HP LH4r Installation and configuration of the HP NetRAID, NetRAID-1 and NetRAI - Page 50

Chapter 4 Planning • 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. Write Back can be used if an Uninterruptible Power Supply maintains the data in cache memory in the event of a power failure. 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 adapter 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. For each logical drive, record the read policy in Worksheet B. Cache Policy HP NetRAID lets you choose one of two cache policies: • Cached I/O: Reads are always cached regardless of randomness. The selected Read policy (Read-Ahead, Normal, or Adaptive) and Write policy (Write-Through and Write-Back) always applies. • Direct I/O: First reads to a new location always come directly from the disk. If a location is read repeatedly, then the Read policy as selected will apply and the read data will be cached. Read data is cached only if accessed repeatedly. With 100% randomness, nothing is cached. For each logical drive, record the cache policy in Worksheet B. 42