Users Guide

The read policies indicate whether the controller must read sequential sectors of the virtual disk searching for data:
• Adaptive Read Ahead — The controller initiates read ahead only if the two most recent reads requests accessed sequential sectors
of the disk. If subsequent read requests access random sectors of the disk, the controller reverts to no read ahead policy. The
controller continues to evaluate whether read requests are accessing sequential sectors of the disk, and initiates read ahead if
necessary.
• Read Ahead — The controller reads sequential sectors of the virtual disk when seeking data. Read ahead policy may improve system
performance if the data is written to the sequential sectors of the virtual disk.
• No Read Ahead — Selecting no read ahead policy indicates that the controller should not use read ahead policy.
The write policies specify if the controller sends a write-request completion signal when the data is in the cache or after it has been
written to the disk.
• Write Through — The controller sends a write-request completion signal only after the data is written to the disk. Write-through
caching provides better data security than write-back caching, since the system assumes that the data is available only after it has
been safely written to the disk.
• Write Back — The controller sends a write-request completion signal as soon as the data is in the controller cache but has not yet
been written to disk. Write back caching may provide improved performance since subsequent read requests can retrieve data quickly
from the cache then from the disk. However, data loss may occur in the event of a system failure which prevents that data from being
written on a disk. Other applications may also experience problems when actions assume that the data is available on the disk.
• Force Write Back — The write cache is enabled regardless of whether the controller has a battery. If the controller does not have a
battery and force write-back caching is used, data loss may occur in the event of a power failure.
The Disk Cache policy applies to readings on a specific virtual disk. These settings do not affect the read-ahead policy.
NOTE:
• Controller non-volatile cache and battery backup of controller cache affects the read-policy or the write policy that a
controller can support. All PERCs do not have battery and cache.
• Read ahead and write back requires cache. Therefore, if the controller does not have cache, it does not allow you to
set the policy value.
Similarly, if the PERC has cache but not battery and the policy is set that requires accessing cache, then data loss
may occur if base of power off. So few PERCs may not allow that policy.
Therefore, depending upon the PERC, the policy value is set.
Deleting virtual disks
Deleting a virtual disk destroys all information including file systems and volumes residing on the virtual disk and removes the virtual disk
from the controller’s configuration. When deleting virtual disks, all assigned global hot spares may be automatically unassigned when the
last virtual disk associated with the controller is deleted. When deleting the last virtual disk of a disk group, all assigned dedicated hot
spares automatically become global hot spares.
If you delete all the VDs for a global hotspare, then the global hotspare gets automatically deleted.
You must have the Login and Server Control privilege to perform delete virtual disks.
When this operation is allowed, you can delete a boot virtual drive. It is done from sideband and the independent of the operating system.
Hence, a warning message appears before you delete the virtual drive.
If you delete a virtual disk and immediately create a new virtual disk with all the same characteristics as the one that was deleted, the
controller recognizes the data as if the first virtual disk were never deleted. In this situation, if you do not want the old data after
recreating a new virtual disk, re-initialize the virtual disk.
Checking virtual disk consistency
This operation verifies the accuracy of the redundant (parity) information. This task only applies to redundant virtual disks. When
necessary, the check consistency task rebuilds the redundant data. If the virtual drive has a degraded status, running a check consistency
may be able to return the virtual drive to ready status. You can perform a consistency check using the web interface or RACADM.
You can also cancel the check consistency operation. The cancel check consistency is a real-time operation.
You must have Login and Server Control privilege to check consistency of virtual disks.
NOTE: Consistency check is not supported when the drives are set up in RAID0 mode.
Managing storage devices 211