Provided by: sanlock_3.8.5-1build1_amd64 
NAME
sanlock - shared storage lock manager
SYNOPSIS
sanlock [COMMAND] [ACTION] ...
DESCRIPTION
sanlock is a lock manager built on shared storage. Hosts with access to the storage can
perform locking. An application running on the hosts is given a small amount of space on
the shared block device or file, and uses sanlock for its own application-specific
synchronization. Internally, the sanlock daemon manages locks using two disk-based lease
algorithms: delta leases and paxos leases.
• delta leases are slow to acquire and demand regular i/o to shared storage. sanlock only
uses them internally to hold a lease on its "host_id" (an integer host identifier from
1-2000). They prevent two hosts from using the same host identifier. The delta lease
renewals also indicate if a host is alive. ("Light-Weight Leases for Storage-Centric
Coordination", Chockler and Malkhi.)
• paxos leases are fast to acquire and sanlock makes them available to applications as
general purpose resource leases. The disk paxos algorithm uses host_id's internally to
represent different hosts, and the owner of a paxos lease. delta leases provide unique
host_id's for implementing paxos leases, and delta lease renewals serve as a proxy for
paxos lease renewal. ("Disk Paxos", Eli Gafni and Leslie Lamport.)
Externally, the sanlock daemon exposes a locking interface through libsanlock in terms of
"lockspaces" and "resources". A lockspace is a locking context that an application
creates for itself on shared storage. When the application on each host is started, it
"joins" the lockspace. It can then create "resources" on the shared storage. Each
resource represents an application-specific entity. The application can acquire and
release leases on resources.
To use sanlock from an application:
• Allocate shared storage for an application, e.g. a shared LUN or LV from a SAN, or files
from NFS.
• Provide the storage to the application.
• The application uses this storage with libsanlock to create a lockspace and resources
for itself.
• The application joins the lockspace when it starts.
• The application acquires and releases leases on resources.
How lockspaces and resources translate to delta leases and paxos leases within sanlock:
Lockspaces
• A lockspace is based on delta leases held by each host using the lockspace.
• A lockspace is a series of 2000 delta leases on disk, and requires 1MB of storage. (See
Storage below for size variations.)
• A lockspace can support up to 2000 concurrent hosts using it, each using a different
delta lease.
• Applications can i) create, ii) join and iii) leave a lockspace, which corresponds to i)
initializing the set of delta leases on disk, ii) acquiring one of the delta leases and
iii) releasing the delta lease.
• When a lockspace is created, a unique lockspace name and disk location is provided by
the application.
• When a lockspace is created/initialized, sanlock formats the sequence of 2000 on-disk
delta lease structures on the file or disk, e.g. /mnt/leasefile (NFS) or /dev/vg/lv
(SAN).
• The 2000 individual delta leases in a lockspace are identified by number:
1,2,3,...,2000.
• Each delta lease is a 512 byte sector in the 1MB lockspace, offset by its number, e.g.
delta lease 1 is offset 0, delta lease 2 is offset 512, delta lease 2000 is offset
1023488. (See Storage below for size variations.)
• When an application joins a lockspace, it must specify the lockspace name, the lockspace
location on shared disk/file, and the local host's host_id. sanlock then acquires the
delta lease corresponding to the host_id, e.g. joining the lockspace with host_id 1
acquires delta lease 1.
• The terms delta lease, lockspace lease, and host_id lease are used interchangeably.
• sanlock acquires a delta lease by writing the host's unique name to the delta lease disk
sector, reading it back after a delay, and verifying it is the same.
• If a unique host name is not specified, sanlock generates a uuid to use as the host's
name. The delta lease algorithm depends on hosts using unique names.
• The application on each host should be configured with a unique host_id, where the
host_id is an integer 1-2000.
• If hosts are misconfigured and have the same host_id, the delta lease algorithm is
designed to detect this conflict, and only one host will be able to acquire the delta
lease for that host_id.
• A delta lease ensures that a lockspace host_id is being used by a single host with the
unique name specified in the delta lease.
• Resolving delta lease conflicts is slow, because the algorithm is based on waiting and
watching for some time for other hosts to write to the same delta lease sector. If
multiple hosts try to use the same delta lease, the delay is increased substantially.
