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       qdisk - a disk-based quorum daemon for CMAN / Linux-Cluster

1. Overview

1.1 Problem

       In  some  situations,  it  may  be  necessary or desirable to sustain a
       majority node failure of a cluster without  introducing  the  need  for
       asymmetric  cluster  configurations  (e.g.  client-server,  or heavily-
       weighted voting nodes).

1.2. Design Requirements

       * Ability to sustain 1..(n-1)/n simultaneous node failures, without the
       danger  of  a simple network partition causing a split brain.  That is,
       we need to be able to ensure that the  majority  failure  case  is  not
       merely the result of a network partition.

       *  Ability  to use external reasons for deciding which partition is the
       the quorate partition in a partitioned cluster.  For  example,  a  user
       may  have  a  service running on one node, and that node must always be
       the master in the event of a network partition.  Or, a node might  lose
       all  network  connectivity  except  the cluster communication path - in
       which case, a user may wish that node to be evicted from the cluster.

       * Integration with CMAN.  We must not require CMAN to run with  us  (or
       without  us).   Linux-Cluster does not require a quorum disk normally -
       introducing new requirements on the base of how Linux-Cluster  operates
       is not allowed.

       * Data integrity.  In order to recover from a majority failure, fencing
       is required.  The fencing  subsystem  is  already  provided  by  Linux-

       *  Non-reliance  on  hardware  or  protocol specific methods (i.e. SCSI
       reservations).  This ensures the quorum disk algorithm can be  used  on
       the widest range of hardware configurations possible.

       *  Little  or  no  memory allocation after initialization.  In critical
       paths during failover, we do not want to  have  to  worry  about  being
       killed  during  a  memory  pressure situation because we request a page
       fault, and the Linux OOM killer responds...

1.3. Hardware Considerations and Requirements

1.3.1. Concurrent, Synchronous, Read/Write Access

       This quorum daemon requires  a  shared  block  device  with  concurrent
       read/write  access  from  all  nodes  in the cluster.  The shared block
       device can be a multi-port SCSI RAID array, a Fiber-Channel RAID SAN, a
       RAIDed  iSCSI target, or even GNBD.  The quorum daemon uses O_DIRECT to
       write to the device.

1.3.2. Bargain-basement JBODs need not apply

       There is a minimum performance requirement inherent  when  using  disk-
       based  cluster  quorum  algorithms, so design your cluster accordingly.
       Using a cheap JBOD with old SCSI2 disks on a multi-initiator  bus  will
       cause problems at the first load spike.  Plan your loads accordingly; a
       node’s inability to write to the quorum disk in a  timely  manner  will
       cause  the  cluster  to  evict  the  node.   Using  host-RAID or multi-
       initiator  parallel  SCSI  configurations  with  the  qdisk  daemon  is
       unlikely  to  work,  and  will  probably  cause administrators a lot of
       frustration.   That  having  been  said,  because  the   timeouts   are
       configurable,  most  hardware  should work if the timeouts are set high

1.3.3. Fencing is Required

       In order to maintain data integrity under all failure scenarios, use of
       this  quorum  daemon  requires adequate fencing, preferably power-based
       fencing.  Watchdog timers and software-based solutions  to  reboot  the
       node   internally,   while  possibly  sufficient,  are  not  considered
       ’fencing’ for the purposes of using the quorum disk.

1.4. Limitations

       * At this time, this daemon supports a maximum of 16  nodes.   This  is
       primarily  a  scalability  issue:  As  we  increase  the node count, we
       increase the amount of synchronous I/O contention on the shared  quorum

       *  Cluster  node  IDs must be statically configured in cluster.conf and
       must be numbered from 1..16 (there can be gaps, of course).

       * Cluster node votes should be more or less equal.

       * CMAN must be running before the qdisk program  can  operate  in  full
       capacity.  If CMAN is not running, qdisk will wait for it.

       *  CMAN’s eviction timeout should be at least 2x the quorum daemon’s to
       give the quorum daemon adequate time to converge on a master  during  a
       failure + load spike situation.

