Provided by: sanlock_2.2-2_amd64 
NAME
sanlock - shared storage lock manager
SYNOPSIS
sanlock [COMMAND] [ACTION] ...
DESCRIPTION
The sanlock daemon manages leases for applications running on a cluster of hosts with
shared storage. All lease management and coordination is done through reading and writing
blocks on the shared storage. Two types of leases are used, each based on a different
algorithm:
"delta leases" are slow to acquire and require regular i/o to shared storage. A delta
lease exists in a single sector of storage. Acquiring a delta lease involves reads and
writes to that sector separated by specific delays. Once acquired, a lease must be
renewed by updating a timestamp in the sector regularly. sanlock uses a delta lease
internally to hold a lease on a host_id. host_id leases prevent two hosts from using the
same host_id and provide basic host liveness information based on the renewals.
"paxos leases" are generally fast to acquire and sanlock makes them available to
applications as general purpose resource leases. A paxos lease exists in 1MB of shared
storage (8MB for 4k sectors). Acquiring a paxos lease involves reads and writes to
max_hosts (2000) sectors in a specific sequence specified by the Disk Paxos algorithm.
paxos leases use host_id's internally to indicate the owner of the lease, and the
algorithm fails if different hosts use the same host_id. So, delta leases provide the
unique host_id's used in paxos leases. paxos leases also refer to delta leases to check
if a host_id is alive.
Before sanlock can be used, the user must assign each host a host_id, which is a number
between 1 and 2000. Two hosts should not be given the same host_id (even though delta
leases attempt to detect this mistake.)
sanlock views a pool of storage as a "lockspace". Each distinct pool of storage, e.g.
from different sources, would typically be defined as a separate lockspace, with a unique
lockspace name.
Part of this storage space must be reserved and initialized for sanlock to store delta
leases. Each host that wants to use the lockspace must first acquire a delta lease on its
host_id number within the lockspace. (See the add_lockspace action/api.) The space
required for 2000 delta leases in the lockspace (for 2000 possible host_id's) is 1MB (8MB
for 4k sectors). (This is the same size required for a single paxos lease.)
More storage space must be reserved and initialized for paxos leases, according to the
needs of the applications using sanlock.
The following steps illustrate these concepts using the command line. Applications may
choose to do these same steps through libsanlock.
1. Create storage pools and reserve and initialize host_id leases
two different LUNs on a SAN: /dev/sdb, /dev/sdc
# vgcreate pool1 /dev/sdb
# vgcreate pool2 /dev/sdc
# lvcreate -n hostid_leases -L 1MB pool1
# lvcreate -n hostid_leases -L 1MB pool2
# sanlock direct init -s LS1:0:/dev/pool1/hostid_leases:0
# sanlock direct init -s LS2:0:/dev/pool2/hostid_leases:0
2. Start the sanlock daemon on each host
# sanlock daemon
3. Add each lockspace to be used
host1:
# sanlock client add_lockspace -s LS1:1:/dev/pool1/hostid_leases:0
# sanlock client add_lockspace -s LS2:1:/dev/pool2/hostid_leases:0
host2:
# sanlock client add_lockspace -s LS1:2:/dev/pool1/hostid_leases:0
# sanlock client add_lockspace -s LS2:2:/dev/pool2/hostid_leases:0
4. Applications can now reserve/initialize space for resource leases, and then acquire the
leases as they need to access the resources.
The resource leases that are created and how they are used depends on the application.
For example, say application A, running on host1 and host2, needs to synchronize access to
data it stores on /dev/pool1/Adata. A could use a resource lease as follows:
5. Reserve and initialize a single resource lease for Adata
# lvcreate -n Adata_lease -L 1MB pool1
# sanlock direct init -r LS1:Adata:/dev/pool1/Adata_lease:0
6. Acquire the lease from the app using libsanlock (see sanlock_register,
sanlock_acquire). If the app is already running as pid 123, and has registered with the
sanlock daemon, the lease can be added for it manually.
# sanlock client acquire -r LS1:Adata:/dev/pool1/Adata_lease:0 -p 123
offsets
offsets must be 1MB aligned for disks with 512 byte sectors, and 8MB aligned for disks
with 4096 byte sectors.
offsets may be used to place leases on the same device rather than using separate devices
and offset 0 as shown in examples above, e.g. these commands above:
# sanlock direct init -s LS1:0:/dev/pool1/hostid_leases:0
# sanlock direct init -r LS1:Adata:/dev/pool1/Adata_lease:0
could be replaced by:
# sanlock direct init -s LS1:0:/dev/pool1/leases:0
# sanlock direct init -r LS1:Adata:/dev/pool1/leases:1048576
failures
If a process holding resource leases fails or exits without releasing its leases, sanlock
will release the leases for it automatically.
