[dm-devel] Active/backup multipath configuration for iSCSI paths

Alexander Murashkin AlexanderMurashkin at msn.com
Wed Mar 28 08:52:09 UTC 2012

On 03/23/2012 02:41 PM, Hart, Brian R. wrote:



> We have a couple of Dell MD storage arrays that when installed

> setup multipath.conf to use this line: prio_callout

> "/sbin/mpath_prio_rdac /dev/%n"


On 03/23/2012 02:41 PM, Bryn M. Reeves wrote:



>Callouts make life difficult when file systems go away so like the

>path checkers before them they were merged into the libmultipath

>shared library. This means that the daemon can lock them into memory

>via mlock(2)/mlockall(2) and not have to worry about being able to

>load a binary from disk when the paths to that storage have failed -

>this is very useful if your root file system is on multipath and you

>need to recover from a failure.


>In RHEL5 this is dealt with using a complex and fragile private

>namespace and RAM-backed file system - the required binaries are

>copied into this ramfs at daemon startup and the daemon unmounts

>unnecessary file systems to avoid blocking when failures occur.


>For this reason the parameter is now just "prio", e.g.:


>device {


>    prio                    rdac



I can see that using shared libraries to handle priorities makes handling
failed paths easier in many situations. But how to configure active/backup
multipath in the following case, for example


-          1st path uses iSCSI over Infiniband

-          2nd path used iSCSI over Gigabit Ethernet

-          iSCSI target does not support ALUA

-          we want Infiniband path to have higher priority


Will it be enough to set path_grouping_policy =  multibus and path_selector
= "service-time 0"? Will Gigabit Ethernet path be never used as long as
Infiniband path is working?


I appreciate your help,

    Alexander Murashkin


Email:  AlexanderMurashkin at msn.com



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