Editing Amiga Fast File System (section)

==Characteristics==
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OFS was the predecessor to FFS. Before FFS was released, [[AmigaOS]] had a single filesystem simply called AmigaDOS: this uses 24 bytes per [[disk sector|sector]] for redundancy data, providing for reconstructing structural data on less reliable media such as floppy disks. When higher-speed media (i.e. [[hard disk]]s) 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. The redundancy was removed with FFS and the data read in from media are passed to the application directly. The previous filesystem, AmigaDOS, was renamed OFS, Old File System, to differentiate between it and FFS. FFS was [[backward-compatible]] and could access devices formatted with OFS.

Given these advantages, FFS was rapidly adopted as the most common filesystem used by almost all Amiga users, although OFS continued to be widely used on floppy disks from third-party 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 [[read-only memory|ROM]] could not boot from these floppies, although they could still read them if they had FFS installed.)

Amiga 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 as a day-to-day filesystem became more problematic in terms of difficulty of maintenance and competitiveness of general performance. Despite this, it is still used on AmigaOS systems and shipped with both [[MorphOS]] and [[AmigaOS 4]].

By the last [[Commodore International|Commodore]] release of AmigaOS, 3.1, FFS was still the only filesystem shipped as standard with the Amiga, but it was already showing its age as technology advanced. FFS (and OFS) stores a "[[bit array|bitmap]]" of the filesystem in a single sector. On write, this is first marked as invalid, then the write is completed, then the bitmap is updated and marked as valid. If a write operation is interrupted by a [[Crash (computing)|crash]] or disk removal, this then allows the 'disk-validator' program to undo the damage. This resembled a very simple form of [[Journaling file system|filesystem journaling]]. To allow the disk to be used again with an invalidated OFS or FFS filesystem, the entire disk has to be completely scanned and the bitmap rebuilt, but only the data being modified during the write would be lost. During this scanning the disk cannot be written to (except by the disk-validator as it performs its function), and read access is very slow.

AmigaOS originally included a disk-validator on every bootable disk, which was prone to being replaced by [[boot sector virus|viruses]] to allow themselves to spread (for example the "Saddam Hussein" virus). Later it became part of the ROM from [[Kickstart (Amiga)|Kickstart]] 2.x onwards, protecting it from malicious replacement. The disk-validator attempted to repair the bitmap on an invalidated drive 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 could feasibly be lost (often without the user being notified or even aware) during the process if their data hashes were corrupted. In some cases the validator could fail and leave the disk in a non-validated state, requiring the user to use a third-party disk tool like DiskSalv to make the volume writable again, or simply save the files by copying them to a fresh [[disk partition|partition]]—a very slow process.

FFS was also originally limited to 32-bit [[address space|addressing]] and therefore about 4&nbsp;GB drives, or at least the first 4&nbsp;GB on a larger drive. Attempting to use FFS partitions beyond this limit caused serious [[data corruption]] all through the drive. FFS belatedly got some third-party 64-bit [[patch (software)|patches]] and then official (but non-Commodore) updates to allow it to circumvent these limitations. The latter were supplied with AmigaOS&nbsp;3.5 and 3.9, from Haage & Partner. The former often were supplied with third-party disk controllers, such as those from [[Phase5]], where the ability to use large-capacity disks was a selling point. The two systems were not mutually compatible.

In terms of support tools, although Commodore itself only shipped with an application called DiskDoctor (and later removed it from AmigaOS disks), FFS had a small selection of third-party tools—most notably 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 and this made it inaccessible to everything but the tool defragmenting it. Most of these tools were not updated when FFS became capable of 64-bit addressing and could only operate on partitions smaller than 4&nbsp;GB; they could not read partitions bigger than 4&nbsp;GB, and would generally corrupt partitions "beyond" the 4 GB boundary. When hard drives in use by Amiga users were reaching 4 GB in size, this became a problem.

For all of these reasons, FFS was often 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 continued to be developed and is now{{When|date=May 2017}} as close to a generic AmigaOS filesystem as FFS, and is the default filesystem for [[AROS Research Operating System|AROS]].