Conditions under which write-through is employed, Virtual disk read cache policies, Conditions under which – Dell PowerEdge RAID Controller H700 User Manual
Page 30: Write-through is employed, Conditions under which forced write-back, With no battery is employed
30
Storage Controller Features
Conditions Under Which Write-Through is Employed
Write-Through caching is used under all conditions in which the battery is
missing or in a low-charge state. Low-charge state is when the battery is not
capable of maintaining data for at least 24 hours in the case of a power loss.
This low-charge state does not apply to controllers with the optional
non-volatile cache (NVC) module present.
Conditions Under Which Forced Write-Back With No Battery is
Employed
Write-Back mode is available when you select Force WB with no battery.
When Forced Write-Back mode is selected, the virtual disk is in Write-Back
mode even if the battery is not present.
CAUTION:
It is recommended that you use a power backup system when forcing
Write-Back to ensure there is no loss of data if the system suddenly loses power.
Virtual Disk Read Cache Policies
The read policy of a virtual disk determines how the controller handles reads
to that virtual disk. The read policies are:
• Always Read Ahead — Allows the controller to read sequentially ahead of
requested data and to store the additional data in cache memory,
anticipating that the data is required 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 — 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 mode.
NOTE:
The default read cache setting for virtual disks is Adaptive Read Ahead.
PERC7.2_UG.book Page 30 Thursday, March 3, 2011 2:14 PM