home | help 
HASTD(8)		     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  ma-
     chines  connected over the TCP/IP network.  Only one machine (cluster node)
     can actively use storage provided by hastd.  This machine	is  called  pri-
     mary.  The hastd daemon operates on block level, which makes it transparent
     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 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 automati-
     cally 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 connec-
     tion 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  daemon  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  possi-
     ble.   Once  this	situation  is manually resolved by an administrator, re-
     source on one of the nodes can be initialized (erasing local  data),  which
     makes  connection to the remote node possible again.  Connection of freshly
     initialized component will trigger full resource synchronization.

     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 fail-
     ure.

     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  speci-
		 fied 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.

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

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.

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.

FreeBSD 8.1			February 1, 2010			HASTD(8)
home | help