xref: /kernel/linux/linux-5.10/Documentation/filesystems/affs.rst

.. SPDX-License-Identifier: GPL-2.0

=============================
Overview of Amiga Filesystems
=============================

Not all varieties of the Amiga filesystems are supported for reading and
writing. The Amiga currently knows six different filesystems:

==============	===============================================================
DOS\0		The old or original filesystem, not really suited for
		hard disks and normally not used on them, either.
		Supported read/write.

DOS\1		The original Fast File System. Supported read/write.

DOS\2		The old "international" filesystem. International means that
		a bug has been fixed so that accented ("international") letters
		in file names are case-insensitive, as they ought to be.
		Supported read/write.

DOS\3		The "international" Fast File System.  Supported read/write.

DOS\4		The original filesystem with directory cache. The directory
		cache speeds up directory accesses on floppies considerably,
		but slows down file creation/deletion. Doesn't make much
		sense on hard disks. Supported read only.

DOS\5		The Fast File System with directory cache. Supported read only.
==============	===============================================================

All of the above filesystems allow block sizes from 512 to 32K bytes.
Supported block sizes are: 512, 1024, 2048 and 4096 bytes. Larger blocks
speed up almost everything at the expense of wasted disk space. The speed
gain above 4K seems not really worth the price, so you don't lose too
much here, either.

The muFS (multi user File System) equivalents of the above file systems
are supported, too.

Mount options for the AFFS
==========================

protect
		If this option is set, the protection bits cannot be altered.

setuid[=uid]
		This sets the owner of all files and directories in the file
		system to uid or the uid of the current user, respectively.

setgid[=gid]
		Same as above, but for gid.

mode=mode
		Sets the mode flags to the given (octal) value, regardless
		of the original permissions. Directories will get an x
		permission if the corresponding r bit is set.
		This is useful since most of the plain AmigaOS files
		will map to 600.

nofilenametruncate
		The file system will return an error when filename exceeds
		standard maximum filename length (30 characters).

reserved=num
		Sets the number of reserved blocks at the start of the
		partition to num. You should never need this option.
		Default is 2.

root=block
		Sets the block number of the root block. This should never
		be necessary.

bs=blksize
		Sets the blocksize to blksize. Valid block sizes are 512,
		1024, 2048 and 4096. Like the root option, this should
		never be necessary, as the affs can figure it out itself.

quiet
		The file system will not return an error for disallowed
		mode changes.

verbose
		The volume name, file system type and block size will
		be written to the syslog when the filesystem is mounted.

mufs
		The filesystem is really a muFS, also it doesn't
		identify itself as one. This option is necessary if
		the filesystem wasn't formatted as muFS, but is used
		as one.

prefix=path
		Path will be prefixed to every absolute path name of
		symbolic links on an AFFS partition. Default = "/".
		(See below.)

volume=name
		When symbolic links with an absolute path are created
		on an AFFS partition, name will be prepended as the
		volume name. Default = "" (empty string).
		(See below.)

Handling of the Users/Groups and protection flags
=================================================

Amiga -> Linux:

The Amiga protection flags RWEDRWEDHSPARWED are handled as follows:

  - R maps to r for user, group and others. On directories, R implies x.

  - W maps to w.

  - E maps to x.

  - D is ignored.

  - H, S and P are always retained and ignored under Linux.

  - A is cleared when a file is written to.

User id and group id will be used unless set[gu]id are given as mount
options. Since most of the Amiga file systems are single user systems
they will be owned by root. The root directory (the mount point) of the
Amiga filesystem will be owned by the user who actually mounts the
filesystem (the root directory doesn't have uid/gid fields).

Linux -> Amiga:

The Linux rwxrwxrwx file mode is handled as follows:

  - r permission will allow R for user, group and others.

  - w permission will allow W for user, group and others.

  - x permission of the user will allow E for plain files.

  - D will be allowed for user, group and others.

