3PAR InForm® OS 2.3.1 Concepts Guide (320-200112 Rev B, February 2010)

8.9
Logical Disk Size and Virtual Volumes
InForm OS Version 2.3.1 3PAR InForm OS Concepts Guide
8.6 Logical Disk Size and Virtual Volumes
Rounding up of logical disk size by the system may occur when creating virtual volumes. When
you create a virtual volume, the system automatically creates the necessary underlying logical
disks to support that volume. When the specified virtual volume size is less than the sum of the
logical disks created to support that volume, those logical disks will have some unused space.
For example, when creating a RAID 5 virtual volume with a default set size of 4 and a total size
of 1024 MB, the system automatically creates two 768 MB logical disks to support that volume.
However, because the requested volume size is only 1024 MB, only 512 MB of each RAID 5
logical disk is actually used by the virtual volume.
8.7 Logical Disk Size and Common Provisioning Groups
When creating a common provisioning group, the system may round up to determine the
initial allocation as well as the growth increment that determines the size of subsequent
allocations that occur as the CPG grows over time. For example, when creating a RAID 5 CPG
with the default set size of 4 and a growth increment of 8192 MB, the system will
automatically create a CPG with an initial user size of 8448 MB and a growth increment of 8448
MB. The system must round up the user space because the CPG requires 11 RAID 5 sets, each
with a size of 768 MG (3 x 256 MB). Together, these 11 RAID 5 sets total 8448 MB (11 x 768 MB).
The CPG does not show any user space until its first virtual volume is created.