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Server Administrator Storage Management 8.2 User’s Guide

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Write Policy

Does my controller support this feature? See Supported Features.

The write policies specify if the controller sends a write-request completion signal as soon as the data is in the cache or after it has been written to the disk.
  • Write Back — The controller sends a write-request completion signal as soon as the data is in the controller cache but has not yet been written to disk. Write back caching may provide improved performance since subsequent read requests can retrieve data quickly from the cache then from the disk. However, data loss may occur in the event of a system failure which prevents that data from being written on a disk. Other applications may also experience problems when actions assume that the data is available on the disk.
    • NOTE: Storage Management does not allow you to select the Write Back policy for controllers that do not have a battery. The only exceptions are PERC S100 and PERC S300. This restriction protects a controller without a battery from the loss of data that may occur in the event of a power failure. On some controllers, the Write Back policy may be available in the controller BIOS even though it is not available in Storage Management.
  • Force Write Back — The write cache is enabled regardless of whether the controller has a battery. If the controller does not have a battery and force write back caching is used, data loss may occur in the event of a power failure.
  • Write Back Enabled — The controller firmware disables the write cache if it does not detect the presence of a charged battery over a specified period. For example, on some controllers, the write cache is disabled if the firmware does not detect a charged battery within 72 hours.
  • Write Through — The controller sends a write-request completion signal only after the data is written to the disk. Write-through caching provides better data security than write-back caching, since the system assumes the data is available only after it has been safely written to the disk.
    • NOTE: Write-through is the default write policy setting when cluster mode is enabled.
  • Write Cache Enabled Protected — The controller writes data to the write cache before writing data to the physical disk. Because it takes less time to write data to the write cache than it does to a disk, enabling write cache can improve system performance. After data is written to the write cache, the system is free to continue with other operations. The controller, in the meantime, completes the write operation by writing the data from the write cache to the physical disk. The Write Cache Enabled Protected policy is available only if the controller has a functional battery. The presence of a functional battery ensures that data can be written from the write cache to the physical disk even when there is power failure.
    • NOTE: Storage Management does not allow you to select the Write Cache Enabled Protected policy for controllers that do not have a battery. This restriction protects a controller without a battery from the data loss that may occur in the event of a power failure. When using the Create Virtual Disk Advanced Wizard on a controller without a battery, the wizard either displays Write Cache Disabled as the only available option or the wizard does not display any option for write policy.
  • Write Cache Disabled — This is the only option available when the controller does not have a functional battery.

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