       *  For  ’all-but-one’  failure  operation,  the  total  number of votes
       assigned to the quorum device should be equal to or  greater  than  the
       total  number  of  node-votes  in the cluster.  While it is possible to
       assign only one (or a few) votes to the quorum device, the  effects  of
       doing so have not been explored.

       *  For  ’tiebreaker’  operation  in  a  two-node  cluster, unset CMAN’s
       two_node flag (or set it to 0), set CMAN’s expected votes to  ’3’,  set
       each  node’s  vote  to  ’1’, and set qdisk’s vote count to ’1’ as well.
       This will allow the cluster to operate if either both nodes are online,
       or a single node & the heuristics.

       *  Currently,  the  quorum disk daemon is difficult to use with CLVM if
       the quorum disk resides on a CLVM  logical  volume.   CLVM  requires  a
       quorate  cluster  to correctly operate, which introduces a chicken-and-
       egg problem for starting the cluster: CLVM needs quorum, but the quorum
       daemon  needs  CLVM  (if  and  only  if the quorum device lies on CLVM-
       managed storage).  One way to work around this  is  to  *not*  set  the
       cluster’s  expected  votes to include the quorum daemon’s votes.  Bring
       all nodes online, and start the quorum daemon *after* the whole cluster
       is  running.  This will allow the expected votes to increase naturally.

2. Algorithms

2.1. Heartbeating & Liveliness Determination

       Nodes update individual status blocks on the quorum  disk  at  a  user-
       defined rate.  Each write of a status block alters the timestamp, which
       is what other nodes use to decide whether a node has hung or not.   If,
       after  a  user-defined number of ’misses’ (that is, failure to update a
       timestamp), a node is declared offline.   After  a  certain  number  of
       ’hits’  (changed  timestamp + "i am alive" state), the node is declared

       The status block contains additional information, such as a bitmask  of
       the  nodes  that node believes are online.  Some of this information is
       used by the master - while some is just for performance recording,  and
       may  be used at a later time.  The most important pieces of information
       a node writes to its status block are:

            - Timestamp
            - Internal state (available / not available)
            - Score
            - Known max score (may be used in the  future  to  detect  invalid
            - Vote/bid messages
            - Other nodes it thinks are online

2.2. Scoring & Heuristics

       The  administrator  can configure up to 10 purely arbitrary heuristics,
       and must exercise caution in doing so.   At  least  one  administrator-
       defined heuristic is required for operation, but it is generally a good
       idea to have more than one heuristic.  By default, only  nodes  scoring
       over  1/2  of the total maximum score will claim they are available via
       the quorum disk, and a node (master or otherwise) whose score drops too
       low will remove itself (usually, by rebooting).

       The  heuristics  themselves  can  be any command executable by ’sh -c’.
       For example, in early testing the following was used:

            <heuristic program="[ -f /quorum ]" score="10" interval="2"/>

       This is a literal sh-ism which tests for the existence of a file called
       "/quorum".  Without that file, the node would claim it was unavailable.
       This is an awful example, and should never, ever be used in production,
       but is provided as an example as to what one could do...

       Typically,  the heuristics should be snippets of shell code or commands
       which help determine a node’s usefulness to  the  cluster  or  clients.
       Ideally,  you  want  to  add traces for all of your network paths (e.g.
       check links, or ping routers), and methods to  detect  availability  of
       shared storage.

2.3. Master Election

       Only  one  master is present at any one time in the cluster, regardless
       of how many partitions exist within the cluster itself.  The master  is
       elected  by  a  simple  voting  scheme  in  which the lowest node which
       believes it is capable of running (i.e. scores high  enough)  bids  for
       master  status.  If the other nodes agree, it becomes the master.  This
       algorithm is run whenever no master is present.

       If another node comes online with a lower node ID while a node is still
       bidding  for  master  status,  it will rescind its bid and vote for the
       lower node ID.  If a master dies or a bidding  node  dies,  the  voting
       algorithm  is  started  over.  The voting algorithm typically takes two
       passes to complete.

