Randall A Sindlinger on 13 Dec 2010 07:47:36 -0800

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Re: [PLUG] 3-way data mirror?

On Sun, Dec 12, 2010 at 06:06:36PM -0500, JP Vossen wrote:
> >Date: Sun, 12 Dec 2010 00:52:00 -0500
> >From: JP Vossen<jp@jpsdomain.org>
> >
> >I have a situation where I have 2 labs that need to keep ~150G of data
> >in sync, but they can't talk to each other.  They can talk to a 3rd
> >machine, which is also backed up, so...
> >
> >What I think I'd like to do is have:
> >
> >	Lab1<-->  COLO<-->  Lab2
> >
> >The problem is that lab1 and lab2 are both read-write, so there is a
> >real possibility of stepping on changes.  COLO is read-only.
> >
> Stuff I forgot:
> 1) I have only limited control over the COLO server.  I can probably
> get something in a repo installed, more than that is iffy.  

unison is in a lot of distros, but doesn't look like it's in RHEL5.
If "repo" also includes third-party ones, it shouldn't be a problem,

> 2) The COLO server is 32-bit RHEL5, the 2 Lab servers are 64-bit
> CentOS-5.5 and CentOS-5.4 (I can upgrade that one).
> 3) AFAIK, the only connectivity is SSH and I can't change that.
> 4) The WAN links are very slow, relative to, say, FiOS.
> 5) The data is a mix of large (DC & DVD ISOs) and small (docs,
> configs, RPMs, etc.).

<Shot in the dark>

Why not split your data?  Put the large ISOs and things that *won't*
change on COLO, and sync them to Lab1 and Lab2 nightly (in case
something gets deleted, etc)  Caveat: making changes to that section
would have to go through you, then, I suppose.

Then, for the smaller stuff, use sshfs.  If COLO doesn't *have* to 
be RO, then use sshfs from Lab1 and Lab2 to COLO, and rsync back
to Lab1 and Lab2 to a separate space if you want to be able to 
handle emergencies where COLO is offline.
If COLO needs to be RO, then perhaps an ssh-tunnel going through
COLO and doing sshfs that way might work.

</Shot in the dark>


> I also thought about DRDB, but I'm pretty sure IT would shoot that
> down for the COLO server.  And I'm not sure if/how that would work
> 3-way.
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