Can one set up a write-through cache with aufs?

Question

Say I have a system with two filesystems; one fast, small and unreliable (ie: ec2 ephemeral disk) and one slow, big and reliable (ie: ebs). Can I union mount the two filesystems so that the fast, unreliable FS acts as a write-through cache in front of the slow, reliable FS?

Answer 1

You could try brsync. It's just like rsync but between branches.

With a bit of integration with inotify (ex.: if you don't write for 1 minute, launch brsync) or crontab and eventually with some kind of transaction/snapshotting management, you could get something fast and reliable (take in mind that you could lose some datas if something go down before sync ends).