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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 the transparent storage of data on two physically separated
     machines connected	over a 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, a
     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 sent to the local component and replicated on
     the remote	(secondary) node if it is available.  Read operations
     (BIO_READ)	are handled locally unless an I/O error	occurs or the 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 the primary
     hastd is unable to	connect	or the connection fails, it will try to	re-es-
     tablish the connection every few seconds.	Once the connection is estab-
     lished, the primary hastd will synchronize	every extent that was modified
     during connection outage to the secondary hastd.

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

     A hastd daemon never picks	its role 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 the primary's failure.

     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 utility appeared	in FreeBSD 8.1.

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

FreeBSD	13.0		       December	21, 2019		  FreeBSD 13.0


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