Users Guide

• 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 Configuration 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:
• Controllers supported on R6525 are:
• PERC H745P Front, PERC H345 Frnt HBA 345
Controllers supported on C6525 are:
• Adapter PERC H745, Adapter PERC H345
• PERC S150
NOTE:
The order of the controllers displayed on Storage Management may differ 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 specified 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 Reconfigure task to expand a virtual disk that uses the full capacity of its member
physical disks.
• Reconfiguring virtual disks — The Reconfigure task is not available when you have more than one virtual disk using the same set of
physical disks. You can, however, reconfigure a virtual disk that is the only virtual disk residing on a set of physical disks.
• Virtual disk names not stored on controller — The names of the virtual disks that you create are not stored on the controller. If you
reboot using a different operating system, the new operating system may rename the virtual disk using its own naming conventions.
• Creating and deleting virtual disks on cluster-enabled controllers — There are particular considerations for creating or deleting a virtual
disk from a cluster-enabled controller.
• Implementing channel redundancy — A virtual disk is channel-redundant when it maintains redundant data on more than one
channel. If one of the channels fails, data is not lost because redundant data resides on another channel.
• Rebuilding data — An failed physical disk that is used by both redundant and nonredundant virtual disks cannot be rebuilt. Rebuilding a
failed physical disk in this situation requires deleting the nonredundant virtual disk.
• Disk group concept consideration for S110 — Disk grouping is a logical grouping of disks attached to a RAID controller on which one or
more virtual disks are created, such that all virtual disks in the disk group use all of the physical disks in the disk group. The current
implementation supports the blocking of mixed disk groups during the creation of logical devices.
Physical disks are bound to disk groups, therefore, there is no RAID level mixing on one disk group.
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