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NAME

lvmlockd — LVM locking daemon

SYNOPSIS

lvmlockd [options]

DESCRIPTION

LVM commands use lvmlockd to coordinate access to shared storage.
When LVM is used on devices shared by multiple hosts, locks will:
• coordinate reading and writing of LVM metadata
• validate caching of LVM metadata
• prevent conflicting activation of logical volumes
lvmlockd uses an external lock manager to perform basic locking.

Lock manager (lock type) options are:
• sanlock: places locks on disk within LVM storage.
• dlm: uses network communication and a cluster manager.

OPTIONS

-h|--help

Show this help information.

-V|--version

Show version of lvmlockd.

-T|--test

Test mode, do not call lock manager.

-f|--foreground

Don’t fork.

-D|--daemon-debug

Don’t fork and print debugging to stdout.

-p|--pid-file path

Set path to the pid file.

-s|--socket-path path

Set path to the socket to listen on.

--adopt-file path

Set path to the adopt file.

-S|--syslog-priority err|warning|debug

Write log messages from this level up to syslog.

-g|--gl-type sanlock|dlm

Set global lock type to be sanlock or dlm.

-i|--host-id num

Set the local sanlock host id.

-F|--host-id-file path

A file containing the local sanlock host_id.

-o|--sanlock-timeout seconds

Override the default sanlock I/O timeout.

-A|--adopt 0|1

Enable (1) or disable (0) lock adoption.

USAGE

Initial set up
Setting up LVM to use lvmlockd and a shared VG for the first time includes some one time set up steps:

1. choose a lock manager
dlm
If dlm (or corosync) are already being used by other cluster software, then select dlm. dlm uses corosync which requires additional configuration beyond the scope of this document. See corosync and dlm documentation for instructions on configuration, set up and usage.

sanlock
Choose sanlock if dlm/corosync are not otherwise required. sanlock does not depend on any clustering software or configuration.

2. configure hosts to use lvmlockd
On all hosts running lvmlockd, configure lvm.conf:
use_lvmlockd = 1

sanlock
Assign each host a unique host_id in the range 1-2000 by setting
/etc/lvm/lvmlocal.conf local/host_id

3. start lvmlockd
Start the lvmlockd daemon.
Use systemctl, a cluster resource agent, or run directly, e.g.
systemctl start lvmlockd

4. start lock manager
sanlock
Start the sanlock and wdmd daemons.
Use systemctl or run directly, e.g.
systemctl start wdmd sanlock

dlm
Start the dlm and corosync daemons.
Use systemctl, a cluster resource agent, or run directly, e.g.
systemctl start corosync dlm

5. create VG on shared devices
vgcreate --shared <vgname> <devices>

The shared option sets the VG lock type to sanlock or dlm depending on which lock manager is running. LVM commands acquire locks from lvmlockd, and lvmlockd uses the chosen lock manager.

6. start VG on all hosts
vgchange --lock-start

Shared VGs must be started before they are used. Starting the VG performs lock manager initialization that is necessary to begin using locks (i.e. creating and joining a lockspace). Starting the VG may take some time, and until the start completes the VG may not be modified or activated.

7. create and activate LVs
Standard lvcreate and lvchange commands are used to create and activate LVs in a shared VG.

An LV activated exclusively on one host cannot be activated on another. When multiple hosts need to use the same LV concurrently, the LV can be activated with a shared lock (see lvchange options -aey vs -asy.) (Shared locks are disallowed for certain LV types that cannot be used from multiple hosts.)

Normal start up and shut down
After initial set up, start up and shut down include the following steps. They can be performed directly or may be automated using systemd or a cluster resource manager/agents.

• start lvmlockd
• start lock manager
• vgchange --lock-start
• activate LVs in shared VGs

The shut down sequence is the reverse:

• deactivate LVs in shared VGs
• vgchange --lock-stop
• stop lock manager
• stop lvmlockd

TOPICS

Protecting VGs on shared devices
The following terms are used to describe the different ways of accessing VGs on shared devices.

shared VG

A shared VG exists on shared storage that is visible to multiple hosts. LVM acquires locks through lvmlockd to coordinate access to shared VGs. A shared VG has lock_type "dlm" or "sanlock", which specifies the lock manager lvmlockd will use.

