Function of LUN prefetch policies

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The controller analyzes the reading trend of data in the future based on read data, and reads data from the disk to the cache in advance to improve the reading hit rate of the host.

Other related questions:
Feature of the LUN variable prefetch policy
1. After receiving a data read request, the storage system prefetches data to the cache based on a multiple of the read length specified in the I/O request. 2. This policy applies to sequential read applications with variable-size data or to multi-user concurrent read applications with an unknown data prefetch amount, for example, multi-user Video on Demand (VOD) initiated by users at different bit rates.

Type of the LUN prefetch policy
The prefetch policy includes: intelligent prefetch, constant prefetch, variable prefetch, and non-prefetch.

Feature of the LUN intelligent prefetch policy
1. Intelligent prefetch analyzes whether the requested data is sequential. If it is, the data following the currently requested data is prefetched from the disk to the cache to improve the cache hit ratio. 2. Intelligent prefetch is applicable to the read applications having only one data flow or to the read applications that cannot be determined as sequential or random, for example, file read/write.

Method used to modify the LUN prefetch policy
Go to the Modify LUN dialog box. In the ISM navigation tree, choose All Devices > SN_XX > Storage Resources > LUN (SN_XX is the name of the target storage system). In the right function pane, select the LUN you want to modify. Click Modify. The Modify LUN dialog box is displayed. Modify the basic and advanced attributes of the LUN.

Feature of the LUN constant prefetch policy
1. After receiving a data read request, the storage system prefetches data to the cache based on the preset prefetch length, regardless of the read length specified in the I/O request. 2. This policy applies to sequential read applications with fixed-size data, for example, streaming media requests initiated by multiple users at the same bit rate.

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