Amiga Fast File System

From Wikipedia, the free encyclopedia
Jump to: navigation, search
AFFS
Introduced (Amiga)
Features
Transparent compression No
Transparent encryption No

The Amiga Fast File System (FFS; not to be confused with the identically named Berkeley Unix FFS) is a file system used on the Amiga personal computer. The previous Amiga filesystem (known originally simply as "DOS" or AmigaDOS) upon the release of FFS became known as Amiga Old File System (OFS). OFS, while fine on floppy disk, soon proved too slow to keep up with era hard drives. FFS was designed as a full replacement for the original Amiga filesystem.

FFS differs from its predecessor mainly in the removal of redundant information. Data blocks contain nothing but data, allowing the filesystem to manage the transfer of large chunks of data directly from the host adapter to the final destination.

Characteristics[edit]

OFS, the predecessor to FFS, used 24 bytes per sector for redundancy data, providing for reconstructing structural data on less reliable media. When higher speed media (i.e. hard disks) became more available to the Amiga, this redundant data posed a bottleneck as all data needed to be realigned to be passed to the application. So the redundancy was removed for FFS and the data read in from media could be passed to the application directly.

FFS was rapidly adopted as the filesystem used by almost all Amiga users, although OFS continued to be widely used on floppy disks by software vendors. This was purely for compatibility with pre-AmigaOS 2 systems in games and applications that did not actually require AmigaOS 2+, as machines running earlier versions of the OS without FFS in the ROM could not boot from these floppies. Many, if not most, Amiga games of the time required booting directly from the game disks.

FFS is simple and efficient, and when introduced was more than adequate, and had many advantages compared to the file systems of other platforms. However, as OFS had done before it, it aged; as drives became larger and the number of files on them increased, its use became increasingly problematic. Despite this, it is still used on Amiga systems even today.

By the last Commodore release of AmigaOS, 3.1, FFS was still the only filesystem shipped as standard with the Amiga, but was already showing its age. FFS (and OFS) stores a "bitmap" of the filesystem in a single sector. On write, this is first marked as invalid, then the write is completed, then the bitmap updated and marked as valid. If a write operation is interrupted by a crash or disk removal, this then allows disk-validator to undo the damage. This is a very simple form of filesystem journaling. To allow the disk to be used again, the entire disk has to be completely scanned and the bitmap rebuilt, but only the data being modified during the write would be lost.

AmigaOS included a disk validator, originally on disk (which was unfortunately prone to being replaced by viruses to allow themselves to spread) or, more sensibly, as part of the ROM (from Kickstart 2.x onwards). The disk validator attempted to repair the bitmap by write-protecting the drive and scanning it; this could take a long time and made it very slow to access the disk until it was finished, especially on slower media. As hard drives got larger and contained more files, the validation process could take many hours. In addition, files and directories were prone to being lost (often without user notification) during the process if their data hashes were corrupted. In some cases the validator could even fail, forcing the user to use a third-party disk tool like DiskSalv to make the volume writable again – and DiskSalv could only be used on drives smaller than 4 GB.

Although originally limited to 32-bit addressing (and therefore about 4 GB drives), FFS belatedly got some third-party 64-bit patches and then real (but non-Commodore) updates to allow it to circumvent these limitations.

In terms of support tools, although Commodore itself only shipped the much-despised DiskDoctor (and later removed it from AmigaOS disks), FFS had a small selection of third-party tools like DiskSalv to maintain the file system and repair and validate it, undelete files, or reverse "quick formats" (filesystem initializations). An OFS or FFS volume had to be locked to defragment or convert to different FFS modes to prevent corruption – this made it inaccessible to everything but the tool defragmenting it. Most of these tools were not updated by the time FFS became capable of 64-bit addressing and could only operate on drives smaller than 4 GB of the drive; they could not read partitions bigger than 4 GB, and would generally corrupt partitions "beyond" the 4 GB boundary. When hard drives were reaching 4 GB in size, this obviously became a real problem.

For these reasons, FFS was being replaced by users in the mid-1990s with more up to date alternatives such as Smart File System (SFS) and Professional File System (PFS), which did not have these limitations and were considered safer, faster and more efficient. SFS in particular kept being developed.

History[edit]

FFS was introduced with version 1.3 of AmigaOS[1] in 1988.[2] It carried the version number of v34, like the rest of the OS.

FFS was backwards compatible with the older filesystem (which became known as OFS), but Amiga machines with older versions of the Kickstart ROM could not boot from FFS-formatted floppy disks. In the case of hard drives, the FFS filesystem could be placed onto the drive RDB (Rigid Disk Block) and executed on boot; it could also be kept on disk for mounting other FFS volumes. Apart from this inability of older systems with FFS in ROM to boot, FFS did not have any compatibility problems with Amiga software, even ones that were considered "system-unfriendly".

FFS operated in several modes, defined by "dostypes". AmigaOS filesystems are identified by a four letter descriptor which is specified either in the RDB or a mountlist or dosdriver; alternatively (as was the case in trackdisk-like devices like floppy disks), the disk itself could be formatted in any dostype specified.

