White Papers

Server configuration
11 Dell EMC SC Series: Red Hat Enterprise Linux Best Practices | CML1031
2.2 SCSI device command timeout
On Linux, the SCSI device command timeout has a default value of 30 seconds. Verify the setting with the
following command.
# cat /sys/block/sdX/device/timeout
Typically, this value does not need to be changed unless instructed or recommended to do so by support or
application-specific directives. If necessary, the value can be changed using the following process.
1. Change the timeout in seconds for a SCSI device by writing to the device’s sysfs file. This does not
persist after system reboots.
# echo 60 > /sys/block/sdX/device/timeout
2. Change and persist the timeout in seconds by the udev rule.
Location of the udev rule files:
/etc/udev/rules.d/60-raw.rules #RHEL 6 and older
/lib/udev/rules.d/60-raw.rules #RHEL 7 and newer
Content of the udev rule:
ACTION=="add", SUBSYSTEMS=="scsi", ATTRS{vendor}=="COMPELNT*",
ATTRS{model}=="Compellent Vol*", RUN+="/bin/sh -c 'echo 60
>/sys$DEVPATH/device/timeout'"
3. Reload udev and multipathd (if applicable) for the changes to take effect immediately. The 2-second
sleep period between commands ensures udev has time to complete before reloading the
multipathd service, picking up the new timeout value of the respective child paths.
# udevadm trigger –-action=add
# sleep 2
# multipath –r > /dev/null
2.3 Serial-attached SCSI
Certain SC and SCv Series models offer 12Gbps serial-attached SCSI (SAS) front-end connectivity. The
front-end SAS support was introduced in SCOS 6.6.x. A Linux system equipped with a compatible SAS HBA
can direct-attach an SC Series storage system through the SAS protocol. Consult the Dell EMC Storage
Compatibility Matrix for the HBA compatibility information.
In direct SAS-connected environments, paths from both controllers are presented to the connected Linux
system (active/optimized and standby). However, only the active/optimized path is used for all active I/O at
any one time. When the active/optimized path becomes unavailable, the storage system dynamically
determines which remaining standby path is the active/optimized path and continues to stream active I/O to
the new active/optimized path.