So, it is best to configure applications to use unique host_id's that will not conflict.
• After sanlock acquires a delta lease, the lease must be renewed until the application
leaves the lockspace (which corresponds to releasing the delta lease on the host_id.)
• sanlock renews delta leases every 20 seconds (by default) by writing a new timestamp
into the delta lease sector.
• When a host acquires a delta lease in a lockspace, it can be referred to as "joining"
the lockspace. Once it has joined the lockspace, it can use resources associated with
the lockspace.
Resources
• A lockspace is a context for resources that can be locked and unlocked by an
application.
• sanlock uses paxos leases to implement leases on resources. The terms paxos lease and
resource lease are used interchangeably.
• A paxos lease exists on shared storage and requires 1MB of space. It contains a unique
resource name and the name of the lockspace.
• An application assigns its own meaning to a sanlock resource and the leases on it. A
sanlock resource could represent some shared object like a file, or some unique role
among the hosts.
• Resource leases are associated with a specific lockspace and can only be used by hosts
that have joined that lockspace (they are holding a delta lease on a host_id in that
lockspace.)
• An application must keep track of the disk locations of its lockspaces and resources.
sanlock does not maintain any persistent index or directory of lockspaces or resources
that have been created by applications, so applications need to remember where they have
placed their own leases (which files or disks and offsets).
• sanlock does not renew paxos leases directly (although it could). Instead, the renewal
of a host's delta lease represents the renewal of all that host's paxos leases in the
associated lockspace. In effect, many paxos lease renewals are factored out into one
delta lease renewal. This reduces i/o when many paxos leases are used.
• The disk paxos algorithm allows multiple hosts to all attempt to acquire the same paxos
lease at once, and will produce a single winner/owner of the resource lease. (Shared
resource leases are also possible in addition to the default exclusive leases.)
• The disk paxos algorithm involves a specific sequence of reading and writing the sectors
of the paxos lease disk area. Each host has a dedicated 512 byte sector in the paxos
lease disk area where it writes its own "ballot", and each host reads the entire disk
area to see the ballots of other hosts. The first sector of the disk area is the
"leader record" that holds the result of the last paxos ballot. The winner of the paxos
ballot writes the result of the ballot to the leader record (the winner of the ballot
may have selected another contending host as the owner of the paxos lease.)
• After a paxos lease is acquired, no further i/o is done in the paxos lease disk area.
• Releasing the paxos lease involves writing a single sector to clear the current owner in
the leader record.
• If a host holding a paxos lease fails, the disk area of the paxos lease still indicates
that the paxos lease is owned by the failed host. If another host attempts to acquire
the paxos lease, and finds the lease is held by another host_id, it will check the delta
lease of that host_id. If the delta lease of the host_id is being renewed, then the
paxos lease is owned and cannot be acquired. If the delta lease of the owner's host_id
has expired, then the paxos lease is expired and can be taken (by going through the
paxos lease algorithm.)
• The "interaction" or "awareness" between hosts of each other is limited to the case
where they attempt to acquire the same paxos lease, and need to check if the referenced
delta lease has expired or not.
• When hosts do not attempt to lock the same resources concurrently, there is no host
interaction or awareness. The state or actions of one host have no effect on others.
• To speed up checking delta lease expiration (in the case of a paxos lease conflict),
sanlock keeps track of past renewals of other delta leases in the lockspace.
Resource Index
The resource index (rindex) is an optional sanlock feature that applications can use to
keep track of resource lease offsets. Without the rindex, an application must keep track
of where its resource leases exist on disk and find available locations when creating new
leases.
The sanlock rindex uses two align-size areas on disk following the lockspace. The first
area holds rindex entries; each entry records a resource lease name and location. The
second area holds a private paxos lease, used by sanlock internally to protect rindex
updates.
The application creates the rindex on disk with the "format" function. Format is a disk-
only operation and does not interact with the live lockspace, so it can be called without
first calling add_lockspace. The application needs to follow the convention of writing
the lockspace at the start of the device (offset 0) and formatting the rindex immediately
following the lockspace area. When formatting, the application must set flags for sector
size and align size to match those for the lockspace.
