Users Guide

selected RAID 10 and if the controller supports uneven RAID 10, then the span count value is not
displayed. The controller automatically sets the appropriate value.
3. In the Select Physical Disks section, select the number of physical disks.
For more information about the fields, see the iDRAC Online Help
4. From the Apply Operation Mode drop-down menu, select when you want to apply the settings.
5. Click Create Virtual Disk.
Based on the selected Apply Operation Mode, the settings are applied.
Creating virtual disks using RACADM
Use racadm storage createvd command. For more information, see the iDRAC RACADM Command
Line Reference Guide
available at dell.com/esmmanuals.
Editing virtual disk cache policies
You can change the read, write, or disk cache policy of a virtual disk.
NOTE: Some of the controllers do not support all read or write policies. Therefore, when a policy is
applied, an error message is displayed.
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 read 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 as soon as 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 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 apply to readings on a specific virtual disk. These settings do not affect the read-
ahead policy.
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