README

See recent updates and contact information at:

	http://locore.cs.washington.edu/yggdrasil/

# How to run the Yxv6 file system

We have tested it using the following setup:

- Cython 0.25.2
- Python 2
- Z3 4.4.2 (git commit e3f0aff318b5873cfe858191b8e73ed716405b59)
- Linux (with FUSE)

Install these packages before proceeding.  Other platforms or
versions may not work.

To compile:

	$ make all prod

To mount:

	$ python2 yav_xv6_main.py -o max_read=4096 -o max_write=4096 -s a -- /dev/sXX

To run verification:

	$ make verify

If your system doesn't have `cython2`, you may want to change it
to `cython` in the makefile (similarly for `python2`).

# What are the guarantees of a verified file system in Yggdrasil like Yxv6

The proof is that a file system implementation is a crash refinement
of its specification.  See the OSDI'16 paper for details.

Note that this does not mean that a verified file system in Yggdrasil
has zero bugs.  There can be bugs in the specification (or things
not modeled by the specification, like error code), the verification
toolchain, and the unverified part (e.g., the glue code to FUSE,
FUSE itself, and the Linux kernel).

# What's new in this version of Yxv6

This implementation of Yxv6 is a clean-up version.  It mostly follows
the design described in the OSDI'16 paper, with a few differences:

- the log size is doubled;
- the garbage collector (for orphan inodes) is more complete;
- ported to a new version of Cython;
- moved more code out of the unverified FUSE layer into the verified part.

You may notice changes in runtime performance and verification time
depending on your platform and tools (e.g., Z3).

# What file system features are missing from Yxv6

Yxv6 is a research prototype.  The implementation has the following
limitations:

- based on FUSE in user space than in the kernel
- Python runtime required (even after compiled by Cython)
- mtime only, no ctime/atime
- file size is limited
- verification time may vary depending on the Z3 version
- no ACL support
- no fallocate support
- no hardlinks

We don't think they are necessarily fundamental limitations of the
toolkit---feel free to send us pull requests if you add some of
these features.