ADMIN+Magazine+Sample+PDF

Tools
OCFS2
you can set for the mount operation.
If OCFS2 detects an error in the data
structure, it will default to readonly.
In certain situations, a reboot
can clear this up. The errors=panic
mount option handles this. Another
interesting option is commit=seconds.
The default value is 5, which means
that OCFS2 writes the data out to disk
every five seconds. If a crash occurs,
a consistent filesystem can be guaranteed
– thanks to journaling – and only
the work from the last five seconds
will be lost. The mount option that
specifies the way data are handled
Listing 2: Starting the OCFS2 Subsystem
# /etc/init.d/o2cb online
Loading filesystem "configfs": OK
Mounting configfs filesystem at /sys/kernel/config: OK
Loading filesystem "ocfs2_dlmfs": OK
Mounting ocfs2_dlmfs filesystem at /dlm: OK
Starting O2CB cluster ocfs2: OK
#
# /etc/init.d/o2cb status
Driver for "configfs": Loaded
Filesystem "configfs": Mounted
Driver for "ocfs2_dlmfs": Loaded
Filesystem "ocfs2_dlmfs": Mounted
Checking O2CB cluster ocfs2: Online
Heartbeat dead threshold = 31
Network idle timeout: 30000
Network keepalive delay: 2000
Network reconnect delay: 2000
Checking O2CB heartbeat: Not active
#
Listing 3: OCFS2 Optimized for Mail Server
# mkfs.ocfs2 ‐T mail ‐L data /dev/sda1
mkfs.ocfs2 1.4.2
Cluster stack: classic o2cb
Filesystem Type of mail
Filesystem label=data
Block size=2048 (bits=11)
Cluster size=4096 (bits=12)
Volume size=1011675136 (246991 clusters) (493982 blocks)
16 cluster groups (tail covers 8911 clusters, rest cover
15872 clusters)
Journal size=67108864
Initial number of node slots: 2
Creating bitmaps: done
Initializing superblock: done
Writing system files: done
Writing superblock: done
Writing backup superblock: 0 block(s)
Formatting Journals: done
Formatting slot map: done
Writing lost+found: done
mkfs.ocfs2 successful
for journaling is also important here.
The latest version lets OCFS2 write all
the data out to disk before updating
the journal. date=writeback forces the
predecessor’s mode.
Inexperienced OCFS2 admins might
wonder why the OCFS2 volume is
not available after a reboot despite an
entry in /etc/fstab. The init script
that comes with the distribution,
/etc/init.d/ocfs2 makes the OCFS2
mount resistant to reboots. Once enabled,
this script scans /etc/fstab for
OCFS2 entries and integrates these
filesystems.
Just as with ext3/​4, the administrator
can modify a couple of filesystem
properties after mounting the filesystem
without destroying data. The
tunefs.ocfs2 tool helps with this. If
the cluster grows unexpectedly and
you want more computers to access
OCFS2 at the same time, too small a
Shared Filesystems
The shared filesystem family is a fairly colorful
bunch. By definition, they all share the ability
to grant multiple computers simultaneous access
to certain data. The differences are in the
way they implement these requirements.
On the one hand are network filesystems, in
which the most popular representative in the
Unix/​Linux camp is Network Filesystem (NFS)
[2]. NFS is available for more or less any operating
system and to all intents and purposes
only asks the operating system to provide a
TCP/​IP stack. The setup is also fairly simple.
The Andrew filesystem (AFS) is another
network filesystem that is available in a free
implementation, OpenAFS [3].
On the other hand are cluster filesystems. Before
computers can access “distributed” data,
History
OCFS2 is a fairly young filesystem. As the “2”
in the name suggests, the current version
is an enhancement. Oracle developed the
predecessor, OCFS, for use in Real Application
Cluster for Oracle databases. The new OCFS2
is designed to fulfill the requirements placed
on a mature filesystem capable of storing arbitrary
data. POSIX compatibility and the typical
– and necessary – performance required
for databases were further criteria.
After two years of development, the programmers
released version 1.0 of OCFS2, and it
made its way into the vanilla kernel (2.6.16)
value for the N option in mkfs.ocfs2
can become a problem. The tunefs.
ocfs2 tool lets you change this in next
to no time. The same thing applies to
the journal size (Listing 4).
Also, you can use this tool to modify
the filesystem label and enable or
disable certain features (see also Listing
8). Unfortunately, the man page
doesn’t tell you which changes are
permitted on the fly and which aren’t.
Thus, you could experience a tunefs.
ocfs2: Trylock failed while opening device
"/dev/sda1" message when you
try to run some commands on OCFS2.
More Detail
As I mentioned earlier, you do not
need to preconfigure the cluster
heartbeat or fencing. When the
cluster stack is initialized, default
values are set for both. However, you
they first need to enter the cluster. The cluster
setup requires additional infrastructure,
such as additional I/​O cards, cluster software,
and, of course, a configuration. Cluster filesystems
are also categorized by the way they
store data. Those based on shared disks allow
multiple computers to read and write to the
same medium. I/​O is handled via Fibre Channel
(“classical SAN”) or TCP/​IP (iSCSI). The most
popular representatives in the Linux camp
here are OCFS2 and the Global Filesystem
(GFS2) [4].
Parallel cluster filesystems are a more recent
invention. They distribute data over computers
in the cluster by striping single files across
multiple storage nodes. Lustre [5] and Ceph
[6] are popular examples of this technology.
just a year later. Version 1.2 became more
widespread, with a great deal of support from
various Enterprise Linux distributions.
OCFS2 has been available for the major players
in the Linux world for some time. This
applies to commercial variants, such as SLES,
RHEL, or Oracle EL, and to the free Debian,
Fedora, and openSUSE systems. Depending on
the kernel version, users either get version
1.4, which was released in 2008, or version 1.2,
which is two years older. The “OCFS2 Choices”
box and Table 3 show you what you need to
watch out for.
38 Admin 01 www.admin-magazine.com