To use the rindex, the application:
• Uses the "create" function to create a new resource lease on disk. This takes the place
of the write_resource function. The create function requires the location of the rindex
and the name of the new resource lease. sanlock finds a free lease area, writes the new
resource lease at that location, updates the rindex with the name:offset, and returns
the offset to the caller. The caller uses this offset when acquiring the resource
lease.
• Uses the "delete" function to remove a resource disk on disk (also corresponding to the
write_resource function.) sanlock clears the resource lease and the rindex entry for
it. A subsequent call to create may use this same disk location for a different
resource lease.
• Uses the "lookup" function to discover the offset of a resource lease given the resource
lease name. The caller would typically call this prior to acquiring the resource lease.
• Uses the "rebuild" function to recreate the rindex if it is damaged or becomes
inconsistent. This function scans the disk for resource leases and creates new rindex
entries to match the leases it finds.
• The "update" function manipulates rindex entries directly and should not normally be
used by the application. In normal usage, the create and delete functions manipulate
rindex entries. Update is mainly useful for testing or repairs.
Expiration
• If a host fails to renew its delta lease, e.g. it looses access to the storage, its
delta lease will eventually expire and another host will be able to take over any
resource leases held by the host. sanlock must ensure that the application on two
different hosts is not holding and using the same lease concurrently.
• When sanlock has failed to renew a delta lease for a period of time, it will begin
taking measures to stop local processes (applications) from using any resource leases
associated with the expiring lockspace delta lease. sanlock enters this "recovery mode"
well ahead of the time when another host could take over the locally owned leases.
sanlock must have sufficient time to stop all local processes that are using the
expiring leases.
• sanlock uses three methods to stop local processes that are using expiring leases:
1. Graceful shutdown. sanlock will execute a "graceful shutdown" program that the
application previously specified for this case. The shutdown program tells the
application to shut down because its leases are expiring. The application must respond
by stopping its activities and releasing its leases (or exit). If an application does
not specify a graceful shutdown program, sanlock sends SIGTERM to the process instead.
The process must release its leases or exit in a prescribed amount of time (see -g), or
sanlock proceeds to the next method of stopping.
2. Forced shutdown. sanlock will send SIGKILL to processes using the expiring leases.
The processes have a fixed amount of time to exit after receiving SIGKILL. If any do
not exit in this time, sanlock will proceed to the next method.
3. Host reset. sanlock will trigger the host's watchdog device to forcibly reset it.
sanlock carefully manages the timing of the watchdog device so that it fires shortly
before any other host could take over the resource leases held by local processes.
Failures
If a process holding resource leases fails or exits without releasing its leases, sanlock
will release the leases for it automatically (unless persistent resource leases were
used.)
If the sanlock daemon cannot renew a lockspace delta lease for a specific period of time
(see Expiration), sanlock will enter "recovery mode" where it attempts to stop and/or kill
any processes holding resource leases in the expiring lockspace. If the processes do not
exit in time, sanlock will force the host to be reset using the local watchdog device.
If the sanlock daemon crashes or hangs, it will not renew the expiry time of the per-
lockspace connections it had to the wdmd daemon. This will lead to the expiration of the
local watchdog device, and the host will be reset.
Watchdog
sanlock uses the wdmd(8) daemon to access /dev/watchdog. wdmd multiplexes multiple
timeouts onto the single watchdog timer. This is required because delta leases for each
lockspace are renewed and expire independently.
sanlock maintains a wdmd connection for each lockspace delta lease being renewed. Each
connection has an expiry time for some seconds in the future. After each successful delta
lease renewal, the expiry time is renewed for the associated wdmd connection. If wdmd
finds any connection expired, it will not renew the /dev/watchdog timer. Given enough
successive failed renewals, the watchdog device will fire and reset the host. (Given the
multiplexing nature of wdmd, shorter overlapping renewal failures from multiple lockspaces
could cause spurious watchdog firing.)
The direct link between delta lease renewals and watchdog renewals provides a predictable
watchdog firing time based on delta lease renewal timestamps that are visible from other
hosts. sanlock knows the time the watchdog on another host has fired based on the delta
lease time. Furthermore, if the watchdog device on another host fails to fire when it
should, the continuation of delta lease renewals from the other host will make this
evident and prevent leases from being taken from the failed host.
