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21 Dell EMC SC Series: Red Hat Enterprise Linux Best Practices | CML1031
3.2.4 Querying WWNs using multipath command
If the system has Linux device-mapper-multipath software enabled, the multipath command displays the
multipath device properties including the WWN. Section 3.6 provides more information on multipathing.
# multipath -ll
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
3.3 Managing persistent devices with volume labels and UUIDs
Volumes discovered in Linux are given device designations such as /dev/sdd and /dev/sdf depending on the
Linux discovery method used by the HBA ports connecting the server to the SAN.
Among other uses, these /dev/sdX device names designate the volumes for mount commands including
entries in the /etc/fstab file. In static disk environments, /dev/sdX device names work well for entries in the
/etc/fstab file. However, the dynamic nature of Fibre Channel or iSCSI connectivity inhibits Linux from
tracking these disk designations persistently across reboots.
There are multiple ways to ensure that these volumes are assigned and referenced by a persistent naming
scheme. This section presents using volume labels or universally unique identifiers (UUIDs) with /dev/sdX-
referenced volumes. Volume labels are exceptionally useful when scripting SC Series snapshot recovery. An
example involves mounting a snapshot view volume of a production SC Series snapshot to a backup server.
In this case, the snapshot view volume may be referenced by its volume label without needing to explicitly
identify the associated /dev/sdX device. The volume label is metadata stored within the volume and is
inherited by snapshot view volumes created from the SC Series snapshot.
Volume labels or UUIDs can also be used in multipath environments. That being said, multipath
/dev/mapper/mpathX device names (or multipath aliases as described in section 3.6.7) are persistent by
default and will not change across reboots. Volume labels, UUIDs, or multipath device names can be used
interchangeably for entries in the /etc/fstab file of a local Linux host.
If snapshot view volumes are used for recovery or relocation of volumes to an alternate Linux system, the use
of volume labels or UUIDs is recommended. These UUID values uniquely identify the volumes, in contrast to
multipath names which may differ (depending on the configuration of the /etc/multipath.conf file).
3.4 Volume size
SC Series storage supports a maximum volume size of 500 TB or the maximum addressable space of the
page pool, whichever is smaller. It also supports a maximum of 2000 volumes in a single system. Many
modern filesystems and applications can now handle volumes larger than 2 TB. Large volumes have no
discernible performance difference compared to small volumes. However, using more volumes can improve
performance by allowing more concurrent streams and I/O queues to the storage system. Also, volumes are
balanced across both SC Series storage controllers. It is a best practice to use two or more volumes on a
Linux system to increase performance.