Provided by: sanlock_3.6.0-4_amd64 bug

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.

       · 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.

       · 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 interchangably.

       · 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 interchangably.

       · 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.

       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

       On  devices  with  512 byte sectors, lockspaces and resources are 1MB in size.  On devices
       with 4096 byte sectors, lockspaces and resources are 8MB in size.  sanlock uses  512  byte
       sectors when shared files are used in place of shared block devices.  Offsets of leases or
       resources must be multiples of 1MB/8MB according to the sector size.

       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.

           # vgcreate vg /dev/sdb
           # lvcreate -n leases -L 1GB vg

       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/test/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/vg/leases:0
           host2:
           # sanlock client add_lockspace -s test:2:/dev/vg/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/vg/leases:1048576
           # sanlock client init -r test:RB:/dev/vg/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/vg/leases:1048576 -p $P
           # sanlock client acquire -r test:RB:/dev/vg/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/vg/leases:1048576 -p $P
           # sanlock client acquire -r test:RB:/dev/vg/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/vg/leases:1048576 -p $P
           # sanlock client release -r test:RB:/dev/vg/leases:2097152 -p $P

       10. Leave the lockspace for the application.

           host1:
           # sanlock client rem_lockspace -s test:1:/dev/vg/leases:0
           host2:
           # sanlock client rem_lockspace -s test:2:/dev/vg/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

       -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.   (Also  see  sanlock  direct
       init.)

       sanlock client init -r RESOURCE

       Tell  the sanlock daemon to initialize a resource lease on disk.  (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 and io timeout are printed.

       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.   (Also  see sanlock direct
       read_leader.)

       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.  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.

   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 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.)  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, 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 suffix
       -r lockspace_name:resource_name:path:offset:lver

       lver leader version

       -r lockspace_name:resource_name:path:offset:SH

       SH indicates shared mode

   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 occured 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  occured  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

SEE ALSO

       wdmd(8)

                                            2015-01-23                                 SANLOCK(8)