       Master deaths take marginally longer to recover  from  than  non-master
       deaths,  because a new master must be elected before the old master can
       be evicted & fenced.

2.4. Master Duties

       The master node decides who is or is not in the  master  partition,  as
       well  as  handles  eviction of dead nodes (both via the quorum disk and
       via the linux-cluster fencing  system  by  using  the  cman_kill_node()

2.5. How it All Ties Together

       When  a  master  is  present,  and  if the master believes a node to be
       online, that node will advertise  to  CMAN  that  the  quorum  disk  is
       available.  The master will only grant a node membership if:

            (a) CMAN believes the node to be online, and
            (b) that node has made enough consecutive, timely writes
                to the quorum disk, and
            (c) the node has a high enough score to consider itself online.

3. Configuration

3.1. The <quorumd> tag

       This tag is a child of the top-level <cluster> tag.

            This is the frequency of read/write cycles, in seconds.

            This  is  the  number  of  cycles  a node must miss in order to be
            declared dead.

            This is the number of cycles a node must be seen in  order  to  be
            declared online.  Default is floor(tko/3).

            This  is the number of cycles a node must wait before initiating a
            bid for master status after heuristic scoring becomes  sufficient.
            The default is 2.  This can not be set to 0, and should not exceed

            This is the number of cycles a node must  wait  for  votes  before
            declaring   itself   master   after  making  a  bid.   Default  is
            floor(tko/2).  This can not be less than 2, must be  greater  than
            tko_up, and should not exceed tko.

            This  is  the number of votes the quorum daemon advertises to CMAN
            when it has a high enough score.

            This controls the verbosity of the quorum  daemon  in  the  system
            logs.  0 = emergencies; 7 = debug.  This option is deprecated.

            This  controls  the syslog facility used by the quorum daemon when
            logging.   For  a  complete  list  of  available  facilities,  see
            syslog.conf(5).   The  default  value  for this is ’daemon’.  This
            option is deprecated.

            Write internal states out to this file  periodically  ("-"  =  use
            stdout).  This is primarily used for debugging.  The default value
            for this attribute is undefined.  This option can be changed while
            qdiskd is running.

            Absolute  minimum  score  to  be  consider one’s self "alive".  If
            omitted, or set to 0, the  default  function  "floor((n+1)/2)"  is
            used,  where  n  is  the total of all of defined heuristics’ score
            attribute.  This must  never  exceed  the  sum  of  the  heuristic
            scores, or else the quorum disk will never be available.

            If  set  to  0  (off),  qdiskd  will *not* reboot after a negative
            transition as a result in a change in  score  (see  section  2.2).
            The  default for this value is 1 (on).  This option can be changed
            while qdiskd is running.

            If set to 0 (off), qdiskd will *not* instruct  to  kill  nodes  it
            thinks  are  dead (as a result of not writing to the quorum disk).
            The default for this value is 1 (on).  This option can be  changed
            while qdiskd is running.

            If set to 1 (on), qdiskd will watch internal timers and reboot the
            node if it takes more than (interval * tko) seconds to complete  a
            quorum  disk  pass.   The default for this value is 0 (off).  This
            option can be changed while qdiskd is running.

            Valid  values  are  ’rr’,  ’fifo’,  and  ’other’.    Selects   the
            scheduling  queue  in the Linux kernel for operation of the main &
            score threads (does not affect the heuristics; they are always run
            in    the    ’other’    queue).     Default    is    ’rr’.     See
            sched_setscheduler(2) for more details.

            Valid values for ’rr’ and  ’fifo’  are  1..100  inclusive.   Valid
            values  for  ’other’  are -20..20 inclusive.  Sets the priority of
            the main & score threads.  The default value is 1 (in the  RR  and
            FIFO  queues,  higher  numbers  denote  higher priority; in OTHER,
            lower values denote higher priority).  This option can be  changed
            while qdiskd is running.

