Is there an async FUSE filesystem for replication?

I have a low-power VIA x86 PC with one SATA port. I want to backup special files (ie: /var) from the SATA drive on this port onto a USB flash disk, but I don't want a RAID1 or fully-synchronous solution for performance reasons.

It is acceptable that data isn't entirely 100% durable/reliable, but I am looking for something more elegant than rsync-ing every 5minutes in crontab.

You could use the lsyncd tool to automate your rsyncs - that uses the filesystem notifier to re-run rsyncs as files change.

Apart from that the other replication options are a lot more heavy-weight, using gluster backed onto both your "real" location and your "backup" would give you mirroring but the extra overhead is noticeable.