FFS dostypes were as follows:[3]

DOS\0: The original Amiga filesystem (OFS). This was left in for compatibility purposes, and the majority floppy disks shipped by software companies or as magazine coverdisks used this dostype so that they would boot on pre-2.x machines like the Amiga 500. It also meant that users with existing OFS-formatted drives could read them once they had installed FFS to the RDB.

DOS\1: The new filesystem, FFS. The first, disk-based releases of FFS did not have any additional modes.

AmigaOS 2.04 made FFS (now v37) part of the Kickstart ROM and introduced new modes for handling international characters in filenames,[4] and for an on-disk directory cache. Each new mode was available with both OFS and FFS dostypes. This odd system was for parity: OFS modes apart from DOS\0 were almost never used but were available nonetheless. (It should be noted that, although OFS, they were still not compatible with Amiga systems without FFS).

The four new dostypes introduced with v37 of FFS:

DOS\2: "International" (OFS-INTL) mode allows OFS to handle filenames with "international characters" - i.e. those not found in English, such as ä and ê.

DOS\3: International mode, for FFS (FFS-INTL). This was the most commonly used FFS mode. (All higher dostypes have international mode always enabled.)

DOS\4: "Directory Cache" (OFS-DC) mode enabled to primitive cache by creating dedicated directory lists instead of having to pick up the linked directory/file entries that lie scattered over the disk. A certain (small) amount of disk space to store the data is allocated.[5] The DirCache option improved directory reading speed drastically but creating, deleting and renaming files became slower. It did not increase the speed of reading individual files.

It became a popular choice on Amiga hard drives, but according to Olaf Barthel, author of FFS2, the use of dircache modes was probably better for floppy disks than it was for hard drives, where it would cause an overall degradation in performance compared to lack of dircache. Despite this it was rarely used on floppy, particularly because of the cache eating precious space, and because of the limited space preventing there being many files to cache in the first place. (According to Barthel it may also have had some serious implementation bugs that would compromise the filesystem over prolonged usage, although the specifics were never released.)

DOS\5: Directory caching with FFS (FFS-DC). Both dircache modes were not backwards compatible with earlier versions of FFS.

Version 40.1 was the last version of FFS released by Commodore, and came with AmigaOS 3.1, both on the OS disks and in the ROM. After this, several Unofficial patches appeared which allowed its use on drives after the first 2 GB of a hard disk using a 64-bit addressing system called TrackDisk64 or TD64 (although the 2 GB limit on file size and the 127 GB limit on partition sizes remained, as it was a limitation of AmigaOS dos.library and all then-current Amiga software) and carried the version number of v44. The version of FFS that came with AmigaOS 3.5 and 3.9 was v45 and differed in that it used a different 64-bit addressing system, New Style Device or NSD.

More recently (from 2003), MorphOS and AmigaOS 4 have introduced support for a slightly updated "FFS2", by Olaf Barthel (FFS v46, v50 respectively). This is compatible with the older FFS. It is PowerPC native, and introduced two more dostypes (which are not capable of being read by the older FFS):

DOS\6: "Long Filename" (OFS-LNFS). This allowed files to have a longer filename (up to 107 characters) than the usual Amiga limit, which was 31 characters.

DOS\7: Long filenames for FFS (FFS-LNFS).

There were no directory caching modes available for LNFS dostypes, and International Mode was always enabled.

Despite the ability to use the long filenames, by this time FFS compared very poorly to other available filesystems on the platforms it was available for. Apart from these extra dostypes, there are little or no functional difference between FFS and FFS2 (although some older non-specified bugs may have been dealt with) and should still not be used except for legacy purposes. Disk validation is still necessary in FFS2 (and may still result in data loss) just as it was on FFS, despite early beliefs to the contrary.

Other implementations[edit]

There were few other implementations which are able to read FFS filesystems, which would normally require an Amiga emulator and a copy of the operating system ROMs to be read. Most notably, support for affs can be compiled into Linux kernels, and offers full read, write and format support on FFS and OFS partitions of all dostypes except DOS\6 and DOS\7 (which are probably incredibly rare).

On the Amiga, the freeware application xfs[6] could, among many filesystems, read and write to devices formatted in OFS or FFS, and was probably the sole Amiga filesystem apart from FFS/FFS2 itself to do so. It did not support DOS\6 or DOS\7, or formatting of devices.

See also[edit]

References[edit]

  1. ^ "wb_13chg.txt". Retrieved Nov 12, 2012. 
  2. ^ 1.3 release confirmed on September 16, 1988 by Carolyn Scheppner of CATS in amiga.dev in BIX. Copy of BIX announcement from USENET
  3. ^ AmigaOS user manual 3.1
  4. ^ All Amiga file systems can handle international characters but prior to the International versions they treated upper and lower cased letters as completely distinct characters.
  5. ^ http://www.amigahistory.co.uk/adfaq.html#p47
  6. ^ http://de4.aminet.net/disk/misc/xfs.readme

External links[edit]