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Multipath I/O is a software solution implemented at the host operating-system level. While Multipath I/O is optional, it provides path redundancy, failover, and performance-enhancing capabilities. It is recommended to deploy Multipath I/O in a production environment or any environment where availability and performance are critical.
The main benefits of using a Multipath I/O solution include the following:
Device Mapper Multipath (DM-Multipath) is a free native Linux multipath I/O solution supported by all Linux distributions.
On Red Hat Enterprise Linux, the multipath software is provided by the device-mapper-multipath package. On SUSE Linux Enterprise Server, the software is provided by the multipath-tools package.
Install or update the multipath software
Red Hat
dnf install device-mapper-multipath
SUSE
zipper install multipath-tools
Note: Dell Technologies recommends using the DM-Multipath solution with ME5 Series storage system.
DM-Multipath 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 ME5 storage 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 ME5 Series device definition 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 "ME5"
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 "3600c0ff00064639baf88726301000000"
alias app-data-01
}
}
Note the following in the prior example:
ME5 storage system 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 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 the section Configure multipath I/O for ME5 shows the proper settings to enable ALUA awareness for ME5 storage systems.
Note: Dell Technologies recommends enabling ALUA awareness in Linux DM-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 paths are configured into groups based on their priorities. 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 active paths from the optimized path group service the host I/Os.
mpatha (3600c0ff00064634cae88726301000000) dm-3 DellEMC,ME5
size=186G features='0' hwhandler='1 alua' wp=rw
|-+- policy='service-time 0' prio=50 status=active
| |- 11:0:10:0 sdb 8:16 active ready running
| |- 11:0:11:0 sdd 8:48 active ready running
| |- 12:0:10:0 sdj 8:144 active ready running
| `- 12:0:11:0 sdl 8:176 active ready running
`-+- policy='service-time 0' prio=10 status=enabled
|- 11:0:12:0 sdf 8:80 active ready running
|- 11:0:13:0 sdh 8:112 active ready running
|- 12:0:12:0 sdn 8:208 active ready running
`- 12:0:13:0 sdp 8:240 active ready running