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

["Amos Shapira" <amos.shapira@xxxxxxxxx>]

On Sun, Apr 6, 2008 at 2:47 PM, Jamie Wilkinson <jaq@xxxxxxxxxxxxxx> wrote:

> This one time, at band camp, Crossfire wrote:
> > Dave Kempe wrote:
> >> Crossfire wrote:
> >>> 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).
> >>>
> >> isn't this just a description of a network filesytem... say NFS?
> >
> > No.  Network Filesystems still have a distinct single storage location.
> > If that storage is taken offline, clients can only error or hang.
> >
> > With a hot real-time replicated filesystem, all involved nodes would
> > have a full local copy at all times and would be able to continue
> > operation.
>
> I agreed with your earlier decision about not using drbd because you
> wouldn't be able to write from multiple nodes to the filesystem; all the
> slaves would have to be mounted read-only.  However if you wanted to get


Can you provide links which support this?

I've been using DRBD for a few months now (just in stand-by mode, but been
following the forums and docs during that time) and all indications are
that:

1. You CAN'T mount a non-cluster-aware file system even read-only on the
secondary node since the primary will change FS-structs under the feet of
the read-only node and cause it to crash (because non-cluster-aware
filesystems assume that they are the only ones who touch that partition).
2. You CAN mount read-write on multiple nodes if you use one of the
cluster-aware filesystems (GFS and OCFS are regularly mentioned, but if you
find any other cluster-aware file system then it sounds like it will work
too).

Ref:
http://www.linux-ha.org/DRBD/FAQ#head-2cad8caa095cfb6e2935261cb595390c742ebd86


> fancy you could still use drbd (which is a great fit for all your other
> requirements) on a multi-node fileserver, and do some nifty failover using
> IP takeover.
>
> Or if you're trying to share the local disk of a lot of nodes, then what
> if
> you used DRBD on them all to replicate the block device, and run a NFS
> server on the nodes thremselves?  Yes you'd get a lot of network traffic
> between them, but it'd work, no? :)


Have you tried this suggestions? From all I read about DRBD this will cause
all secondary nodes to crash.

Cheers,

--Amos