Administrator Guide

Live Volume with Linux/UNIX
85 Dell EMC SC Series: Synchronous Replication and Live Volume | CML1064
10.4.1 DM-Multipath configuration
DM-Multipath comes with built-in default settings for SC Series arrays. These default settings are not reflected
in the /etc/multipath.conf file. To display these settings, execute multipathd -k"show config" and look for the
device section for "COMPELNT".
It is recommended to examine the settings closely especially when Live Volumes are involved because the
built-in default settings might not be appropriate. The following sample configuration demonstrates a version
of the multipath configuration:
defaults {
find_multipaths yes
user_friendly_names yes
polling_interval 5
}
devices {
device {
vendor "COMPELNT"
product "Compellent Vol"
path_grouping_policy "multibus"
path_checker "tur"
features "0"
hardware_handler "0"
prio "const"
failback immediate
rr_weight "uniform"
no_path_retry "24"
fast_io_fail_tmo 5
dev_loss_tmo infinity
path_selector "round-robin 0"
}
}
This configuration is designed with the following assumptions:
• The storage arrays are fully redundant and are protected for complete outage.
• Storage paths are expected to be highly available within an SC Series array and across remote
arrays.
• In the event of path failovers, I/O is queued and retried for up to 24 times. Each retry interval is 5
seconds. Therefore, it tolerates a maximum of 2 minutes (24 x 5 seconds) of path unavailability
before it stops retrying. This timeout needs to be long enough to cover the controller failover within an
SC Series array and the Live Volume swap role between the primary and secondary array. Adjust this
value if necessary to adapt to your environment.
• It is recommended to explicitly include the COMPELNT device settings in /etc/multipath.conf file
even though the built-in defaults might cover it. Several reasons for this recommendation are:
- It prevents an unexpected change of behavior due to unannounced changes on these default
settings in future releases of DM-Multipath.
- It overrides the defaults and allows finer control on COMPELNT-specific settings, such as
no_path_retry, path_grouping_policy and prio.