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26 Dell EMC SC Series: Red Hat Enterprise Linux Best Practices | CML1031
A properly configured SAS volume will return the following multipath -ll output. This SC Series volume is
discovered as a multipath ALUA-capable volume, where each path is capable of I/O. The path represented by
prio=50 (active/optimized path) is used for all active I/O requests. The path represented by prio=1 (standby
path) is a highly available, redundant path and is used when the active/optimized path becomes unavailable.
# multipath -ll
Compelnt_0016 (36000d31000feb3000000000000000016) dm-3 COMPELNT,Compellent Vol
size=100G features='1 queue_if_no_path' hwhandler='1 alua' wp=rw
|-+- policy='service-time 0' prio=50 status=active
| `- 1:0:0:1 sdb 8:16 active ready running
`-+- policy='service-time 0' prio=1 status=enabled
`- 1:0:1:1 sdc 8:32 active ready running
Compelnt_001a (36000d31000feb300000000000000001a) dm-4 COMPELNT,Compellent Vol
size=100G features='1 queue_if_no_path' hwhandler='1 alua' wp=rw
|-+- policy='service-time 0' prio=50 status=active
| `- 1:0:1:2 sdd 8:48 active ready running
`-+- policy='service-time 0' prio=1 status=enabled
`- 1:0:2:2 sde 8:64 active ready running
3.6.6 Multipath aliases
multipathd creates multipath device names in the form of /dev/mapper/mpathX as it discovers new devices.
It is recommended to assign meaningful names (aliases) for the multipath devices though it is not mandatory.
For example, create aliases based on the application type and environment it is in. The following snippet in
the multipaths section assigns an alias of appdata_001 to the SC Series volume with the WWN
36000d3100000650000000000000017f2.
multipaths {
multipath {
wwid "36000d3100000650000000000000017f2"
alias appdata_001
}
}
3.6.7 Multipath queueing
The default dm-multipath settings of SC Series volumes enable the no_path_retry queue option that
instructs dm-multipath to queue all I/O requests indefinitely should all paths become unavailable. After the
paths are recovered, pending I/Os resume execution. This also prevents I/O errors from being propagated to
the applications. The setting is shown in the multipath output. It is recommended to leave this in its default.
# multipath -ll
[snip]
appdata_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