Does Multipath required in Shared SAN environment

[nalina]

we have

DL380 (2 server) with 4 HBA Cards(two each)
HP MSA2000 SAN 01
HP FABRIC switch 02

and I run RHEL 5.2 OS with oracle 11g cluster active/passive.

since I have the redundant paths to storage, do I need to activate redhar Multipath service?

[JKytsi]

You need multipathing when You have multiple paths to same LUN.

[nalina]

In this case yes, I have Multipath to same LUN(first path: node A -> HBA A -> controller A -> LUN 01 and second path controller B: node A -> HBA B -> controller B -> LUN 01).

when fail of one path still I can access the LUN from available path.

I believe I allready have hardware Multipathing. Then why should I use RHEL 5.3 Multipath service?

[Matti_Kurkela]

Yes, you have multiple paths from each server to disks at the hardware level. But without some kind of multipath service, your system does not "know" that and cannot redirect I/O operations to another path if one path fails.

With a multipath HW configuration, you'll need a component in your system that knows that e.g. /dev/sda and /dev/sdk are in fact the same disk: if an I/O operation through /dev/sda fails, the same operation can be retried through /dev/sdk.

Usually, this is done at the OS level, and a new device is set up for the function of "access a particular disk using any available path". With RedHat multipath service, these devices are named like /dev/mapper/mpath or /dev/mapper/. If the applications use these devices, it is possible to add and remove disk paths while the applications are running: the multipath service will automatically re-map the I/O operations to use the appropriate paths.

If your application explicitly uses e.g. /dev/sda in its configuration and the path /dev/sda fails, the application will fail. Then the application configuration must be changed to make it use the other path.

The multipath service will also allow load-balancing between the paths: when both paths are OK but one HBA is busy transferring data from another disk, the multipath service can reroute the request through the other HBA. This will maximize the available data bandwidth, and will give you better performance.

MK