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HASTD(8)		  BSD 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 ma-
     chines 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 re-
     source, 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

     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 au-
     tomatically 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 de-
     tected 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-

     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 argu-

     -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/

     /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.
		       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.

BSD			       February	1, 2010				   BSD


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