Capacity Requirements - Dell PowerVault MD1120 Optimization Manual

A REFERENCE GUIDE FOR OPTIMIZING DELL™ POWERVAULT™ MD1XXX FAMILY SAS SOLUTION
When a physical disk is assigned to a virtual disk as a hot spare, the PERC 5/E and
PERC 6/E controllers will automatically rebuild the virtual disk using the hot spare in the
event of a failed physical disk. This restores the virtual disk to optimal redundancy. The hot
spare disk must be equal to or greater than the largest physical disk size within any of the
virtual disks. Dell recommends assigning hot spares to redundant virtual disks only, (i.e.
RAID 1, RAID 5, RAID 6, RAID 10, RAID 50 and RAID 60.)
Global Hot Spare
A global hot spare is a backup physical disk capable of replacing a failed physical disk in
any virtual disk on PERC 5/E and PERC 6/E controllers. The capacity of a global hot
spare should be greater than or equal to the largest physical disk in all the disk groups.
Dedicated Hot Spare
A dedicated hot spare is a backup physical disk capable of replacing a failed physical disk
in a specific PERC 5/E or PERC 6/E virtual disk. The capacity of a dedicated hot spare
should be greater than or equal to the largest physical disk in the virtual disk to which it is
assigned. Dell recommends assigning dedicated hot spares to all business critical virtual
disks.
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Disk Roaming
Disk roaming is initiated when the physical disks are changed to different ports on the
same controller. The physical disk roaming feature will detect the RAID configuration
from the configuration metadata residing on the physical disk.
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Rebuild
The process of restoring the redundancy of a RAID 1, 5, 6, 10, 50 or 60 virtual disk is
called rebuilding. The rebuilding process can be initiated manually or automatically. Dell
recommends choosing the automatic rebuild option to prevent running in the degraded
mode for an extended period of time.
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Mixed-Size Physical Disk Within RAID 10, RAID 50 or RAID 60
When mixing physical disk sizes in RAID 10, RAID 50 or RAID 60, the controller will not
coerce the larger virtual disks to match the size of the smallest virtual disk. PERC 5/E and
PERC 6/E controllers will stripe across all disks until the smallest one is full, then it will
continue to stripe across the remaining disks until the next one is full, and so on until all the
available space is used. This process can impact performance as the array fills up, since
the numbers of stripes decreases as disks fill up.

Capacity Requirements

The PERC 5/E and PERC 6/E controllers support up to sixty four virtual disks simultaneously
of which each virtual disk can contain up to thirty two physical disks. The maximum number of
total virtual disks that can be supported on each physical disk is sixteen. PERC 5 and PERC
6/E controllers support virtual disks greater than 2 TB in size, but Dell recommends checking
the capacity of the operating systems to support boot volumes of that capacity.
http://www.microsoft.com/windowsserversystem/storage/getstorfacts.mspx
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