From where does default 'scsi timeout' value get set for scsi devices ?

Solution Unverified - Updated

Environment

  • Red Hat Enterprise Linux 5
  • Red Hat Enterprise Linux 6
  • Red Hat Enterprise Linux 7

Issue

  • From where does default 'scsi timeout' value get set for scsi devices ?

Resolution

  • Timeout : The Linux SCSI layer sets a timer on each command. When this timer expires, the SCSI layer will quiesce the host bus adapter (HBA) and wait for all outstanding commands to either time out or complete.

  • Afterwards, the SCSI layer will activate the driver's error handler.

  • When the error handler is triggered, it attempts the following operations in order (until one returns success):

      Abort the command.
  Reset the device.
  Reset the bus.
  Reset the host.

  • If all of these operations fail, the device will be set to the offline state.

  • When this occurs, all IO to that device will be failed, until the problem is corrected and the device is returned to running state.

  • Default scsi device timeout is same across both RHEL5 and RHEL6 kernel.

  • This is defined in kernel scsi sd driver source.

        // **RHEL5** 
        drivers/scsi/sd.c:#define SD_TIMEOUT		(30 * HZ)
        
        // **RHEL6** 
        drivers/scsi/sd.h:#define SD_TIMEOUT		(30 * HZ)
 
       // **RHEL7** 
        drivers/scsi/sd.h:#define SD_TIMEOUT		(30 * HZ)

  • But in RHEL5, its increased to 60 sec using udev rules i.e.

  • If udev is not in use, the default timeout is 30 seconds.

  • The same udev rule could be applied to rhel6 as well to modify the default timeout.

  • Default timeout value in RHEL5 from udev is seen as

      [root@host]# cat /sys/block/sda/device/timeout 
  60

  • Default scsi timeout value for scsi devices get set through udev rule in RHEL5.

  • From /etc/udev/rules.d/50-udev.rules file:

      # sd:           0 TYPE_DISK, 7 TYPE_MOD, 14 TYPE_RBC
  # sr:           4 TYPE_WORM, 5 TYPE_ROM
  # st/osst:      1 TYPE_TAPE
  # sg:           8 changer, [36] scanner
  ACTION=="add", SUBSYSTEM=="scsi" , SYSFS{type}=="0|7|14", \
      RUN+="/bin/sh -c 'echo 60 > /sys$$DEVPATH/timeout'"

  • Also, to change the device timeout value see:

  1. For SAN storage devices (step could also be used):
[root@host]# echo 180 > /sys/class/scsi_device/<H>:<C>:<T>:<L>/device/timeout
  Where these are the meanings of the values:
        H = SCSI host number of the lpfc HBA  
        T = SCSI ID of the SCSI target  
        L = LUN number of the SCSI device.
  1. For internal disks:
[root@host]# echo 180 > /sys/block/sda/device/timeout

This will set device timeout value to 180 seconds.

  • In RHEL6 it was reduced to 30 to allow faster error detection and hence faster recovery from failure of I/O. This is 30 as per industry standard.
  • In RHEL7 it is set to 30 as well.
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