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Multipath – which path should we choose?

Fortunately in Linux  in contrast to our lives we don’t need to choose one path, we are able to choose all paths!

In this article, you can find simple description about configuring multipath under Linux for iSCSI (internet Small Computer System Interface) and FC (Fibre Channel) environments.

At the very beginning we must configure iSCSI or FC target.
We can use trial version of Open-E DSS V6 software.

If we decide to use iSCSI target then it is necessary to connect at least two NICs (Network Interface Cards) on target and initiator sides.
For configuring FC we need to have at least one Qlogic FC HBA (Host Bus Adapter) 2/4/8 Gb with dual-port on target side and also on initiator side.

Next step will be exports of one LU (Logical Unit) via iSCSI or FC target.

After that we must configure our initiator. In case of using iSCSI target we need to connect to it  twice, using iSCSI initiator via both NICs. Using FC target the drives should be visible automatically after loading specific driver.

Now we could check if LU is visible locally on initiators side twice by typing command lsscsi.
Example output for lsscsi command:

[8:0:0:0]    disk    SCST_BIO sB6glXbw6LxyZnCv  101  /dev/sdb
[9:0:0:0]    disk    SCST_BIO sB6glXbw6LxyZnCv  101  /dev/sdc

Now we will generate configuration file for multipath software. To do that properly we need to get WWID (World Wide Identifier) for disks /dev/sdb and /dev/sdc by using command scsi_id.
Example output for command  /lib/udev/scsi_id –page=0x80 –whitelisted –device=/dev/sdb

SSCST_BIOsB6glXbw6LxyZnC2279e18

The same output will be for disk /dev/sdc

Getting this information we are able to generate proper configuration file.

--- /etc/multipath.conf ---
defaults {
udev_dir        /dev
path_grouping_policy    multibus
getuid_callout        "/lib/udev/scsi_id --page=0x80 --whitelisted --device=/dev/%n"
}
multipaths {
multipath {
wwid        SSCST_BIOsB6glXbw6LxyZnC2279e18
alias        multipath
}
}
--- /etc/multipath.conf ---

After multipath start by using script /etc/init.d/multipath-tools start (depending on your Linux distribution), we could type command multipath -l
Example output is:

multipath (SSCST_BIOsB6glXbw6LxyZnC2279e18) dm-0 SCST_BIO,sB6glXbw6LxyZn
[size=3.0G][features=0][hwhandler=0]
\_ round-robin 0 [prio=0][active]
\_ 8:0:0:0 sdb 8:16  [active][undef]
\_ 9:0:0:0 sdc 8:32  [active][undef]

Our new device is available here: /dev/mapper/multipath, we could format it by our favorite file system and mount.

Now we have an access to one LU via two paths! Good luck with testing it! I’m looking forward for some feedback.

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6 Comments

  • marek kaletka

    February 21, 02 2012 08:34:22

    Sounds really easy. Slightly wondering, why is Open-e in last 6 years not able to add multipathing support on iSCSI/FC initiator side in DSS then ?

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    • benjamin kojetz

      February 07, 02 2013 02:09:50

      It was because of priorities I think. But seems that there is possibility to get unofficial update for DSS V7 to enable this feature. I have already tested it and it work very stable. if you are interested in I suggest to contact with their support to get this update.

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  • martino87r

    May 15, 05 2012 09:58:47

    Is multipath supported on the FC target in OpenE?
    I mean is it possible to export a single volume trough 2 FC HBAs and use 2 paths on the initiator side?

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    • robert.bluszcz

      May 22, 05 2012 02:22:09

      Yes, the MPIO does not require any special support or configuration on the target side as it is fully controlled by initiator.

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  • brian.f.summa

    April 27, 04 2013 12:48:44

    What version of dm-multipath does this example cover?

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    • Kasia Kolodziej

      April 28, 04 2013 08:25:52

      Hey Brian, we used the current version that was available at the time of the article. If you need help with dm-multipath you are always welcome to contact us: pre-sales@open-e.com

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