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[SLUG] Network Real-Time Hot Filesystem Replication?


I've just spent some time quickly researching this to no real satisfaction.

What I'm looking for is a way to do real-time hot-replication of a whole filesystem or filesystem tree over 2 nodes (and strictly 2 nodes) without STOMITH[1].

The scenario is I have two identical systems with local (software) RAID1. They will be tethered onto their internet feed via ethernet, and can optionally be tethered to each other via Gig.

I want to be able to set it up so /home (and maybe other filesystems) are replicated from one to the other, in both directions, in real time so they can run in an all-hot redundant cluster.

The environment should be mostly read-oriented, so I can live with write-latent solutions as long as they handle the race/collision gracefully (preferably by actually detecting and reporting it if they can't avoid it).

The options I've investigated so far:

* Lustre (MDS requirements[2] make this not an option)
* GlobalFS (STOMITH requirements make this not an option.  Oriented
  towards shared media too, which I am not using)
* tsync (Naive concurrent operation model, but otherwise viable)
* MogileFS (not quite what I was looking for, but none the less useful).
* OpenAFS (read-only replication only, loss of the node hosting the
  write volume still renders the volume unwritable).

Is anybody aware of any other options that I've missed?

C.

[1] "Shoot The Other Machine In The Head" - the ability for any node to
    forcibly powerdown any other node believed to be malfunctioning.
[2] Single instance MDS only, only clusterable through shared storage.
    d'oh.
[3] People suggesting rsync will be taken out back and shot for not
    reading the requirements.