Coda File System

inode methods

From: Peter J. Braam <>
Date: Mon, 9 Feb 1998 12:28:18 -0500 (EST)
(Ted: could you think about the superblock bit (instead of the mount
option, and tell me if this would be acceptable for mainstreaming in this
form? Thanks. pjb).

Inode Methods for Coda

The fileserver will generally store its files under a directory named
something like "/vicepa". On the whole it is reasonable to assume that
/vicepa is a mount point of a partition by itself and is not used by
other programs. Access to these files should be as fast as possible.
Currently we use a tree system, but an even faster solution would be
to create raw inodes.

Under Mach there were the following system calls for this purpose:

iopen(dev, inode) -- return a filehandle for file inode on device dev.
icreate(dev) -- returns the inode number of a new file on device dev.
istat(dev, inode, statb) -- fill in the statb structure.
iinc(dev,inode) -- increase the disk link count of the file
idec(dev, inode) -- decrease the disk link count (remove file when 0).

Note that at kernel level open maps to a lookup operation which
searches a directory, and iopen would try to bypass this.  Similarly
stat maps to a lookup followed by a read-inode operation to get the
attributes of the file.  istat would bypass the the lookup. icreate is
quite different from create since it must return the inode number of
the new file which has been created.

Device numbers might be on their way out in Linux -- due to devfs so
the arguments should probably change.

Ted Tso and I discussed this over some coffee and he suggested the
following clever solution for the implementation of the inode calls.
Ext2 could be mounted -- either as a mount option or by modifying a
bit in the superblock -- with a special flag "rawinode".  Raw inode
uses some "reserved" filenames which allow for the following:

If open is called on the file: 

fd = open("/mntpnt/__#ino#__4711", flags, mode) 

ext2 will open the file by inode number.  This means that in the
lookup operation in the kernel which intercepts this open, the
directory is NOT searched for name __#ino#__4711 but instead inode
4711 is returned by the lookup.  It may be a good idea to NOT hash a
dentry for this name in the hash table, since we think that using the
inode directly might be faster.  However we will have to create a
dentry for the inode in any case.

To mimic icreate we do something similar:

fd = open("/mntpnt/__#icreate#__, O_CREAT ...) 

This lookup will be called and will -- without consulting -- the
/mntpnt directory return ENOENT, upon which ext2_create is called.
Create treats the name as a special case and finds an unused inode and
allocates it.  Now we face two choices.  Either we put a bogus name
__#inoname#__4712 in the /mntpnt directory (in which case e2fsck will
work fine), or we do not create a name in the directory, in which case
we need to modify fsck a bit (not a problem either) to not consider
the inode lost.  In fact we could ask Ted Tso for a bit in the
superblock to indicate that fsck should not move lose inodes to
lost+found. I think it is better not to create a funny name but to
modify fsck.

Since this open returns a filedescriptor, the inode number (previously
returned by icreate) can be retrieved using fstat(fd, &statbuf).

Now we have enough to do:

iinc and idec are easy too: we simply call
link("/mntpnt/__#ino#__4711", "/mntpnt/__#iinc#__")
and make similar modifications to lookup, link and unlink. 

One will have to be carefull with the dentry's involved in these
special names, but they can probably be easily prevented from entering
the dcache by modifying lookup.  In some cases having them will do no

I think the path forward with this project is now pretty clear:
- use a recent 2.1 kernel.
- hack ext2fs_lookup to treat the case of /mntpnt/__#ino#__4711 as a
special case.  Try to not put a dentry in the hash table for it. 

- hack ext2fs_create to handle /mntpnt/__#icreate#__ as a special case
etc. (this is much more dangerous since you could hose your disk!!!!).

Make sure to do your testing on a partition by itself.

- implement this as a mount option, or perhaps activate this with an
ext2fstools utility which sets a bit in the superblock, after which it
becomes automatic.  Ted will guide us further on this.

- change fsck to make ignore inodes not in a directory 

- then put it to work in Coda. 

Important files for you to read are in the linux kernel: fs/ext2/*,
and the source to fsck and libext2fs.  The kernels can be found on and the sources to fsck and libext2fs can be found by downloading Red Hat srpms for: e2fsprogs. 

Good luck, keep me posted and don't hesitate to ask questions.

- Peter -
Received on 1998-02-09 12:30:07