2 offers superior file system support, which leads to
increased performance. The OS/2 architecture also allows other
file systems to be installed in a modular fashion (Installable
File System or IFS). It is therefore more flexible in design
to accommodate future enhancements; for example, OS/2 now
includes CD-ROM support via an IFS.
OS/2 also provides support for very large disks, a
consideration that is particularly relevant to server
environments. OS/2 supports hard drives up to 2 GB in size.
In Windows 3.1 the limit is only 1 GB.
In addition, OS/2 2.0 provides file I/O services not only to
OS/2 applications but to DOS and Windows applications running
in VDMs. Therefore, DOS and Windows applications can take
advantage of advanced functions, without having to be modified,
since the DOS emulation of MVDM provides a compatible interface
to the file system for DOS applications.
Both major OS/2 file systems have been improved under OS/2 2.0,
allowing better performance not only for applications
themselves, but also in paging performed by system when using
virtual memory.
HIGH PERFORMANCE FILE SYSTEM (HPFS)
-------------------------------------------------------------
The following is an excerpt from A Technical Guide to OS/2 2.0
written by Martin McElroy, IBM United Kingdom.
HPFS was first introduced to OS/2 version 1.2, and is an example
of the kind of advanced function that has not yet been
implemented in less sophisticated systems like DOS. It was
introduced as an alternative to the File Allocation Table (FAT)
system which came from DOS. HPFS is particularly good for
managing large disks and partitions or large files. It provides
fast and consistent performance, outperforming DOS-based FAT
systems in nearly all cases (OS/2 2.0 implements an enhanced FAT
system that uses some similar caching features as HPFS, which
can also give high performance in many circumstances-see below.
HPFS is particularly good in disk utilization compared to FAT.
It uses a highly contiguous file allocation system, which
results in especially good performance (relative to FAT) in
accessing files or data in a cluttered or full partition. It
implements a B-Tree directory structure and search algorithm, as
opposed to sequential under FAT. HPFS also allows for multi-
threaded I/O, caching of directory pointers in memory for
quicker access of last directories used, and read-ahead and lazy
write (lazy write buffers up write requests from applications
and commits them to disk after a given time or during disk
inactivity.) These advanced features allow for substantial
performance increases and greater tuning. In addition, HPFS can
provide write error recovery on the fly with 'hotfix'facilities.
Since it is FAT-compatible at the API level, applications
running under OS/2 can use either system, and do not have to be
written specifically for one or the other. It also presents a
consistent interface to other components of OS/2 like MVDM, to
allow DOS and Windows to use HPFS volumes as if they were FAT.
HPFS also supports the use of long file names, for greater
usability, so instead of having LJS1290.TXT you can have a file
name 'Letter to John Smith September 90'. Obviously applications
need to be coded with this in mind. DOS and Windows applications
can use the 8.3 naming system on HPFS without any difficulty.
HPFS has been enhanced in version 2.0 to add performance-related
features such as command chaining (providing a list of
contiguous sector requests required to fulfill an I/O request)
and scatter/gather facilities such as are supported in Small
Computer Systems Interface (SCSI) adapters to gather physically
discontiguous pages in a data buffer, and perform I/O in a
single operation.
HIGH PERFORMANCE FILE SYSTEM CHANGES
----------------------------------------------------------------
The following is an excerpt from Chapter 6 of the IBM OS/2
Version 2.0 Technical Redbooks, Volume 1: Control Program
(Document Number GG24-3730-00), published by the IBM CORP.
The following changes have been made to the High Performance
File System (HPFS) driver under OS/2 Version 2.0: At
initialization time, the level of support provided by the device
driver is determined using the new device command 1DH-
GetDeviceSupport.
The HPFS driver passes physical addresses for data pointers, in
the appropriate request format, to the device driver. The HPFS
driver now supports command chaining, calling the volume manager
with a list of all contiguous sector requests required to
fulfill an I/O request. This function is supported for all DASD
types.
The HPFS driver supports scatter/gather by passing physical
pointers to each page in the data buffer (physically
discontiguous) as part of the I/O request. This allows I/O
controllers such as the IBM SCSI adapters which support the
scatter/gather capability to perform the I/O in a single
operation.
* The HPFS driver now supports disk caching in the IFS driver,
rather than in the device driver.
* The HPFS driver is able to recognize devices which have
outboard caches (non-system memory), and incorporate them into
the total caching scheme.
* The HPFS file system under OS/2 Version 2.0 supports a maximum
file size of 2GB. The maximum size for an HPFS volume is 512GB.
ENHANCED FAT (aka SUPER FAT)
---------------------------------------------------------------
The following is an excerpt from A Technical Guide to OS/2 2.0,
written by Martin McElroy, IBM United Kingdom.
OS/2 2.0 includes an enhanced version of the FAT file system
which is completely compatible with the FAT system under DOS.
This gives greater performance but full compatibility with
existing FAT systems. It adds features like lazy write and
improved caching to FAT. This means that DOS applications
running with FAT file system under OS/2 2.0 will be
substantially faster for disk-based operations than under DOS.
All of these benefits can be obtained without having to
reformat your hard disk - the OS/2 enhanced FAT driver works
with existing DOS FAT volumes.
FAT FILE SYSTEM CHANGES
---------------------------------------------------------------
The following is an excerpt from Chapter 6 of the IBM OS/2
Version 2.0 Technical Redbooks, Volume 1: Control Program
(Document Number GG24-3730-00), published by the IBM
Corporation.
Changes have also been made to the FAT file system driver under
OS/2 Version 2.0, in order to provide improved performance and
enhanced support for disk hardware devices.
* The FAT driver now supports command chaining.
* The driver attempts to call the volume manager with a list of
all contiguous sector requests required to fulfill an I/O
request, thus allowing multiple page-in and page-out requests
in a single logical operation.
* The FAT driver provides faster allocation of free space on the
logical drive, using a bitmap to track free clusters on the
disk.
Disk caching is now supported within the FAT driver, and has
been removed from the device driver. A cache buffer is provided
to support disk caching with the following features:
* Lazy writing.
* Lazy reading on writes, that is, the ability to write to the
cache and flush the cache to disk, but then to read the updated
information from the cache rather than requiring a physical
disk read operation.
* Asynchronous read-ahead through a multi-purpose asynchronous
read thread.
* Large cache size (theoretical maximum of 64MB, although
practical limitations will necessitate a smaller cache).
* The ability to dynamically enable and disable the cache in
response to a user command.
* Bad sectors are automatically bypassed on reads.
There are a number of advantages in performing caching in the
FAT driver rather than the device driver; more operating system
kernel services are available at this level, and intelligent
read-ahead operations can more easily be performed. Lazy
writing is also more easily implemented at the file system
level than at the device driver level.
The FAT file system under OS/2 Version 2.0 supports a maximum
file size of 2GB. The maximum supported size for a FAT volume
is also 2GB.
12/31/99