Scenarios Where the LUN Write Mode Becomes Write Through

The temperature of a controller exceeds the upper limit.

• If the Controller Enclosure Temperature Exceeds The Upper Limit alarm is generated due to an exception in the equipment room temperature or the internal components of a storage system, LUNs remain in write back mode for a specified period of time (192 hours by default). If the alarm persists after the specified period of time, the mode changes to write through.
• If the Controller Enclosure Temperature Exceeds The Upper Limit alarm is generated due to a fault on a single controller of a controller enclosure, LUNs remain in write back mode for a specified period of time (1 hour by default). If the alarm persists after the specified period of time, the mode changes to write through.

• Impact

  The write mode of all LUNs belonging to the controller enclosure changes to write through after the specified period of time.

• Recommended action

  Check the external refrigerating system, fan modules, and air channels to locate the cause of the over-temperature alarm and rectify faults.

Backup battery units (BBUs) on a controller enclosure malfunction or have reached the end of their service life.

• Dual-controller storage system: If two BBUs malfunction or have reached the end of their service life, an alarm is generated and the write mode changes from write back to write through.
• Four-controller storage system: If two or more BBUs malfunction or have reached the end of their service life, an alarm is generated and the write mode changes from write back to write through.
  NOTE:

  The system generates the BBU Module Is About To Expire alarm three months before a BBU reaches the end of its service life and the BBU Module Reaches Its Expiration Date alarm after a BBU reaches the end of its service life. You can run the show bbu life command on the CLI to query the BBU lifespan.

• Impact

  The write mode of all LUNs belonging to the controller enclosure changes to write through.

• Recommended action
  • Check whether the BBUs are properly installed.
  • Check whether the BBUs are faulty and replace them if necessary.
  • Check whether the BBUs have reached the end of their service life. If yes, replace them with the spare parts.
  • Check whether the charge of the BBUs is insufficient. If the power of the BBUs is insufficient, wait until the BBUs are fully charged.

The coffer disks of a controller enclosure malfunction.

• Dual-controller storage system: If two coffer disks break down, the write mode changes from write back to write through.
• Four-controller storage system: If all coffer disks of controllers A and B or controllers C and D break down (the controllers in the first row are controllers A and B and the controllers in the second row are controllers C and D), the write mode changes from write back to write through.

• Impact

  The write mode of all LUNs belonging to the controller enclosure changes to write through.

• Recommended action

  Check whether the coffer disks are faulty and replace them if necessary.

Controllers malfunction.

LUNs stay in write back mode for the write back hold time (192 hours by default) if only one controller on a storage system is properly working. If faults are not rectified within this period, the write mode of the LUNs changes from write back to write through.

• Impact

  The write mode of all LUNs belonging to the controller enclosure changes to write through if the fault persists for more than the write back hold time.

• Recommended action
  • Replace the faulty controller during off-peak hours and within the write back hold time after the malfunction.
  • If a spare part is unavailable during the write back hold time, extend the time after assessing risks to prevent write through from adversely affecting service performance.

The remaining capacity of a storage pool is smaller than the reserved capacity.

An alarm is generated, indicating that the capacity usage of a storage pool exceeds the threshold and reminding you to expand the storage pool.

• Impact

  The write mode of thin LUNs and thick LUNs with value-added features changes from write back to write through.

• Recommended action

  Expand the storage pool.