22.3. Write Barrier Considerations

One way to alternatively avoid data integrity issues is to ensure that no write caches lose data on power failures. When possible, the best way to configure this is to disable the write cache. On a simple server or desktop with one or more SATA drives (off a local SATA controller Intel AHCI part), you can disable the write cache on the target SATA drives with the following command:

hdparm -W0 /device/

# hdparm -W0 /device/hdparm -W0 /device/hdparm -W0 /device/

Write barriers are also unnecessary whenever the system uses hardware RAID controllers with battery-backed write cache. If the system is equipped with such controllers and if its component drives have write caches disabled, the controller acts as a write-through cache; this informs the kernel that the write cache data survives a power loss.

Most controllers use vendor-specific tools to query and manipulate target drives. For example, the LSI Megaraid SAS controller uses a battery-backed write cache; this type of controller requires the MegaCli64 tool to manage target drives. To show the state of all back-end drives for LSI Megaraid SAS, use:

MegaCli64 -LDGetProp -DskCache -LAll -aALL

# MegaCli64 -LDGetProp -DskCache -LAll -aALL

To disable the write cache of all back-end drives for LSI Megaraid SAS, use:

MegaCli64 -LDSetProp -DisDskCache -Lall -aALL

# MegaCli64 -LDSetProp -DisDskCache -Lall -aALL

High-end arrays have various ways of protecting data in the event of a power failure. As such, there is no need to verify the state of the internal drives in external RAID storage.

NFS clients do not need to enable write barriers, since data integrity is handled by the NFS server side. As such, NFS servers should be configured to ensure data persistence throughout a power loss (whether through write barriers or other means).