Dell Perc5/i RAID5 读写优化

Write Policy:The write policies specify whether the controller sends a write-request completion signal as soon as the data is in the cache or after it has been written to disk.

• Write-Back. When using write-back caching, 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 more quickly retrieve data from the controller cache than they could from the disk. Write-back caching also entails a data security risk, however, since a system failure could prevent the data from being written to disk even though the controller has sent a write-request completion signal. In this case, data may be lost. Other applications may also experience problems when taking actions that assume the data is available on the disk.
• Write-Through. When using write-through caching, 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.

缺少的Cache Policy选项是:
Cache Policy:
The Direct I/O and Cache I/O cache policies apply to reads on a specific virtual disk. These settings do not affect the read-ahead policy. The cache policies are as follows:

• Cache I/O. Specifies that all reads are buffered in cache memory.
• Direct I/O. Specifies that reads are not buffered in cache memory. When using direct I/O, data is transferred to the controller cache and the host system simultaneously during a read request. If a subsequent read request requires data from the same data block, it can be read directly from the controller cache. The direct I/O setting does not override the cache policy settings. Direct I/O is also the default setting.

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两台新上架的X3550 M2。在新机器基准测试的时候，发现磁盘性能比其它同类型机器的性能低很多。

检查发现PD,LD,BBU状态均正常。反馈到厂商那边，回复说需要把这个回写方式要改WriteThrough  为WriteBack。

改了之后，测试通过，前面几次测试都是差的，今天改了写入方式后，性能回到正常。话说之前换了几次RAID卡、硬盘， 浪费了很多时间

又，改这个可以直接用megacli命令去改。但有时候会修改不了。修改不了的时候需要进RAID设置界面那里去改。

# ./MegaCli32  LDSetProp WB -Lall -aALL

结论：电池present状态，并不代表RAID状态OK；比起看BBU状态，直接看Cache Policy更到位。

附：查看CaChe policy

# ./MegaCli ldPDinfo -a0 | grep -i write
Default Cache Policy: WriteBack, ReadAdaptive, Direct, No Write Cache if Bad BBU
Current Cache Policy: WriteBack, ReadAdaptive, Direct, No Write Cache if Bad BBU
Access Policy: Read/Write
Default Cache Policy: WriteBack, ReadAdaptive, Direct, No Write Cache if Bad BBU
Current Cache Policy: WriteBack, ReadAdaptive, Direct, No Write Cache if Bad BBU
Access Policy: Read/Write
Default Cache Policy: WriteBack, ReadAdaptive, Direct, No Write Cache if Bad BBU
Current Cache Policy: WriteBack, ReadAdaptive, Direct, No Write Cache if Bad BBU
Access Policy: Read/Write