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Second extended file system (ext2fs) and MS-DOS
file system support for Lites
-----------------------------------------------
This file describes briefly how support for these two file systems
was implemented in Lites. It also provides some hints for users and
developers.
+ EXT2FS Support
----------------------------------------------------------------------------
The ext2fs support is embedded in the ufs framework, and it is based
on the ffs implementation. The rationale for this is that ffs and ext2fs
are very similar filesystems - both use groups, multilevel indexing,
a similar on-disk inode format and almost identical directory formats.
The main differences are that ext2fs uses smaller blocksizes and doesn't
support fragments. There are minor differences in the directory format.
Furthermore, the allocation policies differ. Ext2fs's policy is mainly
extent-based, aiming for contiguous allocation, whereas ffs' policy
takes various disk parameters into account to aid its strategy.
A first approach to use the Linux code base and provide it with glue
code so that it could use Lites' buffer cache and could be used by
Lites' VFS layer failed. The reasons are threefold: first, the buffer
cache interfaces are different, second, the amount of glue code
for the vfs layer would have been rather large, and finally it would
have been necessary to guarantee all the implicit assumptions the
ext2fs code makes about the Linux kernel.
Therefore, I decided to start with the ffs code. The translation was then
straightforward.
The differences in the DIRECTORY FORMAT required a customized readdir
routine, plus customized routines to enter or remove directory entries.
Since those routines are not part of the VFS abstract interface, the
ufs code dynamically checks the type of the vnode and calls the appropriate
function. A second difference is that ext2fs and ffs have different
notions of a basic unit in a directory (DIRBLKSIZ). These two changes
were the only changes the ufs code suffered; the alternative would have
been to clone large parts of the code.
Linux's ALLOCATION POLICIES are enforced. This includes preferences of
where to allocate blocks and inodes as well as the preallocation of up
to eight contiguous blocks that help lay out ext2fs files contiguously
under contention. The allocation code was the only code that came
from the Linux source. At this point, it includes some i386 specific
asm macros which were also taken from Linux.
PORTING EXT2FS TO OTHER *BSD FLAVOURS: from what was said above it
should be clear that it should be sufficient to trace the changes the
ufs and ffs code experienced and apply them to the ufs/ext2fs code.
(IMHO, it's a pity that there is this jungle of different versions of
ufs in the different *BSD flavours.) Anyone who wants to do that is
requested to #ifdef LITES the changes he/she makes, and provide us
with the changed version. (This is my personal effort to reunite the
BSDs - where it makes sense. I know it's poised to fail).
SUPPORT BINARIES FOR EXT2FS: I aided Ted Ts'o in porting his e2fsprog
package to *BSD. The next release 0.5c will run on NetBSD and FreeBSD
systems. A prelease (as of 8/17/95) can be obtained from:
ftp://tsx-11.mit.edu:/pub/linux/ALPHA/ext2fs/e2fsprogs-0.5c-WIP.tar.gz
UNIMPLEMENTED FEATURES:
- The LINUX SPECIFIC mount options such as resuid/resgid have not been
implemented.
- VFS cluster_read is enabled, cluster_write doesn't work stable and
is disabled. Reallocblks is also not implemented. Due to the small
block size and the Lites implementation of pagemove, write clustering
actually reduces performance.
----------------------------------------------------------------------------
+ MSDOSFS support:
The msdosfs support was taken almost AS IS from the FreeBSD
2.0.5-snapshot 7/26/95. Only minor changes were needed, and these could be
incorporated in a single header file. In addition, FreeBSD extensions
not present in Lites had to be commented out (such as NFS cookies in readdir)
Jordan K. Hubbard cautioned that it doesn't run stable in read-write mode
in FreeBSD. He mentioned that "dd if=/dev/zero of=<dosfile>" would reproduce
the failure. (The disklabel gets supposedly overwritten, causing what he
referred to as partition pizza.) He uses a big DOS partition (>700 MB).
Even though I was not able to reproduce this failure on my small DOS
partition, please be aware that you're using this software at your risk.
Since the changes to the code are really minor, the problem is likely to
occur in Lites as well, provided it doesn't lie elsewhere.
----------------------------------------------------------------------------
If you have any comments/questions, please send mail to gback@cs.utah.edu.
I sincerely appreciate any feedback.
Godmar Back
University of Utah, CSL
August 29, 1995
|
