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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	trans-
     parent to 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>)

     If	(and only if) hastd operates in	primary	role for the given resource,
     corresponding /dev/hast/_name_ disk-like device (GEOM provider) is	cre-
     ated.  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 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 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-

     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.
     /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	11.1		       February	1, 2010			  FreeBSD 11.1


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