HP LH4r Integrated HP NetRaid Controller Configuration Guide - Page 42

Chapter 4 Planning Write Policy When the IOP writes to disk, the data is first written to the cache on the assumption that the IOP will read it again soon. The two Write policies are: • Write-Through: In a write-through cache, data is written to disk at the same time it is cached. This setting provides better security because entries are always copied to disk. This is the default setting. • Write Back: In a write-back cache, data is written to disk only when it is forced out of the cache. Write-back is efficient, because an entry may be written many times in the cache without a disk access. While this setting increases performance, it is not recommended unless your system is equipped with a UPS (uninterruptible power supply). NOTE Write Back is not certified for any logical drive used as a Novell NetWare volume. For each logical drive, record the write policy in Worksheet B. Read Policy The three Read policies for HP NetRAID are: • Read-Ahead: This is a memory caching feature that tells the controller to read sequentially ahead of requested data and cache the additional data in memory, anticipating that the additional data will be requested. ReadAhead supplies sequential data faster, but it is not as efficient when accessing random data. • Normal: This policy does not use the read-ahead memory caching feature. This policy is efficient when most of the data reads are random. • Adaptive: Adaptive policy causes the read-ahead feature to be used if the last two disk accesses were in sequential sectors. This is the default setting. For each logical drive, record the read policy in Worksheet B. 36