HP Surestore Disk Array FC60 HP SureStore E Disk Array FC60 Service Manual (A5 - Page 194
Global Hot Spares, Setting Stripe Segment Size, Storage Efficiency
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• Storage Efficiency - the storage efficiency can range from 50% for RAID 1 and 0/1 up to > 80% for RAID 5. The higher the efficiency, the less cost per megabyte for storing your data. Global Hot Spares Global hot spares provide additional protection against disk failures. The number of global hot spares you use will reflect how much protection you need. Each global hot spare you add will provide protection against the failure of a single disk. In addition to restoring hardware redundancy, a global hot spare also restores disk array performance that may be diminished while the disk array is operating in degraded mode with a failed disk. When adding global hot spares, consider the following: • It is recommended that you add one global hot spare per disk channel. This provides maximum protection against disk failure. A global hot spare can be used to replace any failed disk within the array, regardless of what channel it is on. • When a failed disk is replaced, the data will be copied from the global hot spare disk to the replacement disk and the global hot spare will again be available as protection against another disk failure. • If you choose to limit the number of global hot spares, make sure you are able to respond quickly to replace a failed disk. If an operator is always available to replace a disk, you may not need the added protection offered by multiple global hot spares. Setting Stripe Segment Size Another factor you may have to consider is the stripe segment size you use for a volume. The stripe segment size determines how much data is written to a disk before moving to the next disk in the volume to continue writing. For example, if the stripe segment size is set to 4 Kbytes, the disk array will write 4 Kbytes of data to disk 1, then 4 Kbytes of data to disk 2, then 4 Kbytes of data to disk 3, and so on. For more information, see "Data Striping" on page 47. When creating a volume, the default value used for the stripe segment size is the value currently set for the cache page size (4 Kbytes or 16 Kbytes). The default cache page size is 4 Kbytes. 194 Managing Disk Array Capacity