Lenovo ThinkServer RD240 MegaRAID SAS Software User Guide - Page 255
Stripe size, Read policy, Write policy, I/O policy, Full Initialization, striping, Always read ahead
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◊ Full Initialization: A complete initialization is done on the new configuration. You cannot write data to the new virtual drive until the initialization is complete. This can take a long time if the drives are large. • Stripe size: Stripe sizes of 8, 16, 32, 64, 128, 256, 512, and 1024 Kbytes are supported. The default is 64 Kbytes. For more information, see the striping Glossary entry. • Read policy: Specify the read policy for this virtual drive: ◊ Always read ahead: Read ahead capability allows the controller to read sequentially ahead of requested data and to store the additional data in cache memory, anticipating that the data will be needed soon. This speeds up reads for sequential data, but there is little improvement when accessing random data. ◊ No read ahead: (the default) Disables the read ahead capability. ◊ Adaptive read ahead: When selected, the controller begins using read ahead if the two most recent drive accesses occurred in sequential sectors. If the read requests are random, the controller reverts to No read ahead. • Write policy: Specify the write policy for this virtual drive: ◊ Write back: In this mode the controller sends a data transfer completion signal to the host when the controller cache has received all of the data in a transaction. ◊ Write through: (the default) In this mode the controller sends a data transfer completion signal to the host when the drive subsystem has received all of the data in a transaction. Note: The Write Policy depends on the status of the battery backup unit (BBU). If the BBU is not present, is bad, or is being charged, the default Write Policy will be Write through. This provides better data protection. • I/O policy: The IO policy applies to reads on a specific virtual drive. It does not affect the read ahead cache. ◊ Cached IO: In this mode, all reads are buffered in cache memory. Creating a New Storage Configuration 8-3