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gfs2.txt
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Global File System
------------------
http://sources.redhat.com/cluster/
GFS2 is a cluster file system. It allows a cluster of computers to
simultaneously use a block device that is shared between them (with FC,
iSCSI, NBD, etc). GFS2 reads and writes to the block device like a local
file system, but also uses a lock module to allow the computers coordinate
their I/O so file system consistency is maintained. One of the nifty
features of GFS2 is perfect consistency -- changes made to the file system
on one machine show up immediately on all other machines in the cluster.
GFS2 uses interchangable inter-node locking mechanisms. GFS2 plugs into
one side of a module called "lock_harness" and different lock modules can
plug into the other side of the harness. Each gfs file system selects the
appropriate lock module at mount time. Lock modules include:
lock_nolock -- does no real locking and allows gfs to be used as a
local file system
lock_dlm -- uses a distributed lock manager (dlm) for inter-node locking
The dlm is found at linux/drivers/dlm/
In addition to interfacing with an external locking manager, a gfs lock
module is responsible for interacting with external cluster management
systems. Lock_dlm depends on user space cluster management systems found
at the location above.
To use gfs as a local file system, no external clustering systems are
needed, simply:
$ gfs2_mkfs -p lock_nolock -j 1 /dev/block_device
$ mount -t gfs2 /dev/block_device /dir
GFS2 is not on-disk compatible with previous versions of GFS.
The following man pages can be found at the location above:
gfs2_mkfs to make a filesystem
gfs2_fsck to repair a filesystem
gfs2_grow to expand a filesystem online
gfs2_jadd to add journals to a filesystem online
gfs2_tool to manipulate, examine and tune a filesystem
gfs2_quota to examine and change quota values in a filesystem
gfs2_mount to find mount options
Mount options (from the gfs2_mount man page)
lockproto=LockModuleName
This specifies which inter-node lock protocol is used by the
GFS2 filesystem for this mount, overriding the default lock
protocol name stored in the filesystem's on-disk superblock.
The LockModuleName must be an exact match of the protocol name
presented by the lock module when it registers with the lock
harness. Traditionally, this matches the .o filename of the
lock module, e.g. lock_dlm, lock_gulm, or lock_nolock.
The default lock protocol name is written to disk initially
when creating the filesystem with gfs2_mkfs(8), -p option. It
can be changed on-disk by using the gfs2_tool(8) utility's sb
proto command.
The lockproto mount option should be used only under special
circumstances in which you want to temporarily use a different
lock protocol without changing the on-disk default.
locktable=LockTableName
This specifies the identity of the cluster and of the filesys-
tem for this mount, overriding the default cluster/filesystem
identify stored in the filesystem's on-disk superblock. The
cluster/filesystem name is recognized globally throughout the
cluster, and establishes a unique namespace for the inter-node
locking system, enabling the mounting of multiple GFS2 filesys-
tems.
The format of LockTableName is lock-module-specific. For
lock_gulm and lock_dlm, the format is clustername:fsname. For
lock_nolock, the field is ignored.
The default cluster/filesystem name is written to disk ini-
tially when creating the filesystem with gfs2_mkfs(8), -t
option. It can be changed on-disk by using the gfs2_tool(8)
utility's sb table command.
The locktable mount option should be used only under special
circumstances in which you want to mount the filesystem in a
different cluster, or mount it as a different filesystem name,
without changing the on-disk default.
hostdata=HostIDInfo
This field sends host (the computer on which the filesystem is
being mounted) identity information to the lock module.
The format and behavior of HostIDInfo is lock-module-specific.
For lock_gulm, it overrides the uname(1) -n network node name
used as default by lock_gulm. For lock_nolock, a HostIDInfo of
"jid=x" tells GFS to mount journal number x.
This field is ignored by lock_dlm.
localcaching
This flag tells GFS2 that it is running as a local (not clus-
tered) filesystem, so it can turn on some block caching opti-
mizations that can't be used when running in cluster mode.
This is turned on automatically by the lock_nolock module, but
can be overridden by using the ignore_local_fs option.
localflocks
This flag tells GFS2 that it is running as a local (not clus-
tered) filesystem, so it can allow the kernel VFS layer to do
all flock and fcntl file locking. When running in cluster
mode, these file locks require inter-node locks, and require
the support of GFS2. When running locally, better performance
is achieved by letting VFS handle the whole job.
This is turned on automatically by the lock_nolock module, but
can be overridden by using the ignore_local_fs option.
oopses_ok
When GFS2 detects certain errors (mostly some type of memory
corruption) it will panic the machine rather than risk corrupt-
ing the ondisk state. Using this option will cause GFS2 to
oops instead of panic. This may produce more useful debugging
information than a panic will. The downside of this option is
that an oops on one machine of a cluster filesystem may cause
the filesystem to stall on all machines in the cluster. (Pan-
ics don't have this "feature".) Use this option with care.
This is turned on automatically by the lock_nolock module, but
can be overridden by using the ignore_local_fs option.
debug Causes GFS2 to oops when encountering an error that would cause
the mount to withdraw or print an assertion warning. This
option should probably not be used in a production system.
ignore_local_fs
By default, using the nolock lock module automatically turns on
the localcaching and localflocks optimizations.
ignore_local_fs forces GFS2 to treat the filesystem as if it
were a multihost (clustered) filesystem, with localcaching and
localflocks optimizations turned off.
upgrade
This flag tells GFS2 to upgrade the filesystem's on-disk format
to the version supported by the current GFS2 software installa-
tion on this computer. If you try to mount an old-version disk
image, GFS2 will notify you via a syslog message that you need
to upgrade. Try mounting again, using the -o upgrade option.
When upgrading, only one node may mount the GFS2 filesystem.
num_glockd=Number
Tunes GFS2 to alleviate memory pressure when rapidly aquiring
many locks (e.g. several processes scanning through huge
directory trees). GFS2' glockd kernel daemon cleans up memory
for no-longer-needed glocks. Multiple instances of the daemon
clean up faster than a single instance. The default value is
one daemon, with a maximum of 16. Since this option was intro-
duced, other methods of rapid cleanup have been developed
within GFS2, so this option may go away in the future.
acl Enables POSIX Access Control List acl(5) support within GFS2.
spectator
Mount this filesystem using a special form of read-only mount.
The mount does not use one of the filesystem's journals.
suiddir
Sets owner of any newly created file or directory to be that of
parent directory, if parent directory has S_ISUID permission
attribute bit set. Sets S_ISUID in any new directory, if its
parent directory's S_ISUID is set. Strips all execution bits
on a new file, if parent directory owner is different from
owner of process creating the file. Set this option only if
you know why you are setting it.
quota=[off/account/on]
Turns quotas on or off for a filesystem. Setting the quotas to
be in the "account" state causes the per UID/GID usage statis-
tics to be correctly maintained by the filesystem, limit and
warn values are ignored. The default value is "off".
data=[ordered/writeback]
When data=ordered is set, the user data modified by a transac-
tion is flushed to the disk before the transaction is commited
to disk. This should prevent the user from seeing uninitial-
ized blocks in a file after a crash. Data=writeback mode
writes the user data to the disk at any time after it's dirt-
ied. This doesn't provide the same consistency guarantee as
ordered mode, but it should be slightly faster for some work-
loads. The default is ordered mode.

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