用Perc5/i(Dell™ PowerEdge™ Expandable RAID Controller 5/i)做RAID5的时候VD有Read Policy和Write Policy选项,但是没有发现Cache Policy. (RAID Controller Supported Features)
具体选项是:
Read Policy:
The read policies indicate whether or not the controller should read sequential sectors of the logical drive when seeking data.
- Read-Ahead. When using read-ahead policy, the controller reads sequential sectors of the logical drive when seeking data. Read-ahead policy may improve system performance if the data is actually written to sequential sectors of the logical drive.
- No-Read-Ahead. Selecting no-read-ahead policy indicates that the controller should not use read-ahead policy.
- Adaptive Read-Ahead. When using adaptive read-ahead policy, the controller initiates read-ahead only if the two most recent read requests accessed sequential sectors of the disk. If subsequent read requests access random sectors of the disk, the controller reverts to no-read-ahead policy. The controller continues to evaluate whether read requests are accessing sequential sectors of the disk, and can initiate read-ahead if necessary.
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.
p2p点播对write没有太大的需求,直接选择write-through,但是对于Read Policy,貌似是有点搞头。Adaptive Read-Ahead是在RAID控制器上做缓存策略,但是我怀疑Perc5/i那256MB的DDR内存作为缓存的命中率,这篇文章也证实了我的看法。
但是Dell官方对write policy的说明(Perc5/i-PDF文档下载中文/English;Dell RAID Crontroller Manuals)让人还是感觉很诱惑,毕竟对于优化来说,按说这些东西的优化是没有止境的。
根据dell的说法，对于大数据量的时候，比如real的视频点播，可以使用ReadAhead, 我还没有想到什么时候需要用的wirte-back, 除了IO不够的时候
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