Amiga Fast File System
| Introduced | with 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 was too slow to keep up with hard drives. FFS differs 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
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 became the file system used by almost all Amiga users, although OFS continued to be widely used on floppy disks by software vendors for compatibility with pre-AmigaOS 2 systems (except where the software did not work at all on older systems anyway). In the case of games, software houses tended to be more broad with their support and OFS was common where the even more common custom disk formats were not used.
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. Peculiarly, FFS stores a "bitmap" of the filesystem in a single sector, which is prone to corruption or deletion if a write operation is interrupted by a crash or disk removal. This could result in a situation known as filesystem invalidation. To reverse this, the entire disk had to be scanned and the bitmap rebuilt. 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 by the invalidation. 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 partitions on the first 4 GB of the disk.
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 plethora of third-party tools like DiskSalv to maintain the file system and repair and validate it, undelete files, or reverse "quick formats" (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 partitions on the first 4 GB of the drive - otherwise they would result in corruption of later partitions. When hard drives were reaching 4 GB in size, this obviously became a real problem.
For these reasons, FFS was already being replaced by users by 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 also safer, faster and more efficient – SFS is now considered[by whom?] the default file system of both AmigaOS 4 and MorphOS.
History
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 removables. 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.
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.
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.
The four new dostypes introduced with v37 of FFS:
DOS\2: "International" (OFS-INTL) mode allows OFS to handle filenames with international characters.
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 allows the filesystem to access hard disks more rapidly 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 became a popular choice on Amiga hard drives, but according to Olaf Barthel 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.
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 30 characters.
DOS\7: Long filenames for FFS (FFS-LNFS). There were no directory caching modes for LNFS, and International Mode was 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.
See also
References
- ^ Workbench Nostalgia; history of the AmigaOS GUI: Release 1.3
- ^ 1.3 release confirmed on September 16, 1988 by Carolyn Scheppner of CATS in amiga.dev in BIX. Copy of BIX announcement from USENET
- ^ AmigaOS user manual 3.1
- ^ 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.
- ^ http://www.amigahistory.co.uk/adfaq.html#p47
External links
- The ADFlib Page and precisely ADF File specs
- The ADF specs, in LHA format from Aminet