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

NAME
     hastd — Highly Available Storage daemon

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

DESCRIPTION
     The hastd daemon is responsible for managing highly available GEOM
     providers.

     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 trans‐
     parent 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 oper‐
     ations (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
     progress).

     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 dae‐
     mon 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 adminis‐
     trator, resource on one of the nodes can be initialized (erasing local
     data), which makes connection to the remote node possible again.  Connec‐
     tion of freshly initialized component will trigger full resource synchro‐
     nization.

     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
     failure.

     The hastd daemon can be started with the following command line argu‐
     ments:

     -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 spec‐
		 ified 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 /var/run/hastd.pid.

FILES
     /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/hastd.pid
		       The default location of the hastd PID file.

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

EXAMPLES
     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

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

AUTHORS
     The hastd was developed by Pawel Jakub Dawidek ⟨pjd@FreeBSD.org⟩ under
     sponsorship of the FreeBSD Foundation.

BSD			       February 1, 2010				   BSD
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