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 | NOTE: You may need an additional license to use this feature. |
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On some systems, the Volume Manager supports multiported disk arrays.
It automatically recognizes multiple I/O paths to a particular disk
device within the disk array. The Dynamic Multipathing feature of the
Volume Manager provides greater reliability by providing a path failover
mechanism. In the event of a loss of one connection to a disk, the system
continues to access the critical data over the other sound connections
to the disk. DMP also provides greater I/O throughput by balancing
the I/O load uniformly across multiple I/O paths to the disk device.
In the Volume Manager, all the physical disks connected to
the system are represented as metadevices with one or more physical
access paths. A single physical disk connected to the system is
represented by a metadevice with one path. A disk that is part of
a disk array is represented by a metadevice that has two physical
access paths. You can use the Volume Manager administrative utilities
such as the vxdisk utility to display all the paths of a metadevice
and status information of the various paths.
Path Failover Mechanism |
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DMP enhances system reliability when used with multiported
disk arrays. In the event of the loss of one connection to the disk
array, DMP automatically selects the next I/O paths for the I/O
requests dynamically without action from the administrator.
DMP allows the administrator to indicate to the DMP subsystem
in the Volume Manager whether the connection is repaired or restored.
This is called DMP reconfiguration.The reconfiguration procedure
also allows the detection of newly added devices, as well as devices
that are removed after the system is fully booted (if the operating
system detects them properly).
Load Balancing |
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To provide load balancing across paths, DMP follows the balanced
path mechanism for active/active disk arrays. Load balancing
makes sure that I/O throughput can be increased by utilizing the
full bandwidth of all paths to the maximum.
Sequential IOs starting within a certain range will be sent
down the same path in order to optimize IO throughput by utilizing
the effect of disk track caches. However, large sequential IOs that
do not fall within this range will be distributed across paths to
take the advantage of IO load balancing. For active/passive disk
arrays, I/Os are sent down the primary path until it fails. Once
the primary path fails, I/Os are then switched over to the other
available primary paths or secondary paths. Load balancing across
paths is not done for active/passive disk arrays to avoid the continuous
transfer of ownership of LUNs from one controller to another, thus
resulting in severe I/O slowdown.
Disabling and Enabling DMP |
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| NOTE: After either disabling or enabling DMP, you must reboot
the system for the changes to take effect. |
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Disabling DMP on an HP System:
Stop the vxconfigd daemon using the following command:
Save the /stand/system file as /stand/system.vxdmp by executing the command:
# cp /stand/system /stand/system.vxdmp |
Save the /stand/vmunix file as /stand/vmunix.vxdmp by executing the command:
# cp /stand/vmunix /stand/vmunix.vxdmp |
Edit the /stand/system file and remove the vxdmp entry.
Run the following script
If all the above steps complete successfully, reboot the system.
When the system comes up, DMP should be removed completely from
the system. Verify that DMP was removed by running the vxdmpadm command. The following message is displayed:
vxvm:vxdmpadm: ERROR: vxdmp module is not loaded on the system. Command invalid. |
Also the command vxdisk list <da_name> should not display any multipathing information.
Enabling DMP on a HP System (Where
it is Currently Disabled):
Stop the vxconfigd daemon using the following command:
Save the /stand/system file as /stand/system.vxdmpdis by executing the command:
# cp /stand/system /stand/system.vxdmpdis |
Save the /stand/vmunix file as /stand/vmunix.vxdmpdis by executing the command:
# cp /stand/vmunix /stand/vmunix.vxdmpdis |
Edit the /stand/system file and add the vxdmp entry to it (after the vxvm entry).
Run the following script:
After the above steps complete successfully, reboot the system.
When the system comes up, DMP should be enabled on the system. Verify
this by running the vxdmpadm command. This command shows multipathing information.
Also, the command vxdisk list <da_name> shows multipathing information.
Input/Output (I/O) Controllers |
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This feature allows the administrator to turn off I/Os to
a host I/O controller to perform administrative operations. It can
be used for maintenance of controllers attached to the host or a
disk array supported by the Volume Manager. I/O operations to the
host I/O controller can be turned on after the maintenance task
is completed. This once again enables I/Os to go through this controller.
This operation can be accomplished using the vxdmpadm(1M) utility provided with Volume Manager.
For active/active type disk arrays, Volume Manager uses the balanced path
mechanism to schedule I/Os to a disk with multiple paths
to it. As a result, I/Os may go through any path at any given point
in time.
For active/passive type disk arrays, I/Os will be scheduled
by Volume Manager to the primary path until a failure is encountered.
Therefore, to change an interface card on the disk array or a card
on the host (when possible) that is connected to the disk array,
the I/O operations to the host I/O controller(s) should be disabled.
This allows all I/Os to be shifted over to an active secondary path
or an active primary path on another I/O controller before the hardware
is changed.
After the operation is over, the paths through these controller(s)
can be put back into action by using the enabled option of the vxdmpadm(1M) command.
Volume Manager does not allow you
to disable the last active path to the rootdisk.
Displaying DMP Database Information |
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The vxdmpadm(1M) utility can be used to list DMP database information and
perform other administrative tasks. This command allows the user to
list all the controllers on the systems (connected to disks) and
other related information stored in the DMP database. This information
can be used to locate system hardware and also make a decision regarding which
controllers to enable/disable.
It also provides you with other useful information such as
disk array serial number and the list of DMP devices (disks) that
are connected to the disk array, the list of paths that go through
a particular controllers, etc.
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