Dell PowerVault MD3820f - Manual (Page 27)

RAID level. You can perform a RAID level migration while the system is still running and without
rebooting, which maintains data availability.
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
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