Dell POWERVAULT MD3600I - Manual (Page 25)

Segment Size Migration
Segment size refers to the amount of data (in kilobytes) that the storage array writes on a physical disk in a virtual disk
before writing data on the next physical disk. Valid values for the segment size are 8 KB, 16 KB, 32 KB, 64 KB, 128 KB, and
256 KB.
Dynamic segment size migration enables the segment size of a given virtual disk to be changed. A default segment size
is set when the virtual disk is created, based on such factors as the RAID level and expected usage. You can change the
default value if segment size 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 for
a single I/O. Using a single physical disk for a single request frees disks to service other requests, especially
when you have multiple users accessing a database or storage environment.
• If you use 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. However, you
may need to increase the virtual disk capacity for a standard virtual disk by adding free capacity to the disk group. This
creates more unused space for new virtual disks or to expand existing virtual disks.
Disk Group Expansion
Because the storage array supports hot-swappable 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 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.
Disk Group Operations Limit
The maximum number of active, concurrent disk group processes per installed RAID controller module is one. This limit
is applied to the following disk group processes:
• Virtual disk RAID level migration
• Segment size migration
• Virtual disk capacity expansion
• Disk group expansion
• Disk group defragmentation
If a redundant RAID controller module fails with an existing disk group process, the process on the failed controller is
transferred to the peer controller. A transferred process is placed in a suspended state if there is an active disk group
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