Developers Guide

Server configuration
36 Dell EMC SC Series with Red Hat Enterprise Linux 7x | CML1071
3.3.6 Configured multipath
A properly configured SAS volume will return the following multipath –ll output. This SC Series volume is
discovered as a multipath ALUA-capable volume, where each path is capable of I/O. The path represented by
prio=50 (active/optimized path) is used for all active I/O requests. The path represented by prio=1 (standby
path) is a highly available, redundant path and is used when the active/optimized path becomes unavailable.
# multipath -ll
Compelnt_0016 (36000d31000feb3000000000000000016) dm-3 COMPELNT,Compellent Vol
size=100G features='1 queue_if_no_path' hwhandler='1 alua' wp=rw
|-+- policy='service-time 0' prio=50 status=active
| `- 1:0:0:1 sdb 8:16 active ready running
`-+- policy='service-time 0' prio=1 status=enabled
`- 1:0:1:1 sdc 8:32 active ready running
Compelnt_001a (36000d31000feb300000000000000001a) dm-4 COMPELNT,Compellent Vol
size=100G features='1 queue_if_no_path' hwhandler='1 alua' wp=rw
|-+- policy='service-time 0' prio=50 status=active
| `- 1:0:1:2 sdd 8:48 active ready running
`-+- policy='service-time 0' prio=1 status=enabled
`- 1:0:2:2 sde 8:64 active ready running
3.3.7 SAS queue depth
Currently supported SAS HBAs default to a queue depth of 254 for every volume. This value should be left at
its factory default value unless directed by support or application specific directives.
# lsscsi -L | egrep 'COMPELNT|depth'
[1:0:0:1] disk COMPELNT Compellent Vol 0606 /dev/sdb
queue_depth=254
[1:0:1:1] disk COMPELNT Compellent Vol 0606 /dev/sdc
queue_depth=254
3.3.8 Boot from SAS
Boot from SAS functionality is not validated nor supported in use with Dell EMC 12Gbps SAS HBAs on Linux.
3.4 Managing queue depth
There are two locations to configure queue depth for Fibre Channel HBAs. The first location is within the
BIOS configuration menu of the HBA. This configuration menu is typically accessed during boot or using the
tools provided by the HBA vendor. The queue depth can also be configured using the HBA configuration files
that are managed with the Linux modprobe facility within the operating system. If the queue depth value is
different in these two locations, the lesser of the two values takes precedence. It is recommended to configure
the BIOS to a high value (with 8Gb QLogic HBAs, this value is known as Execution Throttle and defaults to
65535) and subsequently manage the HBA queue depth with its respective operating system configuration
file. A queue depth value of 128 is a recommended starting value. Evaluate, test, and adjust the value
accordingly to meet individual environment needs.