Dell POWERVAULT MD3620I - Manual (Page 46)

46 Planning: MD3600i Series Storage Array Terms and Concepts
Segment Size Migration
Segment size refers to the amount of data (in KB) 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, 256 KB, and 512 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.
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