Markus Klotzbuecher recently announced
release of mini_fo 0.6.0. Mini_fo provides (what has been called in other
systems) a "translucent" or "copy on write" filesystem. A read-only, base
filesystem (possibly from a remote system or CDROM) can be made to appear,
via mini_fo, as a local, writable filesystem. This functionality is useful
for sharing filesystems with local overrides, live CD systems, sandboxing
applications, and more.
At its core, mini_fo performs a simple fan-out operation. Each inode,
dentry, and file structure associated with a mini_fo filesystem contains
(via its private data) pointers to two other structures of the same type.
One of them refers to the file or directory on the base filesystem; the
other, instead, is for a local version of the file or directory on a local
"storage filesystem." Both are hidden from user space, which thinks it is
dealing directly with a file stored in the mini_fo filesystem.
When a mini_fo filesystem is first created, it appears as an exact copy of
the underlying base filesystem. Any operation which reads files or
directories is simply passed through to the base filesystem, with almost no
additional overhead. In this mode, mini_fo functions as a sort of loopback
Things change, however, when a file is opened for writing. In this case,
mini_fo will create a copy of the file on the storage filesystem, with all
of the data moved over. Any subsequent operations on that file will used
the locally-stored version rather than the base version. So any changes
made will appear locally, but they will not be propagated back to the
base. Changes will be persistent across mounts as long as the storage
directory used by mini_fo is not modified by anything except mini_fo.
Modified files are not the full story, of course; mini_fo must also cope
with operations like deletes and renames. To that end, it maintains a set
of lists of files which it knows about locally; there is one list for
modified files, one for deleted files, one for files created locally, etc.
These lists are stored in-kernel as standard linked lists. They are also
written to the storage filesystem in a magic file (named
META_dAfFgHE39ktF3HD2sr, for what it's worth) and reloaded from
that file when the filesystem is mounted.
This release of mini_fo works with both the 2.4 and 2.6 kernels. Its
author claims that it is intended for use with embedded systems, and thus
has a small memory footprint. See the mini_fo web
page for more information.
to post comments)