HP LH4r Installation and configuration of the HP NetRAID, NetRAID-1 and NetRAI - Page 49

Chapter 4 Planning Logical Drive Size You can calculate the usable capacity to assign to each logical drive, or you can let the HP NetRAID Assistant Wizard specify it during configuration. If you calculate logical drive capacity, record it in Worksheet B. If you want a logical drive to occupy less space than is available on one array, you must enter the logical drive size. Stripe Size (Stripe Depth) Stripe size is the amount of data contiguously written to each disk in an array. It is sometimes called "stripe depth." You can specify stripe sizes of 4 KB, 8 KB, 16 KB, 32 KB, 64 KB, and 128 KB for each logical drive. Consider the following when setting your stripe size: • The default stripe size is 8 KB. • A stripe size of 128 KB requires at least 8 MB of memory. • For best performance, choose a stripe size equal to or smaller than the block size used by your operating system. • A large stripe size produces better performance if the system does mostly sequential reads. • Choose a small stripe size if your system performs mostly random reads. • Online conversions of RAID 3 logical drives can be done only if the stripe size is 64 KB. For each logical drive, log the stripe size in Worksheet B. NOTE Consider the stripe size carefully, because once configured, you cannot change the size. 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. If no Uninterruptible Power Supply (like HP NetRAID's Battery Backup unit) is connected, you should use the write-through setting, because otherwise data could be lost from cache memory in a power failure. 41