  - All other flags (suid, sgid, ...) are ignored and will
    not be retained.

Newly created files and directories will get the user and group ID
of the current user and a mode according to the umask.

Symbolic links
==============

Although the Amiga and Linux file systems resemble each other, there
are some, not always subtle, differences. One of them becomes apparent
with symbolic links. While Linux has a file system with exactly one
root directory, the Amiga has a separate root directory for each
file system (for example, partition, floppy disk, ...). With the Amiga,
these entities are called "volumes". They have symbolic names which
can be used to access them. Thus, symbolic links can point to a
different volume. AFFS turns the volume name into a directory name
and prepends the prefix path (see prefix option) to it.

Example:
You mount all your Amiga partitions under /amiga/<volume> (where
<volume> is the name of the volume), and you give the option
"prefix=/amiga/" when mounting all your AFFS partitions. (They
might be "User", "WB" and "Graphics", the mount points /amiga/User,
/amiga/WB and /amiga/Graphics). A symbolic link referring to
"User:sc/include/dos/dos.h" will be followed to
"/amiga/User/sc/include/dos/dos.h".

Examples
========

Command line::

    mount  Archive/Amiga/Workbench3.1.adf /mnt -t affs -o loop,verbose
    mount  /dev/sda3 /Amiga -t affs

/etc/fstab entry::

    /dev/sdb5	/amiga/Workbench    affs    noauto,user,exec,verbose 0 0

IMPORTANT NOTE
==============

If you boot Windows 95 (don't know about 3.x, 98 and NT) while you
have an Amiga harddisk connected to your PC, it will overwrite
the bytes 0x00dc..0x00df of block 0 with garbage, thus invalidating
the Rigid Disk Block. Sheer luck has it that this is an unused
area of the RDB, so only the checksum doesn't match anymore.
Linux will ignore this garbage and recognize the RDB anyway, but
before you connect that drive to your Amiga again, you must
restore or repair your RDB. So please do make a backup copy of it
before booting Windows!

If the damage is already done, the following should fix the RDB
(where <disk> is the device name).

DO AT YOUR OWN RISK::

  dd if=/dev/<disk> of=rdb.tmp count=1
  cp rdb.tmp rdb.fixed
  dd if=/dev/zero of=rdb.fixed bs=1 seek=220 count=4
  dd if=rdb.fixed of=/dev/<disk>

Bugs, Restrictions, Caveats
===========================

Quite a few things may not work as advertised. Not everything is
tested, though several hundred MB have been read and written using
this fs. For a most up-to-date list of bugs please consult
fs/affs/Changes.

By default, filenames are truncated to 30 characters without warning.
'nofilenametruncate' mount option can change that behavior.

Case is ignored by the affs in filename matching, but Linux shells
do care about the case. Example (with /wb being an affs mounted fs)::

    rm /wb/WRONGCASE

will remove /mnt/wrongcase, but::

    rm /wb/WR*

will not since the names are matched by the shell.

The block allocation is designed for hard disk partitions. If more
than 1 process writes to a (small) diskette, the blocks are allocated
in an ugly way (but the real AFFS doesn't do much better). This
is also true when space gets tight.

You cannot execute programs on an OFS (Old File System), since the
program files cannot be memory mapped due to the 488 byte blocks.
For the same reason you cannot mount an image on such a filesystem
via the loopback device.

The bitmap valid flag in the root block may not be accurate when the
system crashes while an affs partition is mounted. There's currently
no way to fix a garbled filesystem without an Amiga (disk validator)
or manually (who would do this?). Maybe later.

If you mount affs partitions on system startup, you may want to tell
fsck that the fs should not be checked (place a '0' in the sixth field
of /etc/fstab).

It's not possible to read floppy disks with a normal PC or workstation
due to an incompatibility with the Amiga floppy controller.

If you are interested in an Amiga Emulator for Linux, look at

http://web.archive.org/web/%2E/http://www.freiburg.linux.de/~uae/