If sanlock is able to stop/kill all processing using an expiring lockspace, the associated
wdmd connection for that lockspace is removed. The expired wdmd connection will no longer
block /dev/watchdog renewals, and the host should avoid being reset.
Storage
The sector size and the align size should be specified when creating lockspaces and
resources (and rindex). The "align size" is the size on disk of a lockspace or a
resource, i.e. the amount of disk space it uses. Lockspaces and resources should use
matching sector and align sizes, and must use offsets in multiples of the align size. The
max number of hosts that can use a lockspace or resource depends on the combination of
sector size and align size, shown below. The host_id of hosts using the lockspace can be
no larger than the max_hosts value for the lockspace.
Accepted combinations of sector size and align size, and the corresponding max_hosts (and
max host_id) are:
sector_size 512, align_size 1M, max_hosts 2000
sector_size 4096, align_size 1M, max_hosts 250
sector_size 4096, align_size 2M, max_hosts 500
sector_size 4096, align_size 4M, max_hosts 1000
sector_size 4096, align_size 8M, max_hosts 2000
When sector_size and align_size are not specified, the behavior matches the behavior
before these sizes could be configured: on devices which report sector size 512,
512/1M/2000 is used, on devices which report sector size 4096, 4096/8M/2000 is used, and
on files, 512/1M/2000 is always used. (Other combinations are not compatible with sanlock
version 3.6 or earlier.)
Using sanlock on shared block devices that do host based mirroring or replication is not
likely to work correctly. When using sanlock on shared files, all sanlock io should go to
one file server.
Example
This is an example of creating and using lockspaces and resources from the command line.
(Most applications would use sanlock through libsanlock rather than through the command
line.)
1. Allocate shared storage for sanlock leases.
This example assumes 512 byte sectors on the device, in which case the lockspace needs
1MB and each resource needs 1MB.
The example shared block device accessible to all hosts is /dev/leases.
2. Start sanlock on all hosts.
The -w 0 disables use of the watchdog for testing.
# sanlock daemon -w 0
3. Start a dummy application on all hosts.
This sanlock command registers with sanlock, then execs the sleep command which
inherits the registered fd. The sleep process acts as the dummy application. Because
the sleep process is registered with sanlock, leases can be acquired for it.
# sanlock client command -c /bin/sleep 600 &
4. Create a lockspace for the application (from one host).
The lockspace is named "test".
# sanlock client init -s test:0:/dev/leases:0
5. Join the lockspace for the application.
Use a unique host_id on each host.
host1:
# sanlock client add_lockspace -s test:1:/dev/leases:0
host2:
# sanlock client add_lockspace -s test:2:/dev/leases:0
6. Create two resources for the application (from one host).
The resources are named "RA" and "RB". Offsets are used on the same device as the
lockspace. Different LVs or files could also be used.
# sanlock client init -r test:RA:/dev/leases:1048576
# sanlock client init -r test:RB:/dev/leases:2097152
7. Acquire resource leases for the application on host1.
Acquire an exclusive lease (the default) on the first resource, and a shared lease
(SH) on the second resource.
# export P=`pidof sleep`
# sanlock client acquire -r test:RA:/dev/leases:1048576 -p $P
# sanlock client acquire -r test:RB:/dev/leases:2097152:SH -p $P
8. Acquire resource leases for the application on host2.
Acquiring the exclusive lease on the first resource will fail because it is held by
host1. Acquiring the shared lease on the second resource will succeed.
# export P=`pidof sleep`
# sanlock client acquire -r test:RA:/dev/leases:1048576 -p $P
# sanlock client acquire -r test:RB:/dev/leases:2097152:SH -p $P
9. Release resource leases for the application on both hosts.
The sleep pid could also be killed, which will result in the sanlock daemon releasing
its leases when it exits.
