freebsd/sbin/hastd/hastd.8

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.\" This software was developed by Pawel Jakub Dawidek under sponsorship from
.\" the FreeBSD Foundation.
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.Dd February 1, 2010
.Dt HASTD 8
.Os
.Sh NAME
.Nm hastd
.Nd "Highly Available Storage daemon"
.Sh SYNOPSIS
.Nm
.Op Fl dFh
.Op Fl c Ar config
.Op Fl P Ar pidfile
.Sh DESCRIPTION
The
.Nm
daemon is responsible for managing highly available GEOM providers.
.Pp
.Nm
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
.Nm .
This machine is called primary.
The
.Nm
daemon operates on block level, which makes it transparent to file
systems and applications.
.Pp
There is one main
.Nm
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
.Xr setproctitle 3 ) ,
which describes its role and resource it controls.
The exact format is:
.Bd -literal -offset indent
hastd: <resource name> (<role>)
.Ed
.Pp
If (and only if)
.Nm
operates in primary role for the given resource, a corresponding
.Pa /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
.Dv ( 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
.Dv ( 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).
.Pp
The
.Nm
daemon uses the GEOM Gate class to receive I/O requests from the
in-kernel GEOM infrastructure.
The
.Nm geom_gate.ko
module is loaded automatically if the kernel was not compiled with the
following option:
.Bd -ragged -offset indent
.Cd "options GEOM_GATE"
.Ed
.Pp
The connection between two
.Nm
daemons is always initiated from the one running as primary to the one
running as secondary.
When the primary
.Nm
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
.Nm
will synchronize every extent that was modified during connection outage
to the secondary
.Nm .
.Pp
It is possible that in the case of a connection outage between the nodes the
.Nm
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
.Nm
daemon as this will lead most likely to data corruption or loss of
important changes.
Even though it cannot be fixed by
.Nm
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.
.Pp
A
.Nm
daemon never picks its role automatically.
The role has to be configured with the
.Xr hastctl 8
control utility by additional software like
.Nm ucarp
or
.Nm heartbeat
that can reliably manage role separation and switch secondary node to
primary role in case of the primary's failure.
.Pp
The
.Nm
daemon can be started with the following command line arguments:
.Bl -tag -width ".Fl P Ar pidfile"
.It Fl c Ar config
Specify alternative location of the configuration file.
The default location is
.Pa /etc/hast.conf .
.It Fl d
Print or log debugging information.
This option can be specified multiple times to raise the verbosity
level.
.It Fl F
Start the
.Nm
daemon in the foreground.
By default
.Nm
starts in the background.
.It Fl h
Print the
.Nm
usage message.
.It Fl P Ar pidfile
Specify alternative location of a file where main process PID will be
stored.
The default location is
.Pa /var/run/hastd.pid .
.El
.Sh FILES
.Bl -tag -width ".Pa /var/run/hastd.pid" -compact
.It Pa /etc/hast.conf
The configuration file for
.Nm
and
.Xr hastctl 8 .
.It Pa /var/run/hastctl
Control socket used by the
.Xr hastctl 8
control utility to communicate with
.Nm .
.It Pa /var/run/hastd.pid
The default location of the
.Nm
PID file.
.El
.Sh EXIT STATUS
Exit status is 0 on success, or one of the values described in
.Xr sysexits 3
on failure.
.Sh EXAMPLES
Launch
.Nm
on both nodes.
Set role for resource
.Nm shared
to primary on
.Nm nodeA
and to secondary on
.Nm nodeB .
Create file system on
.Pa /dev/hast/shared
provider and mount it.
.Bd -literal -offset indent
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
.Ed
.Sh SEE ALSO
.Xr sysexits 3 ,
.Xr geom 4 ,
.Xr hast.conf 5 ,
.Xr ggatec 8 ,
.Xr ggated 8 ,
.Xr ggatel 8 ,
.Xr hastctl 8 ,
.Xr mount 8 ,
.Xr newfs 8 ,
.Xr g_bio 9
.Sh AUTHORS
The
.Nm
was developed by
.An Pawel Jakub Dawidek Aq Mt pjd@FreeBSD.org
under sponsorship of the FreeBSD Foundation.