If the sanlock daemon cannot renew a lockspace host_id for a specific period of time
(usually because storage access is lost), sanlock will kill any process holding a resource
lease within the lockspace.
If the sanlock daemon crashes or gets stuck, it will no longer renew the expiry time of
its per-host_id connections to the wdmd daemon, and the watchdog device will reset the
host.
watchdog
sanlock uses the wdmd(8) daemon to access /dev/watchdog. A separate wdmd connection is
maintained with wdmd for each host_id being renewed. Each host_id connection has an
expiry time for some seconds in the future. After each successful host_id renewal,
sanlock updates the associated expiry time in wdmd. If wdmd finds any connection expired,
it will not pet /dev/watchdog. After enough successive expired/failed checks, the
watchdog device will fire and reset the host.
After a number of failed attempts to renew a host_id, sanlock kills any process using that
lockspace. Once all those processes have exited, sanlock will unregister the associated
wdmd connection. wdmd will no longer find the expired connection, and will resume petting
/dev/watchdog (assuming it finds no other failed/expired tests.) If the killed processes
did not exit quickly enough, the expired wdmd connection will not be unregistered, and
/dev/watchdog will reset the host.
Based on these known timeout values, sanlock on another host can calculate, based on the
last host_id renewal, when the failed host will have been reset by its watchdog (or killed
all the necessary processes).
If the sanlock daemon itself fails, crashes, get stuck, it will no longer update the
expiry time for its host_id connections to wdmd, which will also lead to the watchdog
resetting the host.
safety
sanlock leases are meant to guarantee that two process on two hosts are never allowed to
hold the same resource lease at once. If they were, the resource being protected may be
corrupted. There are three levels of protection built into sanlock itself:
1. The paxos leases and delta leases themselves.
2. If the leases cannot function because storage access is lost (host_id's cannot be
renewed), the sanlock daemon kills any pids using resource leases in the lockspace.
3. If the pids do not exit after being killed, or if the sanlock daemon fails, the
watchdog device resets the host.
OPTIONS
COMMAND can be one of three primary top level choices
sanlock daemon start daemon
sanlock client send request to daemon (default command if none given)
sanlock direct access storage directly (no coordination with daemon)
sanlock daemon [options]
-D no fork and print all logging to stderr
-Q 0|1 quiet error messages for common lock contention
-R 0|1 renewal debugging, log debug info for each renewal
-L pri write logging at priority level and up to logfile (-1 none)
-S pri write logging at priority level and up to syslog (-1 none)
-U uid user id
-G gid group id
-t num max worker threads
-w 0|1 use watchdog through wdmd
-h 0|1 use high priority features (realtime scheduling, mlockall)
-a 0|1 use async i/o
-o sec io timeout in seconds
sanlock client action [options]
sanlock client status
Print processes, lockspaces, and resources being manged by the sanlock daemon. Add -D to
show extra internal daemon status for debugging. Add -o p to show resources by pid, or -o
s to show resources by lockspace.
sanlock client host_status -s LOCKSPACE
Print state of host_id delta leases read during the last renewal. Only lockspace_name is
used from the LOCKSPACE argument. Add -D to show extra internal daemon status for
debugging.
sanlock client log_dump
Print the sanlock daemon internal debug log.
sanlock client shutdown
Ask the sanlock daemon to exit. Without the force option (-f 0), the command will be
ignored if any lockspaces exist. With the force option (-f 1), any registered processes
will be killed, their resource leases released, and lockspaces removed.
sanlock client init -s LOCKSPACE
sanlock client init -r RESOURCE
Tell the sanlock daemon to initialize storage for lease areas. (See sanlock direct init.)
sanlock client align -s LOCKSPACE
Tell the sanlock daemon to report the required lease alignment for a storage path. Only
path is used from the LOCKSPACE argument.
sanlock client add_lockspace -s LOCKSPACE
Tell the sanlock daemon to acquire the specified host_id in the lockspace. This will
allow resources to be acquired in the lockspace.
sanlock client inq_lockspace -s LOCKSPACE
Ask to the sanlock daemon weather the lockspace is acquired or not.
sanlock client rem_lockspace -s LOCKSPACE
Tell the sanlock daemon to release the specified host_id in the lockspace. Any processes
holding resource leases in this lockspace will be killed, and the resource leases not
released.
sanlock client command -r RESOURCE -c path args
Register with the sanlock daemon, acquire the specified resource lease, and exec the
command at path with args. When the command exits, the sanlock daemon will release the
lease. -c must be the final option.
