Re: [SLUG] Network Real-Time Hot Filesystem Replication?

[Crossfire <xfire@xxxxxxxx>]

Mick Pollard wrote:
> On Sat, 05 Apr 2008 09:52:55 +1100
> Crossfire <xfire@xxxxxxxx> wrote:
>
> > 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.
> Have you had a look at http://www.drbd.org/ ?
> It basically mirrors a blockdevice over ethernet. 
> A raid1 of sorts.

DRBD doesn't actually solve the problem - it either provides a warm 
replication of a normal filesystem, or provides an pseudo-shared storage 
device.

Warm replication is a no-go since I will need effectively local access 
to the filesystem on both nodes so there isn't a song-and-dance routine 
to perform w.r.t mounts, etc, during a failure.

As a psuedo-shared storage device, I doubt it's of any particular use 
due to (drastically-higher) latency incurred by having to replicate the 
changes between the storage pools rather than the storage pool being 
shared.  I'd also be concerned about split-brain recovery with cluster 
filesystems (split-brain is not actually possible with a real shared 
drive, but completely possible with DRBD). Initial comments I've seen 
from people trying to use it with OCFS2 also seem poor.

C.