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#linux-cluster (irc://irc.freenode.net/linux-cluster) at irc.freenode.net first. Don't forget to add your answer here if you found something out (this is no help desk).

When a machine in a GFS cluster goes down unexpectedly, how is data loss prevented -- are full redundant copies kept, or some sort of parity information? (Any references about how GFS achieves robustness against node crashes appreciated.)

Try not to confuse machines (nodes) with the storage media itself, which is separate from the nodes, and shared by (up to) all nodes in the cluster. For example, think of a JBOD disk array connected to two nodes by fibre channel; each node has direct access to the storage, but the storage is external to the nodes. When a node crashes, the data on the disks remains available to all surviving nodes. GFS has a journal system for filesystem metadata. The journal is a redundant copy of the metadata, written to shared disk in a special journal area before the "real" metadata gets written to disk in its proper location. Each node has its own journal area; when a node crashes, another node comes in and replays the journal, basically writing the journal's copy into the "real" location on disk. If all nodes crash at once, all of the journals will be replayed when mounting the filesystem again after reboots. Note that metadata journaling does not protect the actual file data itself, but just protects the filesystem layout (block allocation, inode info, etc.) (hmm, does GFS now provide an option for data journaling? Yes, see inherit_jdata).

Which are the absolute minimum hardware requirements for a test setup?

You can use gnbd (block device over tcp) instead of a san

There seems to be a voting mechanism, does that only apply to fencing or also to failover?

I think voting is needed to detect split-brain scenarios. cman uses a traditional vote-based quorum algorithm (same as vms clusters)

I get the following error

can't open cluster socket: Address family not supported by protocol

Check to make sure you have the proper modules loaded (The usage.txt doc has a list of what all should be loaded)

I'm using devfs, and gnbd is acting up

/dev/gnbd_ctl needs to be chmod 600

Where can I get detailed information on the cluster.conf file ?

[Thank you for your help]

Is it possible to run a GFS root filesystem ?

Possibly yes, since SSIC-Linux clusters do (or did) so. See http://ssic-linux.sourceforge.net.

My guess is that it could be done using a combination of the following:

• linux terminal server for booting new cluster nodes
  • using dhcp and tftp to serve kernel and configuration data and initial ramdisk
• creating an initial ramdisk which would have all the cluster tools, do the initial cluster configuration, and remount it's root as a shared GFS device

Yes it is. Have a look at http://www.open-sharedroot.org .

I get an 'Operation not permitted' error when exporting a GNBD device

See GNBD installation#GNBD export, at the end of the section for an explanation.

I get the error 'cman_tool: node name <node> is ambiguous' when trying to join the cluster.

This is caused by there being multiple matches for the hostname, with the result that the cluster software cannot work out which IP address to use for within the cluster. Ideally this should be fixed by changing /etc/hosts (or DNS, NIS, or other naming scheme) such that there is only a single match for the name.

How stable is the GFS 6.1 tree?

(Replace this with an answer please :) - We'd like to try out GFS in a production enviroment, well knowing the risks. How stable are the current GFS releases, compared to CVS, etc?

Can I implement a storage similar to GoogleFS with GFS, I mean merging several hard disks from several different machines into one (or more), and having redundancy too?

No, this is the job for a distributed filesystem like coda, InterMezzo or OpenAFS.

What advantages does GFS have over OCFS2? (if any)

(Replace this with an answer please)