            Ordinarily,  cluster membership is left up to CMAN, not qdisk.  If
            this parameter is set to 1 (on), qdiskd will tell  CMAN  to  leave
            the  cluster  if it is unable to initialize the quorum disk during
            startup.  This can be used to prevent cluster participation  by  a
            node  which  has  been disconnected from the SAN.  The default for
            this value is 0 (off).  This option can be changed while qdiskd is

            If  this  parameter  is set to 1 (on), qdiskd will use values from
            /proc/uptime for internal timings.  This is  a  bit  less  precise
            than  gettimeofday(2), but the benefit is that changing the system
            clock will not affect qdiskd’s behavior  -  even  if  paranoid  is
            enabled.   If  set to 0, qdiskd will use gettimeofday(2), which is
            more precise.  The default for this value is 1 (on / use  uptime).

            This  is  the device the quorum daemon will use.  This device must
            be the same on all nodes.

            This overrides the device field if  present.   If  specified,  the
            quorum  daemon  will  read  /proc/partitions  and  check for qdisk
            signatures on  every  block  device  found,  comparing  the  label
            against  the  specified  label.   This is useful in configurations
            where the block device name differs on a per-node basis.

            This overrides the  label  advertised  to  CMAN  if  present.   If
            specified,  the quorum daemon will register with this name instead
            of the actual device name.

            If we receive an I/O error during a cycle, we do not poll CMAN and
            tell  it we are alive.  If specified, this value will cause qdiskd
            to exit after the specified number of  consecutive  cycles  during
            which  I/O  errors  occur.   The  default is 0 (no maximum).  This
            option can be changed while qdiskd is running.


3.2. The <heuristic> tag

       This tag is a child of  the  <quorumd>  tag.   Heuristics  may  not  be
       changed while qdiskd is running.

            This  is the program used to determine if this heuristic is alive.
            This can be anything which may  be  executed  by  /bin/sh  -c.   A
            return  value  of  zero indicates success; anything else indicates
            failure.  This is required.

            This is the weight of this heuristic.  Be careful when determining
            scores for heuristics.  The default score for each heuristic is 1.

            This is the frequency (in seconds) at which we poll the heuristic.
            The default interval for every heuristic is 2 seconds.

            After  this  many  failed  attempts  to  run  the heuristic, it is
            considered DOWN, and its score is removed.  The  default  tko  for
            each  heuristic  is  1, which may be inadequate for things such as

3.3. Examples

3.3.1. 3 cluster nodes & 3 routers

        <cman expected_votes="6" .../>
            <clusternode name="node1" votes="1" ... />
            <clusternode name="node2" votes="1" ... />
            <clusternode name="node3" votes="1" ... />
        <quorumd interval="1" tko="10" votes="3" label="testing">
            <heuristic  program="ping  A  -c1  -t1"   score="1"   interval="2"
            <heuristic   program="ping   B  -c1  -t1"  score="1"  interval="2"
            <heuristic  program="ping  C  -c1  -t1"   score="1"   interval="2"

3.3.2. 2 cluster nodes & 1 IP tiebreaker

        <cman two_node="0" expected_votes="3" .../>
            <clusternode name="node1" votes="1" ... />
            <clusternode name="node2" votes="1" ... />
        <quorumd interval="1" tko="10" votes="1" label="testing">
            <heuristic   program="ping   A  -c1  -t1"  score="1"  interval="2"

3.4. Heuristic score considerations

       * Heuristic timeouts should be set high enough to  allow  the  previous
       run of a given heuristic to complete.

       *  Heuristic  scripts  returning anything except 0 as their return code
       are considered failed.

       * The worst-case for improperly configured quorum heuristics is a  race
       to fence where two partitions simultaneously try to kill each other.

3.5. Creating a quorum disk partition

       The  mkqdisk  utility  can  create and list currently configured quorum
       disks visible to the local node; see mkqdisk(8) for more details.


       mkqdisk(8), qdiskd(8), cman(5), syslog.conf(5), gettimeofday(2)

                                  20 Feb 2007                         QDisk(5)