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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.
Reconfiguration Of Virtual Disks
An online virtual disk can be reconfigured in ways that expands its capacity and/or change its RAID level.
NOTE: Spanned virtual disks such as RAID 10, 50, and 60 cannot be reconfigured.
NOTE: Reconfiguring Virtual Disks typically impacts disk performance until the reconfiguration operation is
complete.
Online Capacity Expansion (OCE) can be done in two ways:
• If there is a single virtual disk in a disk group and free space is available, the virtual disk’s capacity can be
expanded within that free space.
• If a virtual disk is created and it does not use the maximum size of the disk group, free space is available.
Free space is also available when a disk group’s physical disks are replaced by larger disks using the Replace Member
feature. A virtual disk's capacity can also be expanded by performing an OCE operation to add more physical disks.
RAID Level Migration (RLM) refers to changing a virtual disk’s RAID level. Both RLM and OCE can be done at the same
time so that a virtual disk can simultaneously have its RAID level changed and its capacity increased. When a RLM/OCE
operation is complete, a reboot is not required. See the following table for a list of RLM/OCE possibilities. The source
RAID level column indicates the virtual disk RAID level before the RLM/OCE and the target RAID level column indicates
the RAID level after the operation has completed.
NOTE: If the controller already contains the maximum number of virtual disks, you cannot perform a RAID level
migration or capacity expansion on any virtual disk.
NOTE: The controller changes the write cache policy of all virtual disks undergoing a RLM/OCE to Write-Through
until the RLM/OCE is complete.
Table 3. RAID Level Migration
Source RAID
Level
Target RAID Level Number of
Physical Disks
(Beginning)
Number of
Physical Disks
(End)
Capacity
Expansion
Possible
Description
RAID 0 RAID 0 1 2 or more Yes Increases
capacity by
adding disks
RAID 0 RAID 1 1 2 No Converts non-
redundant virtual
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