Developers Guide

Server configuration
41 Dell EMC SC Series with Red Hat Enterprise Linux 7x | CML1071
3.6.2 Multipath queueing
The compiled defaults of presented and discovered SC Series storage volumes feature the 1
queue_if_no_path option that instructs multipath to automatically queue and retry all I/O requests should all
paths become unavailable. This is observed from the following sample output, and should be left in its default
state.
# multipath -ll
[snip]
vol_001 (36000d3100000650000000000000017f2) dm-3 COMPELNT,Compellent Vol
size=50G features='1 queue_if_no_path' hwhandler='0' wp=rw
`-+- policy='service-time 0' prio=1 status=active
|- 1:0:3:1 sdf 8:80 active ready running
|- 1:0:5:1 sdk 8:160 active ready running
|- 4:0:4:1 sdg 8:96 active ready running
`- 4:0:5:1 sdj 8:144 active ready running
[snip]
The exception to using this default is in business or application scenarios where I/O is not queued but instead
quickly failed if all paths become unavailable. In these circumstances, I/O queueing can be disabled in
/etc/multipath.conf either within the defaults clause, to specific devices within the multipath clause, or applied
at runtime.
Within the defaults clause, insert the following into /etc/multipath.conf, then restart the multipath service.
devices {
device {
vendor "COMPELNT"
product "“Compellent Vol"
features 0
no_path_retry fail
}
Multipath queueing applies to all I/O requests for all transports including Fibre Channel, iSCSI, and SAS
presented SC Series volumes.
To implement this to specific device at runtime, issue the following command for the desired multipath device.
# dmsetup message /dev/mapper/vol_001 0 "fail_if_no_path"
3.7 Multipath environments
During an SC Series path or controller failover event with the SC Series storage operating in legacy port
mode, any ports experiencing failure will trigger their respective WWPN identities to momentarily disappear
from the SAN fabric before relocating these WWPN identities to a reserve port within the same fault domain
on the alternate active controller.
During an SC path failover event with SC Series storage operating in virtual port mode, the WWPN
identities of any NPIV ports experiencing failure will be relocated to another active NPIV port within the same
fault domain on the same controller. During a controller failover event, the WWPN identities of any NPIV
ports experiencing failure will be relocated to the active NPIV ports within the same fault domain on the
alternate active controller.