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

8.8
Logical Disk Size and RAID Types
3PAR InForm OS Concepts Guide InForm OS Version 2.3.1
8.5 Logical Disk Size and RAID Types
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. A
RAID 5 set with three chunklets has a total of two chunklets of space for data and one
chunklet of space for parity. 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, for each
data chunklet in the RAID set. For example, if you specify a size of 750 MB when creating a
RAID 5 logical disk with the default set size of 4 chunklets, the result is a logical disk with a
user space of 768 MB (3 x 256 MB). The minimum size for a RAID 5 logical disk is 768 MB.
For RAID MP logical disks, the total size of the logical disk is determined by the number of data
chunklets in the RAID set. A RAID MP set, or double parity set, must contain at least 8
chunklets. RAID MP set sizes of 8 and 16 chunklets are supported. The system default is 8
chunklets. A RAID MP set with 8 chunklets has a total of 6 chunklets of space for data and 2
chunklets of space for parity (6+2). When creating a RAID MP logical disk with this default 6+2
layout, the system automatically rounds up so that the logical disk size reflects one chunklet, or
256 MB, for each data chunklet in the RAID set. For example, if you specify a size of 1,500 MB
when creating a RAID MP logical disk with the default set size of 8 chunklets, the result is a
logical disk with a user space of 1,536 MB (6 x 256 MB). The minimum size for a RAID MP
logical disk is 1,536 MB.