Users Guide

Considerations Before Creating Virtual Disks
Dierent controllers have particular characteristics in the way they implement virtual disks. These characteristics may include use of disk
space, limitations on the number of virtual disks per controller, and so on. It can be helpful to understand these characteristics before
creating virtual disks on the controller.
The following sections describe controller information that applies to virtual disks:
• Virtual Disk Considerations for Controllers
• Problems Associated With Using The Same Physical Disks For Both Redundant And Non-Redundant Virtual Disks
• Virtual Disk Considerations On Systems Running Linux
• Number Of Physical Disks Per Virtual Disk
• Number Of Virtual Disks Per Controller
• Calculation For Maximum Virtual Disk Size
You may also want to review the following sections:
• RAID Controller Read, Write, Cache, And Disk Cache Policy
• Understanding Hot spares
• Controller - Supported Stripe Sizes
• Time Delay In Displaying Conguration Changes
NOTE
: In addition to this document, review the hardware documentation that is provided with the controllers. Reviewing the
hardware documentation along with this document may provide a better understanding of the controller limitations.
Virtual Disk Considerations For Controllers
In addition to the considerations described in this section, you should also be aware of the controller limitations described in Number of
Physical Disks per Virtual Disk for the following controllers:
• PERC H730P MX
• PERC H745P MX
• PERC S140
NOTE
: The order of the controllers displayed on Storage Management may dier with the order of the controllers displayed in
the Human Interface (HII) and PERC Option ROM. The order of the controllers does not cause any limitation.
The following considerations apply when creating virtual disks:
• Creating virtual disks on controllers — When you create a virtual disk, you specify which physical disks are to be included in the virtual
disk. The virtual disk you create spans the specied physical disks. Depending on the size of the virtual disk, the virtual disk may not
consume all of the space on the physical disks. Any leftover space on the physical disks cannot be used for a second virtual disk unless
the physical disks are of equal size. In addition, when the physical disks are of equal size and you use the leftover space for a second
virtual disk, this new virtual disk cannot expand to include any physical disks not included in the original virtual disk.
• Space allocation when deleting and creating virtual disks on controllers — When you delete a virtual disk, you free up or make available
space on the physical disks that was being used by the deleted virtual disk. If you have created several virtual disks on a disk group,
then deleting virtual disks can result in pockets of free space residing in various locations on the physical disks. When you create a new
virtual disk, the controller must decide which free space on the physical disks to allocate to the new virtual disk. The PERC controllers
look for the largest area of free space and allocate this space to the new virtual disk.
• SCSI limitation of 2TB — Virtual disks created on a PERC controller cannot be created from physical disks with an aggregate size
greater than 2TB. This is a limitation of the controller implementation. For example, you cannot select more than 30 physical disks that
are 73GB in size, regardless of the size of the resulting virtual disk. When attempting to select more than 30 disks of this size, a pop-up
message is displayed indicating that the 2TB limit has been reached, and that you should select a smaller number of physical disks. The
2TB limit is an industry-wide SCSI limitation.
• Expanding virtual disks — You can only use the Recongure task to expand a virtual disk that uses the full capacity of its member
physical disks.
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Virtual Disks