Using Your RAID Enclosure 41
Segment Size Migration
Segment size refers to the amount of data (in kilobytes) that the RAID controller module writes on a
single physical disk in a virtual disk before writing data on the next physical disk. Valid values for the
segment size are 8, 16, 32, 64, 128, 256, and 512 KB.
Dynamic segment size migration enables the segment size of a given virtual disk to be changed. A
default segment size was set when the virtual disk was created, based on such factors as the RAID level
and expected usage. You can change the default value if actual usage does not match your needs.
When considering a segment-size change, two scenarios illustrate different approaches to the
limitations:
• If I/O activity stretches beyond the segment size, you can increase it to reduce the number of disks
required to satisfy a single I/O. Using a single physical disk for a single request frees other disks to
service other requests, especially when you have multiple users accessing a database or storage
environment.
• If you are using the virtual disk in a single-user, large I/O environment (such as for multimedia
application storage), performance can be optimized when a single I/O request is serviced with a single
data stripe (the segment size multiplied by the number of physical disks in the disk group used for data
storage). In this case, multiple disks are used for the same request, but each disk is only accessed once.
Virtual Disk Capacity Expansion
When you configure a virtual disk, you select a capacity based on the amount of data you expect to store.
For example, if a disk group will contain a virtual disk that stores larger multimedia files and another
virtual disk that stores smaller text files, the multimedia file virtual disk will obviously require more
capacity.
However, you might need to eventually increase the virtual disk capacity for a standard virtual disk by
adding free capacity to the disk group. This creates more unused space for you to create new virtual disks,
or to expand your existing virtual disks.
Disk Group Expansion
Because the storage array supports hot pluggable physical disks, you can add two physical disks at a time
for each disk group while the storage array remains online. Data remains accessible on virtual disk groups,
virtual disks, and physical disks throughout the entire modification operation. The data and increased
unused free space are dynamically redistributed across the disk group. RAID characteristics are also
reapplied to the disk group as a whole.
Disk Group Defragmentation
Defragmenting consolidates the free capacity in the disk group into one contiguous area.
Defragmentation does not change the way in which the data is stored on the virtual disks.