# sanlock client release -r test:RA:/dev/leases:1048576 -p $P
# sanlock client release -r test:RB:/dev/leases:2097152 -p $P
10. Leave the lockspace for the application.
host1:
# sanlock client rem_lockspace -s test:1:/dev/leases:0
host2:
# sanlock client rem_lockspace -s test:2:/dev/leases:0
11. Stop sanlock on all hosts.
# sanlock shutdown
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)
Daemon Command
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
-H num renewal history size
-t num max worker threads
-g sec seconds for graceful recovery
-w 0|1 use watchdog through wdmd
-h 0|1 use high priority (RR) scheduling
-l num use mlockall (0 none, 1 current, 2 current and future)
-b sec seconds a host id bit will remain set in delta lease bitmap
-e str local host name used in delta leases
Client Command
sanlock client action [options]
sanlock client status
Print processes, lockspaces, and resources being managed 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
Print state of host_id delta leases read during the last renewal. State of all lockspaces
is shown (use -s to select one). Add -D to show extra internal daemon status for
debugging.
sanlock client gets
Print lockspaces being managed by the sanlock daemon. The LOCKSPACE string will be
followed by ADD or REM if the lockspace is currently being added or removed. Add -h 1 to
also show hosts in each lockspace.
sanlock client renewal -s LOCKSPACE
Print a history of renewals with timing details. See the Renewal history section below.
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. With the wait
option (-w 1), the command will wait for a result from the daemon indicating that it has
shut down and is exiting, or cannot shut down because lockspaces exist (command fails).
sanlock client init -s LOCKSPACE
Tell the sanlock daemon to initialize a lockspace on disk. The -o option can be used to
specify the io timeout to be written in the host_id leases. The -Z and -A options can be
used to specify the sector size and align size, and both should be set together. (Also
see sanlock direct init.)
sanlock client init -r RESOURCE
Tell the sanlock daemon to initialize a resource lease on disk. The -Z and -A options can
be used to specify the sector size and align size, and both should be set together. (Also
see sanlock direct init.)
sanlock client read -s LOCKSPACE
Tell the sanlock daemon to read a lockspace from disk. Only the LOCKSPACE path and offset
are required. If host_id is zero, the first record at offset (host_id 1) is used. The
complete LOCKSPACE is printed. Add -D to print other details. (Also see sanlock direct
read_leader.)
sanlock client read -r RESOURCE
Tell the sanlock daemon to read a resource lease from disk. Only the RESOURCE path and
offset are required. The complete RESOURCE is printed. Add -D to print other details.
(Also see sanlock direct read_leader.)
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. The -o option can be used to specify the
io timeout of the acquiring host, and will be written in the host_id lease.
sanlock client inq_lockspace -s LOCKSPACE
Inquire about the state of the lockspace in the sanlock daemon, whether it is being added
or removed, or is joined.
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 convert -r RESOURCE -p pid
Tell the sanlock daemon to convert the mode of the specified resource lease for the given
pid. If the existing mode is exclusive (default), the mode of the lease can be converted
to shared with RESOURCE:SH. If the existing mode is shared, the mode of the lease can be
converted to exclusive with RESOURCE (no :SH suffix).
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 client set_event -s LOCKSPACE -i host_id -g gen -e num -d num
Set an event for another host. When the sanlock daemon next renews its delta lease for
the lockspace it will: set the bit for the host_id in its bitmap, and set the generation,
event and data values in its own delta lease. An application that has registered for
events from this lockspace on the destination host will get the event that has been set
when the destination sees the event during its next delta lease renewal.
sanlock client set_config -s LOCKSPACE
Set a configuration value for a lockspace. Only lockspace_name is used from the LOCKSPACE
argument. The USED flag has the same effect on a lockspace as a process holding a
resource lease that will not exit. The USED_BY_ORPHANS flag means that an orphan resource
lease will have the same effect as the USED.
-u 0|1 Set (1) or clear (0) the USED flag.
-O 0|1 Set (1) or clear (0) the USED_BY_ORPHANS flag.
sanlock client format -x RINDEX
Create a resource index on disk. Use -Z and -A to set the sector size and align size to
match the lockspace.
sanlock client create -x RINDEX -e resource_name
Create a new resource lease on disk, using the rindex to find a free offset.
sanlock client delete -x RINDEX -e resource_name[:offset]
Delete an existing resource lease on disk.
sanlock client lookup -x RINDEX -e resource_name
Look up the offset of an existing resource lease by name on disk, using the rindex. With
no -e option, lookup returns the next free lease offset. If -e specifes both name and
offset, the lookup verifies both are correct.
sanlock client update -x RINDEX -e resource_name[:offset] [-z 0|1]
Add (-z 0) or remove (-z 1) an rindex entry on disk.
sanlock client rebuild -x RINDEX
Rebuild the rindex entries by scanning the disk for resource leases.
