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HASTD(8)                FreeBSD System Manager's Manual               HASTD(8)

     hastd - Highly Available Storage daemon

     hastd [-dFh] [-c config] [-P pidfile]

     The hastd daemon is responsible for managing highly available GEOM

     hastd allows to transparently store data on two physically separated
     machines connected over the TCP/IP network.  Only one machine (cluster
     node) can actively use storage provided by hastd.  This machine is called
     primary.  The hastd daemon operates on block level, which makes it
     transparent for file systems and applications.

     There is one main hastd daemon which starts new worker process as soon as
     a role for the given resource is changed to primary or as soon as a role
     for the given resource is changed to secondary and remote (primary) node
     will successfully connect to it.  Every worker process gets a new process
     title (see setproctitle(3)), which describes its role and resource it
     controls.  The exact format is:

           hastd: <resource name> (<role>)

     When (and only when) hastd operates in primary role for the given
     resource, corresponding /dev/hast/_name_ disk-like device (GEOM provider)
     is created.  File systems and applications can use this provider to send
     I/O requests to.  Every write, delete and flush operation (BIO_WRITE,
     BIO_DELETE, BIO_FLUSH) is send to local component and synchronously
     replicated to the remote (secondary) node if it is available.  Read
     operations (BIO_READ) are handled locally unless I/O error occurs or
     local version of the data is not up-to-date yet (synchronization is in

     The hastd daemon uses the GEOM Gate class to receive I/O requests from
     the in-kernel GEOM infrastructure.  The geom_gate.ko module is loaded
     automatically if the kernel was not compiled with the following option:

           options GEOM_GATE

     The connection between two hastd daemons is always initiated from the one
     running as primary to the one running as secondary.  When primary hastd
     is unable to connect or connection fails, it will try to re-establish
     connection every few seconds.  Once connection is established, primary
     hastd will synchronize every extent that was modified during connection
     outage to the secondary hastd.

     It is possible that in case of connection outage between the nodes hastd
     primary role for the given resource will be configured on both nodes.
     This in turn leads to incompatible data modifications.  Such condition is
     called split-brain and cannot be automatically resolved by the hastd
     daemon as this will lead most likely to data corruption or lost of
     important changes.  Even though it cannot be fixed by hastd itself, it
     will be detected and further connection between independently modified
     nodes will not be possible.  Once this situation is manually resolved by
     an administrator, resource on one of the nodes can be initialized
     (erasing local data), which makes connection to the remote node possible
     again.  Connection of freshly initialized component will trigger full
     resource synchronization.

     The hastd daemon itself never picks his role up automatically.  The role
     has to be configured with the hastctl(8) control utility by additional
     software like ucarp or heartbeat that can reliably manage role separation
     and switch secondary node to primary role in case of original primary

     The hastd daemon can be started with the following command line

     -c config         Specify alternative location of the configuration file.
                       The default location is /etc/hast.conf.

     -d                Print or log debugging information.  This option can be
                       specified multiple times to raise the verbosity level.

     -F                Start the hastd daemon in the foreground.  By default
                       hastd starts in the background.

     -h                Print the hastd usage message.

     -P pidfile        Specify alternative location of a file where main
                       process PID will be stored.  The default location is

     /etc/hast.conf        The configuration file for hastd and hastctl(8).
     /var/run/hastctl      Control socket used by the hastctl(8) control
                           utility to communicate with hastd.
     /var/run/    The default location of the hastd PID file.

     Exit status is 0 on success, or one of the values described in
     sysexits(3) on failure.

     Launch hastd on both nodes.  Set role for resource shared to primary on
     nodeA and to secondary on nodeB.  Create file system on /dev/hast/shared
     provider and mount it.

           nodeB# hastd
           nodeB# hastctl role secondary shared

           nodeA# hastd
           nodeA# hastctl role primary shared
           nodeA# newfs -U /dev/hast/shared
           nodeA# mount -o noatime /dev/hast/shared /shared

     sysexits(3), geom(4), hast.conf(5), ggatec(8), ggated(8), ggatel(8),
     hastctl(8), mount(8), newfs(8), g_bio(9).

     The hastd was developed by Pawel Jakub Dawidek <> under
     sponsorship of the FreeBSD Foundation.

FreeBSD 11.0-PRERELEASE        February 1, 2010        FreeBSD 11.0-PRERELEASE


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