When the lock manager for the lock type is not available (e.g. not started or failed), lvmlockd is unable to acquire locks for LVM commands. In this situation, LVM commands are only allowed to read and display the VG; changes and activation will fail.

local VG

A local VG is meant to be used by a single host. It has no lock type or lock type "none". A local VG typically exists on local (non-shared) devices and cannot be used concurrently from different hosts.

If a local VG does exist on shared devices, it should be owned by a single host by having the system ID set, see lvmsystemid(7). The host with a matching system ID can use the local VG and other hosts will ignore it. A VG with no lock type and no system ID should be excluded from all but one host using lvm.conf filters. Without any of these protections, a local VG on shared devices can be easily damaged or destroyed.

clvm VG

A clvm VG (or clustered VG) is a VG on shared storage (like a shared VG) that requires clvmd for clustering and locking. See below for converting a clvm/clustered VG to a shared VG.

Shared VGs from hosts not using lvmlockd
Hosts that do not use shared VGs will not be running lvmlockd. In this case, shared VGs that are still visible to the host will be ignored (like foreign VGs, see lvmsystemid(7)).

The --shared option for reporting and display commands causes shared VGs to be displayed on a host not using lvmlockd, like the --foreign option does for foreign VGs.

Creating the first sanlock VG
When use_lvmlockd is first enabled in lvm.conf, and before the first sanlock VG is created, no global lock will exist. In this initial state, LVM commands try and fail to acquire the global lock, producing a warning, and some commands are disallowed. Once the first sanlock VG is created, the global lock will be available, and LVM will be fully operational.

When a new sanlock VG is created, its lockspace is automatically started on the host that creates it. Other hosts need to run ’vgchange --lock-start’ to start the new VG before they can use it.

Creating the first sanlock VG is not protected by locking, so it requires special attention. This is because sanlock locks exist on storage within the VG, so they are not available until after the VG is created. The first sanlock VG that is created will automatically contain the "global lock". Be aware of the following special considerations:

See below for more information about managing the sanlock global lock.

Using shared VGs
In the ’vgs’ command, shared VGs are indicated by "s" (for shared) in the sixth attr field, and by "shared" in the "--options shared" report field. The specific lock type and lock args for a shared VG can be displayed with ’vgs -o+locktype,lockargs’.

Shared VGs need to be "started" and "stopped", unlike other types of VGs. See the following section for a full description of starting and stopping.

Removing a shared VG will fail if other hosts have the VG started. Run vgchange --lock-stop <vgname> on all other hosts before vgremove. (It may take several seconds before vgremove recognizes that all hosts have stopped a sanlock VG.)

Starting and stopping VGs
Starting a shared VG (vgchange --lock-start) causes the lock manager to start (join) the lockspace for the VG on the host where it is run. This makes locks for the VG available to LVM commands on the host. Before a VG is started, only LVM commands that read/display the VG are allowed to continue without locks (and with a warning).

Stopping a shared VG (vgchange --lock-stop) causes the lock manager to stop (leave) the lockspace for the VG on the host where it is run. This makes locks for the VG inaccessible to the host. A VG cannot be stopped while it has active LVs.

When using the lock type sanlock, starting a VG can take a long time (potentially minutes if the host was previously shut down without cleanly stopping the VG.)

A shared VG can be started after all the following are true:

• lvmlockd is running
• the lock manager is running
• the VG’s devices are visible on the system

A shared VG can be stopped if all LVs are deactivated.

All shared VGs can be started/stopped using:
vgchange --lock-start
vgchange --lock-stop

Individual VGs can be started/stopped using:
vgchange --lock-start <vgname> ...
vgchange --lock-stop <vgname> ...