Direct Command
sanlock direct action [options]
-o sec io timeout in seconds
sanlock direct init -s LOCKSPACE
sanlock direct init -r RESOURCE
Initialize storage for a lockspace or resource. Use the -Z and -A flags to specify the
sector size and align size. The max hosts that can use the lockspace/resource (and the
max possible host_id) is determined by the sector/align size combination. Possible
combinations are: 512/1M, 4096/1M, 4096/2M, 4096/4M, 4096/8M. Lockspaces and resources
both use the same amount of space (align_size) for each combination. When initializing a
lockspace, sanlock initializes delta leases for max_hosts in the given space. When
initializing a resource, sanlock initializes a single paxos lease in the space. With -s,
the -o option specifies the io timeout to be written in the host_id leases. With -r, the
-z 1 option invalidates the resource lease on disk so it cannot be used until
reinitialized normally.
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 dump path[:offset[:size]]
Read disk sectors and print leader records for delta or paxos leases. Add -f 1 to print
the request record values for paxos leases, host_ids set in delta lease bitmaps, and
rindex entries.
sanlock direct format -x RINDEX
sanlock direct lookup -x RINDEX -e resource_name
sanlock direct update -x RINDEX -e resource_name[:offset] [-z 0|1]
sanlock direct rebuild -x RINDEX
Access the resource index on disk without going through the sanlock daemon. This
precludes using the internal paxos lease to protect rindex modifications. See client
equivalents for descriptions.
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 to use 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 to use leases
offset offset on path (bytes)
RESOURCE option string with suffix
-r lockspace_name:resource_name:path:offset:lver
lver leader version
-r lockspace_name:resource_name:path:offset:SH
SH indicates shared mode
RINDEX option string
-x lockspace_name:path:offset
lockspace_name name of lockspace
path path to storage to use for leases
offset offset on path (bytes) of rindex
Defaults
sanlock help shows the default values for the options above.
sanlock version shows the build version.
OTHER
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 set_bitmap_seconds.)
force_mode determines the action the resource lease owner should take:
• FORCE (1): 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. The kill
signal is SIGKILL (or SIGTERM if SIGKILL is restricted.)
• GRACEFUL (2): run the program configured by sanlock_killpath against the process holding
the resource lease. If no killpath is defined, then FORCE is used.
Persistent and orphan resource leases
A resource lease can be acquired with the PERSISTENT flag (-P 1). If the process holding
the lease exits, the lease will not be released, but kept on an orphan list. Another
local process can acquire an orphan lease using the ORPHAN flag (-O 1), or release the
orphan lease using the ORPHAN flag (-O 1). All orphan leases can be released by setting
the lockspace name (-s lockspace_name) with no resource name.
Renewal history
sanlock saves a limited history of lease renewal information in each lockspace. See
sanlock.conf renewal_history_size to set the amount of history or to disable (set to 0).
IO times are measured in delta lease renewal (each delta lease renewal includes one read
and one write).
For each successful renewal, a record is saved that includes:
• the timestamp written in the delta lease by the renewal
• the time in milliseconds taken by the delta lease read
• the time in milliseconds taken by the delta lease write
Also counted and recorded are the number io timeouts and other io errors that occur
between successful renewals.
Two consecutive successful renewals would be recorded as:
timestamp=5332 read_ms=482 write_ms=5525 next_timeouts=0 next_errors=0
timestamp=5353 read_ms=99 write_ms=3161 next_timeouts=0 next_errors=0
Those fields are:
• timestamp is the value written into the delta lease during that renewal.
• read_ms/write_ms are the milliseconds taken for the renewal read/write ios.
• next_timeouts are the number of io timeouts that occurred after the renewal recorded on
that line, and before the next successful renewal on the following line.
• next_errors are the number of io errors (not timeouts) that occurred after renewal
recorded on that line, and before the next successful renewal on the following line.
