topic Re: multipath for I/O in Operating System - Linux

multipath for I/O

Tonatiuh — Thu, 17 Aug 2006 22:22:12 GMT

Red Hat Enterprise Linux AS 4

I have 2 QLogic FC (Fibre Channel) adapters in the same server. It access to an external storage.

The 2 FC adapters reports same LUN of the external storage as two devices to the server (/dev/sdb1 and /dev/sdc1), but both are two paths for the same divice.

Can I configure this two devices in a RAID to present certain software just one device to point?

Does this RAID configuration take care of the multipath feature of the device and manage this without any data corruption?

For the moment, the use of a multipathing management software (like HP Secure Path, etc.) is not an option (it will be the option in a later stage).

Re: multipath for I/O

Uwe Zessin — Fri, 18 Aug 2006 00:37:21 GMT

If I remember correctly, Secure Path is not supported on RHEL4 and won't be in the future.

You can download HP branded device drivers for the Fibre Channel adapters and turn on the embedded multipath feature.

For early adopters, HP has published a document on how to play with the device mapper...

Re: multipath for I/O

Atul Gautam — Fri, 18 Aug 2006 00:46:39 GMT

Hi..

And don't forget to check your "gcc" version coz at times an upgrade for gcc which is 3.4 is needed.

--
Atul

Re: multipath for I/O

Kodjo Agbenu — Fri, 18 Aug 2006 04:20:08 GMT

Hello,

There seems to be several points here :

1) I/O multi-pathing
2) RAID configuration
3) device naming

1) The driver provided by HP or QLogic does multi-pathing. Upont properly configured, the LUN will be seen only once (ex : /dev/sdb1) by the Linux operating system.

2) RAID means you need to spread data on several LUNs to reach more performance and/or redundancy. It is generally not recommended to configure software RAID with LUNs that belong to a FC storage (MSA, EVA, XP). Unless you are a storage performance expert, rely on the disk array firmware for performance & availability.

3) You may take care of one annoying behaviour of Linux OS : in standard configuration, scsi/FC devices are renamed after each reboot, in the order they are discovered.
Example : you have LUN17 and LUN18 in the storage array that are presented to Linux. They will be named /dev/sda and /dev/sdb.
Then you insert LUN16 in the array : after subsequent reboot, LUN16 will be /dev/sda, LUN17 => /dev/sdb and LUN18 => /dev/sdc.

You can easily imagine the problem if you store vital Oracle data for example...

As a simple and reliable method avoiding this, I would recommend usage of LVM. Upon LVM driver initialization, it scans all disk devices and re-creates its structure, even if physical path of devices have changed.

Summary :
=> Rely on driver provided by HP or QLogic for multi-pathing.
=> Rely on RAID capabilities inside the storage array and do not configure software RAID that would lower down performances.
=> Rely on LVM for dealing with device renaming potential issue.

Hope this helps.
Good lcuk.
Kodjo

Re: multipath for I/O

Tonatiuh — Fri, 18 Aug 2006 09:04:56 GMT

Kodjo,

The was something that sounds very interesting.

1) The driver provided by HP or QLogic does multi-pathing. Upont properly configured, the LUN will be seen only once (ex : /dev/sdb1) by the Linux operating system.

Must I underestand that I can config the multipath (to be presented only as one device) only configuring somethings in the QLogic driver?

Could you be mor specific (syntax, samples, etc.) about how to configure the QLogic driver to do it?