To make vgchange not wait for start to complete:
vgchange --lock-start --lock-opt nowait ...

lvmlockd can be asked directly to stop all lockspaces:
lvmlockctl -S|--stop-lockspaces

To start only selected shared VGs, use the lvm.conf activation/lock_start_list. When defined, only VG names in this list are started by vgchange. If the list is not defined (the default), all visible shared VGs are started. To start only "vg1", use the following lvm.conf configuration:

activation {
lock_start_list = [ "vg1" ]
...
}

Internal command locking
To optimize the use of LVM with lvmlockd, be aware of the three kinds of locks and when they are used:

Global lock

The global lock is associated with global information, which is information not isolated to a single VG. This includes:

• The global VG namespace.
• The set of orphan PVs and unused devices.
• The properties of orphan PVs, e.g. PV size.

The global lock is acquired in shared mode by commands that read this information, or in exclusive mode by commands that change it. For example, the command ’vgs’ acquires the global lock in shared mode because it reports the list of all VG names, and the vgcreate command acquires the global lock in exclusive mode because it creates a new VG name, and it takes a PV from the list of unused PVs.

When an LVM command is given a tag argument, or uses select, it must read all VGs to match the tag or selection, which causes the global lock to be acquired.

VG lock

A VG lock is associated with each shared VG. The VG lock is acquired in shared mode to read the VG and in exclusive mode to change the VG or activate LVs. This lock serializes access to a VG with all other LVM commands accessing the VG from all hosts.

The command ’vgs <vgname>’ does not acquire the global lock (it does not need the list of all VG names), but will acquire the VG lock on each VG name argument.

LV lock

An LV lock is acquired before the LV is activated, and is released after the LV is deactivated. If the LV lock cannot be acquired, the LV is not activated. (LV locks are persistent and remain in place when the activation command is done. Global and VG locks are transient, and are held only while an LVM command is running.)

lock retries

If a request for a global or VG lock fails due to a lock conflict with another host, lvmlockd automatically retries for a short time before returning a failure to the LVM command. If those retries are insufficient, the LVM command will retry the entire lock request a number of times specified by global/lvmlockd_lock_retries before failing. If a request for an LV lock fails due to a lock conflict, the command fails immediately.

Managing the global lock in sanlock VGs
The global lock exists in one of the sanlock VGs. The first sanlock VG created will contain the global lock. Subsequent sanlock VGs will each contain a disabled global lock that can be enabled later if necessary.

The VG containing the global lock must be visible to all hosts using sanlock VGs. For this reason, it can be useful to create a small sanlock VG, visible to all hosts, and dedicated to just holding the global lock. While not required, this strategy can help to avoid difficulty in the future if VGs are moved or removed.

The vgcreate command typically acquires the global lock, but in the case of the first sanlock VG, there will be no global lock to acquire until the first vgcreate is complete. So, creating the first sanlock VG is a special case that skips the global lock.

vgcreate determines that it’s creating the first sanlock VG when no other sanlock VGs are visible on the system. It is possible that other sanlock VGs do exist, but are not visible when vgcreate checks for them. In this case, vgcreate will create a new sanlock VG with the global lock enabled. When the another VG containing a global lock appears, lvmlockd will then see more than one VG with a global lock enabled. LVM commands will report that there are duplicate global locks.

If the situation arises where more than one sanlock VG contains a global lock, the global lock should be manually disabled in all but one of them with the command:

lvmlockctl --gl-disable <vgname>

(The one VG with the global lock enabled must be visible to all hosts.)

An opposite problem can occur if the VG holding the global lock is removed. In this case, no global lock will exist following the vgremove, and subsequent LVM commands will fail to acquire it. In this case, the global lock needs to be manually enabled in one of the remaining sanlock VGs with the command:

lvmlockctl --gl-enable <vgname>

(Using a small sanlock VG dedicated to holding the global lock can avoid the case where the global lock must be manually enabled after a vgremove.)

Internal lvmlock LV
A sanlock VG contains a hidden LV called "lvmlock" that holds the sanlock locks. vgreduce cannot yet remove the PV holding the lvmlock LV. To remove this PV, change the VG lock type to "none", run vgreduce, then change the VG lock type back to "sanlock". Similarly, pvmove cannot be used on a PV used by the lvmlock LV.

To place the lvmlock LV on a specific device, create the VG with only that device, then use vgextend to add other devices.