The command 'sanlock client renewal -s lockspace_name' reports the full history of
renewals saved by sanlock, which by default is 180 records, about 1 hour of history when
using a 20 second renewal interval for a 10 second io timeout.
INTERNALS
Disk Format
• This example uses 512 byte sectors.
• Each lockspace is 1MB. It holds 2000 delta_leases, one per sector, supporting up to
2000 hosts.
• Each paxos_lease is 1MB. It is used as a lease for one resource.
• The leader_record structure is used differently by each lease type.
• To display all leader_record fields, see sanlock direct read_leader.
• A lockspace is often followed on disk by the paxos_leases used within that lockspace,
but this layout is not required.
• The request_record and host_id bitmap are used for requests/events.
• The mode_block contains the SHARED flag indicating a lease is held in the shared mode.
• In a lockspace, the host using host_id N writes to a single delta_lease in sector N-1.
No other hosts write to this sector. All hosts read all lockspace sectors when renewing
their own delta_lease, and are able to monitor renewals of all delta_leases.
• In a paxos_lease, each host has a dedicated sector it writes to, containing its own
paxos_dblock and mode_block structures. Its sector is based on its host_id; host_id 1
writes to the dblock/mode_block in sector 2 of the paxos_lease.
• The paxos_dblock structures are used by the paxos_lease algorithm, and the result is
written to the leader_record.
0x000000 lockspace foo:0:/path:0
(There is no representation on disk of the lockspace in general, only the sequence of
specific delta_leases which collectively represent the lockspace.)
delta_lease foo:1:/path:0
0x000 0 leader_record (sector 0, for host_id 1)
magic: 0x12212010
space_name: foo
resource_name: host uuid/name
...
host_id bitmap (leader_record + 256)
delta_lease foo:2:/path:0
0x200 512 leader_record (sector 1, for host_id 2)
magic: 0x12212010
space_name: foo
resource_name: host uuid/name
...
host_id bitmap (leader_record + 256)
delta_lease foo:3:/path:0
0x400 1024 leader_record (sector 2, for host_id 3)
magic: 0x12212010
space_name: foo
resource_name: host uuid/name
...
host_id bitmap (leader_record + 256)
delta_lease foo:2000:/path:0
0xF9E00 leader_record (sector 1999, for host_id 2000)
magic: 0x12212010
space_name: foo
resource_name: host uuid/name
...
host_id bitmap (leader_record + 256)
0x100000 paxos_lease foo:example1:/path:1048576
0x000 0 leader_record (sector 0)
magic: 0x06152010
space_name: foo
resource_name: example1
0x200 512 request_record (sector 1)
magic: 0x08292011
0x400 1024 paxos_dblock (sector 2, for host_id 1)
0x480 1152 mode_block (paxos_dblock + 128)
0x600 1536 paxos_dblock (sector 3, for host_id 2)
0x680 1664 mode_block (paxos_dblock + 128)
0x800 2048 paxos_dblock (sector 4, for host_id 3)
0x880 2176 mode_block (paxos_dblock + 128)
0xFA200 paxos_dblock (sector 2001, for host_id 2000)
0xFA280 mode_block (paxos_dblock + 128)
0x200000 paxos_lease foo:example2:/path:2097152
0x000 0 leader_record (sector 0)
magic: 0x06152010
space_name: foo
resource_name: example2
0x200 512 request_record (sector 1)
magic: 0x08292011
0x400 1024 paxos_dblock (sector 2, for host_id 1)
0x480 1152 mode_block (paxos_dblock + 128)
0x600 1536 paxos_dblock (sector 3, for host_id 2)
0x680 1664 mode_block (paxos_dblock + 128)
0x800 2048 paxos_dblock (sector 4, for host_id 3)
0x880 2176 mode_block (paxos_dblock + 128)
0xFA200 paxos_dblock (sector 2001, for host_id 2000)
0xFA280 mode_block (paxos_dblock + 128)
Lease ownership
Not shown in the leader_record structures above are the owner_id, owner_generation and
timestamp fields. These are the fields that define the lease owner.
