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, 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 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 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 possible. Once this situation is manually resolved by an administrator, resource 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 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.