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Multipathing is a software solution implemented at the host operating-system level. Although multipathing is optional, it provides path redundancy, failover, and performance-enhancing capabilities. Therefore, it is highly recommended to deploy the solution in a production environment or any environments where availability and performance are critical.
The main benefits of using an MPIO solution include the following:
The native Linux multipath solution is supported and bundled with most popular Linux distributions in use today. Because the software is widely and readily available at no additional cost, many administrators prefer using it compared to other third-party solutions.
Note: Dell Technologies recommends using the native Linux multipath solution with ME4 Series storage.
The native Linux multipath software comes with a set of default settings for an extensive list of storage models from different vendors. The default settings allow the software to work with the ME4 Series system right out of the box. However, these settings might not be optimal for all situations and should be reviewed and modified if necessary. It is highly recommended to create a specific ME4 Series device section in the configuration file. An example configuration is shown as follows:
defaults {
find_multipaths yes
user_friendly_names yes
}
blacklist {
}
devices {
device {
vendor "DellEMC"
product "ME4"
path_grouping_policy "group_by_prio"
path_checker "tur"
hardware_handler "1 alua"
prio "alua"
failback immediate
rr_weight "uniform"
path_selector "service-time 0"
}
}
multipaths {
multipath {
wwid "3600c0ff0003c589f1e4b6b5b01000000"
alias app-data-01
}
}
Note the following in the prior example:
ME4 Series storage uses Unified LUN Presentation (ULP), which can expose all LUNs through all host ports on both controllers. The storage system appears as an active-active system to the host. The host can choose any available path to access a LUN regardless of disk group ownership. When ULP is in use, the controllers' operating/redundancy mode is shown as active-active ULP. ULP uses the Asymmetric Logic Unit Access (ALUA) extensions to negotiate paths with the ALUA-aware hosts. If the host is not ALUA-aware, all paths are treated as equal by the host even though some paths might have lower latency than others.
To make the Linux host ALUA-aware, set the path_grouping_policy, hardware_handler, and prio parameters accordingly in the /etc/multipath.conf file. The prior example in Configure multipath for ME4 Series storage section shows the proper settings to enable ALUA awareness for ME4 Series storage.
Note: Dell Technologies recommends enabling ALUA awareness in Linux multipath to optimize the I/O paths.
After modifying the multipath.conf file, reload the configuration.
# multipath –r
When the ALUA setting is enabled, the path information should look similar to the following example. Two path groups are formed. Each group consists of one or more paths. The optimized path group has a higher priority, 50 in the following example, than the non-optimized path group which has a lower priority of 10. The optimized path group also has active status while the status of the non-optimized path group is enabled.
# multipath -ll mpathb
mpathb (3600c0ff0003c589f1e4b6b5b01000000) dm-6 DellEMC ,ME4
size=200G features='0' hwhandler='1 alua' wp=rw
|-+- policy='service-time 0' prio=50 status=active
| |- 18:0:5:0 sdd 8:48 active ready running
| `- 19:0:5:0 sdj 8:144 active ready running
`-+- policy='service-time 0' prio=10 status=enabled
|- 18:0:4:0 sdl 8:176 active ready running
`- 19:0:4:0 sdi 8:128 active ready running