5
5-18
EMC Fibre Channel Storage System Model FC4700 Configuration Planning Guide
Planning File Systems and LUNs
For example,
•A five-disk RAID 5 or RAID 3 Group of 18-Gbyte disks holds
72 Gbytes;
•An eight-disk RAID 1/0 Group of 18-Gbyte disks also holds
72Gbytes;
•A RAID 1 mirrored pair of 18-Gbyte disks holds 18 Gbytes;
and
•An individual disk of an 18-Gbyte disk also holds 18 Gbytes.
Each disk in the RAID Group must have the same capacity;
otherwise, you will waste disk storage space.
LUN size. Enter the user-available capacity in gigabytes (Gbytes)
of the LUN. You can make this the same size as the RAID Group,
described previously. Or, for a RAID 5, RAID 1, RAID 1/0, or
RAID 0 Group, you can make the LUN smaller than the RAID
Group. You might do this if you wanted a RAID 5 Group with a
large capacity and wanted to place many smaller capacity LUNs
on it; for example, to specify a LUN for each user. However,
having multiple LUNs per RAID Group may adversely impact
performance. If you want multiple LUNs per RAID Group, then
use a RAID Group/LUN series of entries for each LUN.
Disk IDs. Enter the IDs of all disks that will make up the LUN or
hot spare. These are the same disk IDs you specified on the
previous worksheet. For example, for a RAID 5 Group in the DPE
(enclosure 0, disks 2 through 6), enter 003, 004, 005, 006, and 007.
RAID type. Copy the RAID type from the previous worksheet.
For example, RAID 5 or hot spare. For a hot spare (not strictly
speaking a LUN at all), skip the rest of this LUN entry and
continue to the next LUN entry (if any).
If this is a RAID 3 Group, specify the amount of SP memory for
that group. To work efficiently, each RAID 3 Group needs at least
6 Mbytes of memory.
Caching. If you want to use caching (entry on page 5-16), you can
specify whether you want caching — read and write, read, or
write for this LUN. Generally, write caching improves
performance far more than read caching. The ability to specify
caching on a LUN basis provides additional flexibility, since you
can use caching for only the units that will benefit from it. Read
and write caching recommendations follow.