Dell 341-7212 User Guide - Page 25

Understanding Virtual Disk Parameters, striping, Always Read Ahead, No Read Ahead, Adaptive Read Ahead

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Understanding Virtual Disk Parameters This section describes the Virtual Disk Parameters that you can set when you use the Guided Configuration or Manual Configuration modes of the Configuration Wizard. You do not necessarily need to change these parameters when you create a storage configuration; you can leave them at their default settings. • Stripe Size: A stripe is a segment of data spread across multiple physical disks. Data segments with stripe sizes of 8, 16, 32, 64, and 128 Kbytes are supported. The default is 64 KB. For more information, see the Glossary entry for striping. • Read Policy: 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: Disables the read-ahead capability. Adaptive Read Ahead: When selected, the controller begins using readahead if the two most recent disk accesses occurred in sequential sectors. If the read requests are random, the controller reverts to No Read Ahead. • Write Policy: Write back: In this mode the controller sends a data transfer completion signal to the host when the controller cache has received all the data in a transaction. This setting is recommended in standard mode. Write through: In this mode the controller sends a data transfer completion signal to the host when the disk subsystem has received all the data in a transaction. NOTE: If you select Write back, you need to clear the check box "Use Write through for failed or missing battery?". Otherwise, Write through is used because no battery is present. CAUTION: Using Write Back can result in data loss if a power failure occurs since the data is not stored in battery backed cache. • IO Policy: The IO Policy applies to reads on a specific virtual disk. It does not affect the Read-ahead cache. Cached IO: In this mode, all reads are buffered in cache memory. Configuration 25

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Configuration
25
Understanding Virtual Disk Parameters
This section describes the Virtual Disk Parameters that you can set when you use
the Guided Configuration or Manual Configuration modes of the Configuration
Wizard. You do not necessarily need to change these parameters when you create
a storage configuration; you can leave them at their default settings.
•
Stripe Size:
A stripe is a segment of data spread across multiple physical disks. Data
segments with stripe sizes of 8, 16, 32, 64, and 128 Kbytes are supported.
The default is 64 KB. For more information, see the Glossary entry for
striping
.
•
Read Policy:
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:
Disables the read-ahead capability.
Adaptive Read Ahead:
When selected, the controller begins using read-
ahead if the two most recent disk accesses occurred in sequential sectors. If
the read requests are random, the controller reverts to
No Read Ahead
.
•
Write Policy:
Write back:
In this mode the controller sends a data transfer completion
signal to the host when the controller cache has received all the data in a
transaction. This setting is recommended in standard mode.
Write through:
In this mode the controller sends a data transfer completion
signal to the host when the disk subsystem has received all the data in a
transaction.
NOTE:
If you select Write back, you need to clear the check box "Use Write
through for failed or missing battery?". Otherwise, Write through is used
because no battery is present.
CAUTION:
Using Write Back can result in data loss if a power failure
occurs since the data is not stored in battery backed cache.
•
IO Policy:
The IO Policy applies to reads on a specific virtual disk. It does
not affect the Read-ahead cache.
Cached IO:
In this mode, all reads are buffered in cache memory.