The delta_lease at sector N for host_id N+1 has leader_record.owner_id N+1. The
leader_record.owner_generation is incremented each time the delta_lease is acquired. When
a delta_lease is acquired, the leader_record.timestamp field is set to the time of the
host and the leader_record.resource_name is set to the unique name of the host. When the
host renews the delta_lease, it writes a new leader_record.timestamp. When a host
releases a delta_lease, it writes zero to leader_record.timestamp.
When a host acquires a paxos_lease, it uses the host_id/generation value from the
delta_lease it holds in the lockspace. It uses this host_id/generation to identify itself
in the paxos_dblock when running the paxos algorithm. The result of the algorithm is the
winning host_id/generation - the new owner of the paxos_lease. The winning
host_id/generation are written to the paxos_lease leader_record.owner_id and
leader_record.owner_generation fields and leader_record.timestamp is set. When a host
releases a paxos_lease, it sets leader_record.timestamp to 0.
When a paxos_lease is free (leader_record.timestamp is 0), multiple hosts may attempt to
acquire it. The paxos algorithm, using the paxos_dblock structures, will select only one
of the hosts as the new owner, and that owner is written in the leader_record. The
paxos_lease will no longer be free (non-zero timestamp). Other hosts will see this and
will not attempt to acquire the paxos_lease until it is free again.
If a paxos_lease is owned (non-zero timestamp), but the owner has not renewed its
delta_lease for a specific length of time, then the owner value in the paxos_lease becomes
expired, and other hosts will use the paxos algorithm to acquire the paxos_lease, and set
a new owner.
FILES
/etc/sanlock/sanlock.conf
• quiet_fail = 1
See -Q
• debug_renew = 0
See -R
• logfile_priority = 4
See -L
• logfile_use_utc = 0
Use UTC instead of local time in log messages.
• syslog_priority = 3
See -S
• names_log_priority = 4
Log resource names at this priority level (uses syslog priority numbers). If this is
greater than or equal to logfile_priority, each requested resource name and location is
recorded in sanlock.log.
• use_watchdog = 1
See -w
• high_priority = 1
See -h
• mlock_level = 1
See -l
• sh_retries = 8
The number of times to try acquiring a paxos lease when acquiring a shared lease when
the paxos lease is held by another host acquiring a shared lease.
• uname = sanlock
See -U
• gname = sanlock
See -G
• our_host_name = <str>
See -e
• renewal_read_extend_sec = <seconds>
If a renewal read i/o times out, wait this many additional seconds for that read to
complete at the start of the subsequent renewal attempt. When not configured, sanlock
waits for an additional io_timeout seconds for a previous timed out read to complete.
• renewal_history_size = 180
See -H
• paxos_debug_all = 0
Include all details in the paxos debug logging.
• debug_io = <str>
Add debug logging for each i/o. "submit" (no quotes) produces debug output at
submission time, "complete" produces debug output at completion time, and
"submit,complete" (no space) produces both.
• max_sectors_kb = <str>|<num>
Set to "ignore" (no quotes) to prevent sanlock from checking or changing max_sectors_kb
for the lockspace disk when starting a lockspace. Set to "align" (no quotes) to set
max_sectors_kb for the lockspace disk to the align size of the lockspace. Set to a
number to set a specific number of KB for all lockspace disks.
• debug_clients = 0
Enable or disable debug logging for all client connections to the sanlock daemon.
• debug_cmd = +|-<name>
Enable (+name) or disable (-name) debug logging at the command processing level for
specifically named commands, e.g. "debug_cmd = +acquire", or "debug_cmd =
-inq_lockspace". Repeat this line for each command name. Use a plus prefix before the
name to enable and a minus prefix to disable. By default sanlock disables some command
level debugging for commands that are often repetitive and fill the in memory debug
buffer. This only affects debug logging, not errors or warnings, and disabling command
level debugging for a command does not disable lower level debugging for that command.
Special values +all and -all can be used to enable or disable all commands, and can be
used before or after other debug_cmd lines.
• write_init_io_timeout = <seconds>
The io timeout to use when initializing ondisk lease structures for a lockspace or
resource. This timeout is not used as a part of either lease algorithm (as the standard
io_timeout is.)
• max_worker_threads = <num>
See -t
SEE ALSO
wdmd(8)
2015-01-23 SANLOCK(8)