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Huawei V2 and V3 Server RAID Controller Card User Guide 36

This document describes the appearances and features of the redundant array of independent disks (RAID) controller cards, how to configure properties for the RAID controller cards, and how to install drivers. The RAID controller cards include the LSI SAS2208, LSI SAS2308, LSI SAS3008, LSI SAS3108, LSI SoftRAID, PM8060, and PM8068.
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Virtual Drive Read and Write Policies

Virtual Drive Read and Write Policies

When creating a virtual drive, you need to define its data read and write policies to standardize subsequent data read and write operations on it.

Data Read Policy

The RAID controller card supports the following data read policies:

  • Read-ahead policy: This policy is usually specified as Always Read Ahead, Read Ahead, or Ahead on the configuration screen. If this policy is used, the RAID controller card caches the data that follows the data being read for faster access. This policy reduces drive seeks and shortens read time.

    The read-ahead policy is applicable only when the RAID controller card supports power failure protection for data. If the supercapacitor of the RAID controller that adopts the read-ahead policy is abnormal, data may get lost.

  • Non-read-ahead policy: If this policy is used, the RAID controller card does not read data ahead. Instead, it reads data from a virtual drive only when it receives a data read command.

Data Write Policy

The RAID controller card supports the following data write policies:

  • Write-back: This policy is usually specified as Write Back on the configuration screen. If this policy is used, data is directly written to the cache. The controller card then updates accumulated cache data to hard drives in batches. The overall data write speed is higher since writing to the cache is faster than writing to the hard drive. After the controller cache receives all data that needs to be transmitted, the controller signals the host that the data transmission is complete.

    The write-back policy is applicable only when the RAID controller card supports power failure protection for data. If the supercapacitor of the RAID controller that adopts the write-back policy is abnormal, data may get lost.

  • Write-through: This policy is usually specified as Write Through on the configuration screen. If this policy is used, the RAID controller card writes data directly into a virtual drive, without passing through the cache. After the drive subsystem receives all data that needs to be transmitted, the controller signals the host that the data transmission is complete.

    This policy does not require the RAID controller card to support BBU, and BBU faults do not affect data write. However, the data write speed is low.

  • BBU-related write-back: This policy is usually specified as Write Back with BBU on the configuration screen. This policy involves two scenarios: If the BBU of the RAID controller card is present and normal, the write operation from the RAID controller card to a virtual drive will go through the cache (write-back mode). If the BBU of the RAID controller card is absent or faulty, the write operation from the RAID controller card to a virtual drive does not go through the cache (write-through mode).

For the LSI SAS3108:

  • If the firmware version is 4.270.00-4382 or earlier, the options of the Write Policy parameter are Write Back, Write Through, and Write Back with BBU.
  • If the firmware version is 4.650.00-6121 or later, the options of the Write Policy parameter are Always Write Back, Write Through, and Write Back (write-back with BBU).
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Updated: 2019-11-01

Document ID: EDOC1000004186

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