Developers Guide

Server configuration
40 Dell EMC SC Series with Red Hat Enterprise Linux 7x | CML1071
An example of the multipath -ll command output is shown as follows.
[root@dean ~]# multipath -ll
boot_drive (36000d31000035e000000000000000059) dm-0 COMPELNT,Compellent Vol
size=146G features='1 queue_if_no_path' hwhandler='0' wp=rw
`-+- policy='service-time 0' prio=1 status=active
|- 1:0:1:0 sdc 8:32 active ready running
|- 1:0:2:0 sde 8:64 active ready running
|- 4:0:1:0 sdb 8:16 active ready running
`- 4:0:2:0 sdd 8:48 active ready running
[snip]
Note: With RHEL 7.x, multipath defaults to service-time 0 instead of the round-robin 0 policy; RHEL 6.x
releases continues to default to round-robin 0. The service-time 0 policy states, "Send the next bunch of I/O
down the path with the shortest estimated service time, which is determined by dividing the total size of the
outstanding I/O to each path by its relative throughput", and is recommended to be left in this state unless
instructed by support or application specific directives. The service-time 0 policy compensates for any
unbalanced I/O patterns and improves throughput. With more balanced I/O patterns, service-time 0 operates
akin to the round-robin 0 policy. This policy works with Fibre Channel, iSCSI, and SAS protocols.
3.6.1 Multipath aliases
In the prior example /etc/multipath.conf file, SC Series storage WWID values are further identified by an alias
key value (vol_XXX) within the each respective multipath clause. Multipath aliases serve to better identify the
function of the volume, and also maintain device name persistence across reboots and reconfiguration. This
means that multipath aliases are consistent and safe for use in scripting, mount commands, /etc/fstab syntax,
and more.
If multipath aliases are not defined in /etc/multipath.conf, volumes will default to their default naming scheme
of /dev/mapper/mpathX while also maintaining persistence across reboots. Designating aliases is
recommended and extremely useful when associating multipath device names with more descriptive labels
(for example, business function or usage).
After defining aliases or updating any clauses inside /etc/multipath.conf, restart the multipathd daemon as
follows.
# systemctl restart multipathd.service
With RHEL 7.x, the older command syntax may still be used to pass the command constructs into the new
systemd service management framework.
# service multipathd restart