sanlock client acquire -r RESOURCE -p pid
sanlock client release -r RESOURCE -p pid
Tell the sanlock daemon to acquire or release the specified resource lease for the given
pid. The pid must be registered with the sanlock daemon. acquire can optionally take a
versioned RESOURCE string RESOURCE:lver, where lver is the version of the lease that must
be acquired, or fail.
sanlock client inquire -p pid
Print the resource leases held the given pid. The format is a versioned RESOURCE string
"RESOURCE:lver" where lver is the version of the lease held.
sanlock client request -r RESOURCE -f force_mode
Request the owner of a resource do something specified by force_mode. A versioned
RESOURCE:lver string must be used with a greater version than is presently held. Zero
lver and force_mode clears the request.
sanlock client examine -r RESOURCE
Examine the request record for the currently held resource lease and carry out the action
specified by the requested force_mode.
sanlock client examine -s LOCKSPACE
Examine requests for all resource leases currently held in the named lockspace. Only
lockspace_name is used from the LOCKSPACE argument.
sanlock direct action [options]
-a 0|1 use async i/o
-o sec io timeout in seconds
sanlock direct init -s LOCKSPACE
sanlock direct init -r RESOURCE
Initialize storage for 2000 host_id (delta) leases for the given lockspace, or initialize
storage for one resource (paxos) lease. Both options require 1MB of space. The host_id
in the LOCKSPACE string is not relevant to initialization, so the value is ignored. (The
default of 2000 host_ids can be changed for special cases using the -n num_hosts and -m
max_hosts options.)
sanlock direct read_leader -s LOCKSPACE
sanlock direct read_leader -r RESOURCE
Read a leader record from disk and print the fields. The leader record is the single
sector of a delta lease, or the first sector of a paxos lease.
sanlock direct read_id -s LOCKSPACE
sanlock direct live_id -s LOCKSPACE
read_id reads a host_id and prints the owner. live_id reads a host_id once a second until
it the timestamp or owner change (prints live 1), or until host_dead_seconds (prints live
0). (host_dead_seconds is derived from the io_timeout option. The live 0|1 conclusion
will not match the sanlock daemon's conclusion unless the configured timeouts match.)
sanlock direct dump path[:offset]
Read disk sectors and print leader records for delta or paxos leases. Add -f 1 to print
the request record values for paxos leases, and host_ids set in delta lease bitmaps.
LOCKSPACE option string
-s lockspace_name:host_id:path:offset
lockspace_name name of lockspace
host_id local host identifier in lockspace
path path to storage reserved for leases
offset offset on path (bytes)
RESOURCE option string
-r lockspace_name:resource_name:path:offset
lockspace_name name of lockspace
resource_name name of resource
path path to storage reserved for leases
offset offset on path (bytes)
RESOURCE option string with version
-r lockspace_name:resource_name:path:offset:lver
lver leader version or SH for shared lease
Defaults
sanlock help shows the default values for the options above.
sanlock version shows the build version.
USAGE
Request/Examine
The first part of making a request for a resource is writing the request record of the
resource (the sector following the leader record). To make a successful request:
• RESOURCE:lver must be greater than the lver presently held by the other host. This
implies the leader record must be read to discover the lver, prior to making a request.
• RESOURCE:lver must be greater than or equal to the lver presently written to the
request record. Two hosts may write a new request at the same time for the same lver,
in which case both would succeed, but the force_mode from the last would win.
• The force_mode must be greater than zero.
• To unconditionally clear the request record (set both lver and force_mode to 0), make
request with RESOURCE:0 and force_mode 0.
The owner of the requested resource will not know of the request unless it is explicitly
told to examine its resources via the "examine" api/command, or otherwise notfied.
The second part of making a request is notifying the resource lease owner that it should
examine the request records of its resource leases. The notification will cause the lease
owner to automatically run the equivalent of "sanlock client examine -s LOCKSPACE" for the
lockspace of the requested resource.
The notification is made using a bitmap in each host_id delta lease. Each bit represents
each of the possible host_ids (1-2000). If host A wants to notify host B to examine its
resources, A sets the bit in its own bitmap that corresponds to the host_id of B. When B
next renews its delta lease, it reads the delta leases for all hosts and checks each
bitmap to see if its own host_id has been set. It finds the bit for its own host_id set
in A's bitmap, and examines its resource request records. (The bit remains set in A's
bitmap for request_finish_seconds.)
force_mode determines the action the resource lease owner should take:
1 (KILL_PID): kill the process holding the resource lease. When the process has exited,
the resource lease will be released, and can then be acquired by anyone.
SEE ALSO
wdmd(8)
2011-08-05 SANLOCK(8)