LV activation
In a shared VG, LV activation involves locking through lvmlockd, and the following values are possible with lvchange/vgchange -a:

Manually repairing a shared VG
Some failure conditions may not be repairable while the VG has a shared lock type. In these cases, it may be possible to repair the VG by forcibly changing the lock type to "none". This is done by adding "--lock-opt force" to the normal command for changing the lock type: vgchange --lock-type none VG. The VG lockspace should first be stopped on all hosts, and be certain that no hosts are using the VG before this is done.

Recover from lost PV holding sanlock locks
In a sanlock VG, the sanlock locks are held on the hidden "lvmlock" LV. If the PV holding this LV is lost, a new lvmlock LV needs to be created. To do this, ensure no hosts are using the VG, then forcibly change the lock type to "none" (see above). Then change the lock type back to "sanlock" with the normal command for changing the lock type: vgchange --lock-type sanlock VG. This recreates the internal lvmlock LV with the necessary locks.

Locking system failures
lvmlockd failure

If lvmlockd fails or is killed while holding locks, the locks are orphaned in the lock manager. Orphaned locks must be cleared or adopted before the associated resources can be accessed normally. If lock adoption is enabled, lvmlockd keeps a record of locks in the adopt-file. A subsequent instance of lvmlockd will then adopt locks orphaned by the previous instance. Adoption must be enabled in both instances (--adopt|-A 1). Without adoption, the lock manager or host would require a reset to clear orphaned lock state.

dlm/corosync failure

If dlm or corosync fail, the clustering system will fence the host using a method configured within the dlm/corosync clustering environment.

LVM commands on other hosts will be blocked from acquiring any locks until the dlm/corosync recovery process is complete.

sanlock lease storage failure

If the PV under a sanlock VG’s lvmlock LV is disconnected, unresponsive or too slow, sanlock cannot renew the lease for the VG’s locks. After some time, the lease will expire, and locks that the host owns in the VG can be acquired by other hosts. The VG must be forcibly deactivated on the host with the expiring lease before other hosts can acquire its locks. This is necessary for data protection.

When the sanlock daemon detects that VG storage is lost and the VG lease is expiring, it runs the command lvmlockctl --kill <vgname>. This command emits a syslog message stating that storage is lost for the VG, and that LVs in the VG must be immediately deactivated.

If no LVs are active in the VG, then the VG lockspace will be removed, and errors will be reported when trying to use the VG. Use the lvmlockctl --drop command to clear the stale lockspace from lvmlockd.

If the VG has active LVs, they must be quickly deactivated before the locks expire. After all LVs are deactivated, run lvmlockctl --drop <vgname> to clear the expiring lockspace from lvmlockd.

If all LVs in the VG are not deactivated within about 40 seconds, sanlock uses wdmd and the local watchdog to reset the host. The machine reset is effectively a severe form of "deactivating" LVs before they can be activated on other hosts. The reset is considered a better alternative than having LVs used by multiple hosts at once, which could easily damage or destroy their content.

sanlock lease storage failure automation

When the sanlock daemon detects that the lease storage is lost, it runs the command lvmlockctl --kill <vgname>. This lvmlockctl command can be configured to run another command to forcibly deactivate LVs, taking the place of the manual process described above. The other command is configured in the lvm.conf lvmlockctl_kill_command setting. The VG name is appended to the end of the command specified.

The lvmlockctl_kill_command should forcibly deactivate LVs in the VG, ensuring that existing writes to LVs in the VG are complete and that further writes to the LVs in the VG will be rejected. If it is able to do this successfully, it should exit with success, otherwise it should exit with an error. If lvmlockctl --kill gets a successful result from lvmlockctl_kill_command, it tells lvmlockd to drop locks for the VG (the equivalent of running lvmlockctl --drop). If this completes in time, a machine reset can be avoided.

One possible option is to create a script my_vg_kill_script.sh:
#!/bin/bash
VG=$1
# replace dm table with the error target for top level LVs
dmsetup wipe_table -S "uuid=~LVM && vgname=$VG && lv_layer=\"\""
# check that the error target is in place
dmsetup table -c -S "uuid=~LVM && vgname=$VG && lv_layer=\"\"" |grep -vw error
if [[ $? -ne 0 ]] ; then
exit 0
fi
exit 1

Set in lvm.conf:
lvmlockctl_kill_command="/usr/sbin/my_vg_kill_script.sh"

(The script and dmsetup commands should be tested with the actual VG to ensure that all top level LVs are properly disabled.)

