ADMIN

To do this, you need to
change the mount type from
cluster to local. After doing
so, only a single computer
can mount the filesystem,
and it doesn’t need the cluster
stack to do so.
In all of these actions, you
need to be aware that the
filesystem can be mounted
by more than one computer.
Certain actions that involve,
say, tunefs.ocfs2, will not
work if another computer accesses
the filesystem at the
same time.
The example in Listing 6
shows the user attempting
to modify the label. This
process fails, although the
filesystem is offline (on this
computer). In this case,
mounted.ocfs2 will help: It
checks the OCFS2 header to
identify the computer that is
online with the filesystem.
The most important filesystem
structure data are contained
in the superblock. Just
like other Linux filesystems,
OCFS2 creates backup copies
of the superblock; however,
the approach the OCFS2
developers took is slightly
unusual.
OCFS2 creates a maximum
of six copies at non-configurable
offsets: 1, 4, 16, 64,
and 256GB and 1TB. Needless
to say, OCFS2 volumes
smaller than 1GB (!) don’t
have a copy of the superblock.
To be fair, mkfs.ocfs2
does tell you this when you
generate the filesystem. You
need to watch out for the
Writing backup superblock:
... line.
A neat side effect of these
static backup superblocks is
that you can reference them
by number during a filesystem
check. The example in
Listing 7 shows a damaged
primary superblock that is
preventing mounting and
a simple fsck.ocfs2 from
working. The first backup
makes it possible to restore.
Basically, Yes, but …
On the whole, it is easy to
set up an OCFS2 cluster. The
software is available for a
number of Linux distributions.
Because OCFS2 works
just as well with iSCSI and
Fibre Channel, the hardware
side is not too difficult either.
Setting up the cluster framework
is a fairly simple task
that you can handle with
simple tools like Vi.
Although OCFS2 doesn’t
include sophisticated fencing
technologies, in contrast
to other cluster filesystems,
fencing is not necessary in
many areas. The lack of a
cluster-capable volume manager
makes it easier for the
user to become immersed in
the world of OCFS2. Because
OCFS2 is simpler and less
complex than other cluster
filesystems, it is well worth
investigating.
n
Info
[1] OCFS2: [http:// oss. oracle. com/​
projects/ ocfs2/]
[2] First NFS RFC: [http:// tools. ietf.​
org/ html/ rfc1094]
[3] OpenAFS:
[http:// www. openafs. org/]
[4] GFS: [http:// sources. redhat.​
com/ cluster/ gfs/]
[5] Lustre: [http:// wiki. lustre. org]
[6] Ceph:
[http:// ceph. newdream. net/]
[7] DRBD: [http:// www. drbd. org/]
The Author
Udo Seidel is a teacher of math and
physics and has been an avid supporter
of Linux since 1996. After
completing his PhD, he worked as a
Linux/​Unix trainer, system administrator,
and senior solutions engineer.
He now works as the head of a
Linux/​Unix team for Amadeus Data
Processing GmbH in Erding, Germany.
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Admin 01
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