dynamic inode allocation

Theodore Tso tytso at mit.edu
Mon Sep 1 18:37:44 UTC 2008

On Mon, Sep 01, 2008 at 01:18:31PM -0400, Mag Gam wrote:
> This maybe a newbie question but how come other file systems such as
> ReiserFS and Veritas' Vxfs dynamically allocate inodes and filesystems
> such as ext2/ext3 and JFS we need to allocate them when creating the
> filesystem? Is there a performance or maintenance gain when pre
> allocating?

Having a static inode table is definitely much simpler than a dynamic
inode table, and that's why ext2 originally used a static inode
allocation system.  Ext2 drew much of its initial design inspiration
from the BSD Fast Filesystem, and it (along with most traditional Unix
filesystems) used a static inode table.  

One of the advantages of having a static inode table is you can always
reliably find it.  With a dynamic inode table, it can often be much
more difficult to find it in the face of filesystem corruption, caused
by either hardware or software failure.  For example, with Reiserfs,
the inodes are stored in a B-Tree.  If the root node, or a relatively
high-level node of the B-tree is lost, the only way to recover all of
the inodes is by looking at each block, and trying to determine if it
"looks" like part of the filesystem B-tree or not.  This is what the
reiserfs's fsck program will do if the filesystem is sufficiently
damaged.  Unfortuntaely, this means that if you store reiserfs
filesystem image (for example, for use by vmware, or qemu, or kvm, or
xen) in a reiserfs filesystem, and the filesystem gets damaged, the
recovery procedure will take every single block that looks like it
could have been part Reiserfs B-tree, and stich them together into a
new-btree.  The result, if you have Reiserfs filesystem images is
those blocks will get treated as if they were part of the containing
filesystem, and the result is not pretty.

These problems can be solved (although they were not for Reiserfs),
but it means a lot more complexity.

							- Ted

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