lvmlockd

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

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.

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

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

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

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.

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.

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.)

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

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.

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.

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:

• The first vgcreate command needs to be given the path to a device that has not yet been initialized with pvcreate.  The pvcreate initialization will be done by vgcreate.  This is because the pvcreate command requires the global lock, which will not be available until after the first sanlock VG is created.
• Because the first sanlock VG will contain the global lock, this VG needs to be accessible to all hosts that will use sanlock shared VGs.  All hosts will need to use the global lock from the first sanlock VG.

• The device and VG name used by the initial vgcreate will not be protected from concurrent use by another vgcreate on another host.

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

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 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" ]
    ...
}

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.

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.)

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.

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

y|ey

The command activates the LV in exclusive mode, allowing a single host to activate the LV.  Before activating the LV, the command uses lvmlockd to acquire an exclusive lock on the LV.  If the lock cannot be acquired, the LV is not activated and an error is reported.  This would happen if the LV is active on another host.

sy

The command activates the LV in shared mode, allowing multiple hosts to activate the LV concurrently.  Before activating the LV, the command uses lvmlockd to acquire a shared lock on the LV.  If the lock cannot be acquired, the LV is not activated and an error is reported. This would happen if the LV is active exclusively on another host.  If the LV type prohibits shared access, such as a snapshot, the command will report an error and fail. The shared mode is intended for a multi-host/cluster application or file system. LV types that cannot be used concurrently from multiple hosts include thin, cache, raid, mirror, and snapshot.

n

The command deactivates the LV.  After deactivating the LV, the command uses lvmlockd to release the current lock on the LV.

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.

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.

lvmlockd failure

If lvmlockd fails or is killed while holding locks, the locks are orphaned in the lock manager.

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.

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

If no LVs are active in the VG, then the lockspace with an expiring lease 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 when the lock storage is lost, the LVs must be quickly deactivated before the lockspace lease expires.  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.

In the future, the lvmlockctl kill command may automatically attempt to forcibly deactivate LVs before the sanlock lease expires.  Until then, the user must notice the syslog message and 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.

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:

• Start the VG on the host changing the lock type
  vgchange --lock-start <vgname>
• Stop the VG on all other hosts:
  vgchange --lock-stop <vgname>
• Change the VG lock type to none on the host where the VG is started:
  vgchange --lock-type none <vgname>
• Change the dlm cluster name on the hosts or move the VG to the new cluster.  The new dlm cluster must now be running on the host.  Verify the new name by:
  cat /sys/kernel/config/dlm/cluster/cluster_name
• Change the VG lock type back to dlm which sets the new cluster name:
  vgchange --lock-type dlm <vgname>
• Start the VG on hosts to use it:
  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:

• Ensure the VG is not being used by any hosts.
• The new dlm cluster must be running on the host making the change. The current dlm cluster name can be seen by:
  cat /sys/kernel/config/dlm/cluster/cluster_name
• Change the VG lock type to none:
  vgchange --lock-type none --lock-opt force <vgname>
• Change the VG lock type back to dlm which sets the new cluster name:
  vgchange --lock-type dlm <vgname>
• Start the VG on hosts to use it:
  vgchange --lock-start <vgname>

All LVs must be inactive to change the lock type.

lvmlockd must be configured and running as described in Usage.

• Change a local VG to a shared VG with the command:
  vgchange --lock-type sanlock|dlm <vgname>
• Start the VG on hosts to use it:
  vgchange --lock-start <vgname>

All LVs must be inactive to change the lock type.

• Start the VG on the host making the change:
  vgchange --lock-start <vgname>
• Stop the VG on all other hosts:
  vgchange --lock-stop <vgname>
• Change the VG lock type to none on the host where the VG is started:
  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.

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".

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.

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)

(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):

• lvm.conf is configured to use lvmlockd by setting use_lvmlockd=1. clvmd used locking_type=3.
• vgcreate --shared creates a shared VG.  vgcreate --clustered y created a clvm/clustered VG.
• lvmlockd adds the option of using sanlock for locking, avoiding the need for network clustering.
• lvmlockd defaults to the exclusive activation mode whenever the activation mode is unspecified, i.e. -ay means -aey, not -asy.
• lvmlockd commands always apply to the local host, and never have an effect on a remote host.  (The activation option 'l' is not used.)
• lvmlockd saves the cluster name for a shared VG using dlm.  Only hosts in the matching cluster can use the VG.
• lvmlockd requires starting/stopping shared VGs with vgchange --lock-start and --lock-stop.
• vgremove of a sanlock VG may fail indicating that all hosts have not stopped the VG lockspace.  Stop the VG on all hosts using vgchange --lock-stop.
• vgreduce or pvmove of a PV in a sanlock VG will fail if it holds the internal "lvmlock" LV that holds the sanlock locks.
• lvmlockd uses lock retries instead of lock queueing, so high lock contention may require increasing global/lvmlockd_lock_retries to avoid transient lock failures.
• lvmlockd includes VG reporting options lock_type and lock_args, and LV reporting option lock_args to view the corresponding metadata fields.
• In the 'vgs' command's sixth VG attr field, "s" for "shared" is displayed for shared VGs.
• If lvmlockd fails or is killed while in use, locks it held remain but are orphaned in the lock manager.  lvmlockd can be restarted with an option to adopt the orphan locks from the previous instance of lvmlockd.