3PAR InForm® OS 2.2.4 Concepts Guide (320-200085 Rev B, March 2009)

7.3
Logical Disk Size and RAID Type
InForm OS Version 2.2.4 3PAR InForm OS Concepts Guide
Group (CPG). These administration LDs are separate from the admin volume, a single base
volume created on each system during installation and set up to store system
administrative data such as the system event log.
7.4 Logical Disk Size and RAID Type
A logical disk is a collection of physical disk chunklets, each 256 MB, that have been arranged
as rows of RAID sets. For this reason, the total size of a logical disk is a multiple of the user
space within a single logical disk RAID set.
For RAID 0 and RAID 1 logical disks, the user space within each RAID set is equal to one
chunklet, or 256 MB. Therefore, for RAID 0 and RAID 1 logical disks, the total logical disk size is
always a multiple of 256 MB. When creating a RAID 0 or RAID 1 logical disk, the system
automatically rounds up to the next multiple of 256 MB when you specify a logical disk size
that is not a multiple of 256 MB. For example, if you specify a size of 1000 MB when creating a
RAID 0 logical disk, the result will be a RAID 0 logical disk with a user space of 1024 MB.
For RAID 5 logical disks, the total size of the logical disk is determined by the number of data
chunklets in the RAID set. A RAID 5 set, or parity set, must contain at least three chunklets: two
for data and one for parity (2+1). The system default is four chunklets: three for data and one
for parity (3+1). When creating a RAID 5 logical disk with this default 3+1 layout, the system
automatically rounds up so that the logical disk size reflects one chunklet, or 256 MB, per data
chunklet in the RAID set (3). For example, if you specify a size of 500 MB when creating a RAID
5 logical disk with the default set size of 3, the result is a logical disk with a size of 768 MB (3 x
256 MB).
7.4.1 For Virtual Volumes
Rounding up of logical disk size by the system can also 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 3
and a total size of 1024 MB, the system will automatically create two 768 MB logical disks (3 x
256 MB) to support that volume as explained previously. 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.