Lenovo ThinkServer RD240 MegaRAID SAS Software User Guide - Page 111

cache has received all of the data in a transaction., This setting is recommended in Standard mode.

Page 111 highlights

◊ Read Only: Allow read-only access. This type of access is the default. ◊ Blocked: Do not allow access. • Read Policy: Specify the read policy for this virtual drive: ◊ Normal: This option disables the read ahead capability. This is the default. ◊ Ahead: This option enables read ahead capability, which 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. ◊ Adaptive: When Adaptive read ahead is 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 Normal (no read ahead). • Write Policy: Specify the write policy for this virtual drive: ◊ WBack: In Writeback 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. This setting is recommended in Standard mode. ◊ WThru: In Writethrough 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. This is the default. ◊ Bad BBU: Select this mode if you want the controller to use Writeback mode but the controller has no BBU or the BBU is bad. If you do not choose this option, the controller firmware automatically switches to Writethrough mode if it detects a bad or missing BBU. Caution: Lenovo allows Writeback mode to be used with or without a battery. To protect the entire system, it is recommended that you use either a battery to protect the controller cache or an uninterruptible power supply (UPS). If you do not use a Creating a Storage Configuration 4-35

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Creating a Storage Configuration
4-35
Read Only
:
Allow read-only access. This type of access is
the default.
Blocked
:
Do not allow access.
Read Policy:
Specify the read policy for this virtual drive:
Normal
:
This option disables the read ahead capability.
This is the default.
Ahead
:
This option enables read ahead capability, which
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.
Adaptive
:
When Adaptive read ahead is 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
Normal
(no
read ahead).
Write Policy:
Specify the write policy for this virtual drive:
WBack
:
In Writeback 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.
This setting is recommended in Standard mode.
WThru
:
In Writethrough 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.
This is the default.
Bad BBU
: Select this mode if you want the controller to use
Writeback mode but the controller has no BBU or the BBU
is bad. If you do not choose this option, the controller
firmware automatically switches to Writethrough mode if it
detects a bad or missing BBU.
Caution
:
Lenovo allows Writeback mode to be used with or without a
battery. To protect the entire system, it is recommended that
you use
either
a battery to protect the controller cache or
an uninterruptible power supply (UPS). If you do not use a