If the lvmlockctl_kill_command is not configured, or fails, lvmlockctl --kill will emit syslog messages as described in the previous section, notifying the user to manually deactivate the VG before sanlock resets the machine.

sanlock daemon failure

If the sanlock daemon fails or exits while a lockspace is started, the local watchdog will reset the host. This is necessary to protect any application resources that depend on sanlock leases.

Changing dlm cluster name
When a dlm VG is created, the cluster name is saved in the VG metadata. To use the VG, a host must be in the named dlm cluster. If the dlm cluster name changes, or the VG is moved to a new cluster, the dlm cluster name saved in the VG must also be changed.

To see the dlm cluster name saved in the VG, use the command:
vgs -o+locktype,lockargs <vgname>

To change the dlm cluster name in the VG when the VG is still used by the original cluster:

vgchange --lock-start <vgname>

vgchange --lock-stop <vgname>

vgchange --lock-type none <vgname>

cat /sys/kernel/config/dlm/cluster/cluster_name

vgchange --lock-type dlm <vgname>

vgchange --lock-start <vgname>

To change the dlm cluster name in the VG when the dlm cluster name has already been changed on the hosts, or the VG has already moved to a different cluster:

cat /sys/kernel/config/dlm/cluster/cluster_name

vgchange --lock-type none --lock-opt force <vgname>

vgchange --lock-type dlm <vgname>

vgchange --lock-start <vgname>

Changing a local VG to a shared VG
All LVs must be inactive to change the lock type.

lvmlockd must be configured and running as described in USAGE.

vgchange --lock-type sanlock|dlm <vgname>

vgchange --lock-start <vgname>

Changing a shared VG to a local VG
All LVs must be inactive to change the lock type.

vgchange --lock-start <vgname>

vgchange --lock-stop <vgname>

vgchange --lock-type none <vgname>

If the VG cannot be started with the previous lock type, then the lock type can be forcibly changed to none with:
vgchange --lock-type none --lock-opt force <vgname>

To change a VG from one lock type to another (i.e. between sanlock and dlm), first change it to a local VG, then to the new type.

Changing a clvm/clustered VG to a shared VG
All LVs must be inactive to change the lock type.

First change the clvm/clustered VG to a local VG. Within a running clvm cluster, change a clustered VG to a local VG with the command:

vgchange -cn <vgname>

If the clvm cluster is no longer running on any nodes, then extra options can be used to forcibly make the VG local. Caution: this is only safe if all nodes have stopped using the VG:

vgchange --lock-type none --lock-opt force <vgname>

After the VG is local, follow the steps described in "changing a local VG to a shared VG".

Extending an LV active on multiple hosts
With lvmlockd and dlm, a special clustering procedure is used to refresh a shared LV on remote cluster nodes after it has been extended on one node.

When an LV holding gfs2 or ocfs2 is active on multiple hosts with a shared lock, lvextend is permitted to run with an existing shared LV lock in place of the normal exclusive LV lock.

After lvextend has finished extending the LV, it sends a remote request to other nodes running the dlm to run ’lvchange --refresh’ on the LV. This uses dlm_controld and corosync features.

Some special --lockopt values can be used to modify this process. "shupdate" permits the lvextend update with an existing shared lock if it isn’t otherwise permitted. "norefresh" prevents the remote refresh operation.

Limitations of shared VGs
Things that do not yet work in shared VGs:
• using external origins for thin LVs
• splitting snapshots from LVs
• splitting mirrors in sanlock VGs
• pvmove of entire PVs, or under LVs activated with shared locks
• vgsplit and vgmerge (convert to a local VG to do this)

lvmlockd changes from clvmd
(See above for converting an existing clvm VG to a shared VG.)

While lvmlockd and clvmd are entirely different systems, LVM command usage remains similar. Differences are more notable when using lvmlockd’s sanlock option.

Visible usage differences between shared VGs (using lvmlockd) and clvm/clustered VGs (using clvmd):

SEE ALSO

lvm(8), lvmlockctl(8)