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
     transparent 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
     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 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
     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 the pri-
     mary hastd is unable to connect or the connection fails, it will try to re-establish the connection every few seconds.  Once the connection
     is established, 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 possible 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 arguments:

     -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 /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