Red Hat ENTERPRISE LINUX 5 - GLOBAL NETWORK BLOCK DEVICE Use Manual page 16

Chapter 3. GNBD Driver and Command Usage
pathname
Specifies a storage device to export.
gnbdname
Specifies an arbitrary name selected for the GNBD. It is used as the device name on GNBD
clients. This name must be unique among all GNBDs exported in a network.
-o
Export the device as read-only.
-c
Enable caching. Reads from the exported GNBD and takes advantage of the Linux page cache.
By default, the gnbd_export command does not enable caching.
Note
When you configure GNBD servers with device-mapper multipath, do not specify the -
c option. All GNBDs that are part of a logical volume must run with caching disabled.
Note
If you have been using GFS 5.2 or earlier and do not want to change your GNBD
setup you should specify the -c option. Before GFS Release 5.2.1, Linux caching was
enabled by default for gnbd_export. If the -c option is not specified, GNBD runs
with a noticeable performance decrease. Also, if the -c option is not specified, the
exported GNBD runs in timeout mode, using the default timeout value (the -t option).
For more information about the gnbd_export command and its options, refer to the
gnbd_export man page.
-u uid
Manually sets the Universal Identifier for an exported device. This option is used with -e. The
UID is used by device-mapper multipath to determine which devices belong in a multipath map. A
device must have a UID to be multipathed. However, for most SCSI devices the default Get UID
command, /usr/sbin/gnbd_get_uid, will return an appropriate value.
Note
The UID refers to the device being exported, not the GNBD itself. The UIDs of two
GNBD devices should be equal, only if they are exporting the same underlying device.
This means that both GNBD servers are connected to the same physical device.
Warning
This option should only be used for exporting shared storage devices, when the -U
command option does not work. This should almost never happen for SCSI devices.
If two GNBD devices are not exporting the same underlying device, but are given the
same UID, data corruption will occur.
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