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EMC Fibre Channel Storage-System Configuration Planning
RAID Types and Tradeoffs
We do not recommend using RAID 3 in the same storage-system
chassis with RAID 5 or RAID 1/0.
A RAID 1 mirrored pair has its disks locked in synchronization, but
the SP can read data from the disk whose read/write heads are closer
to it. Therefore, RAID 1 read performance can be twice that of an
individual disk while write performance remains the same as that of
an individual disk.
A RAID 0 Group (nonredundant individual access array) or RAID
1/0 Group (mirrored RAID 0 Group) can have as many I/O
operations occurring simultaneously as there are disks in the group.
Since RAID 1/0 locks pairs of RAID 0 disks the same way as RAID 1
does, the performance of RAID 1/0 equals the number of disk pairs
times the RAID 1 performance number. If you want high throughput
for a specific LUN, use a RAID 1/0 or RAID 0 Group. A RAID 1/0
Group requires at least six disks; a RAID 0 Group, at least three disks.
An individual unit needs only one I/O operation per read or write
operation.
RAID types 5, 1, 1/0, and 0 allow multiple LUNs per RAID Group. If
you create multiple LUNs on a RAID Group, the LUNs share the
RAID Group disks, and the I/O demands of each LUN affect the I/O
service time to the other LUNs. For best performance, you may want
to use one LUN per RAID Group.
Storage Flexibility
Certain RAID Group types — RAID 5, RAID 1, RAID 1/0, and RAID
0 — let you create up to 32 LUNs in each group. This adds flexibility,
particularly with large disks, since it lets you apportion LUNs of
various sizes to different servers, applications, and users. Conversely,
with RAID 3, there can be only one LUN per RAID Group, and the
group must include five or nine disks — a sizable block of storage to
devote to one server, application, or user. However, the nature of
RAID 3 makes it ideal for that single-threaded type of application.
Data Availability and Disk Space Usage
If data availability is critical and you cannot afford to wait hours to
replace a disk, rebind it, make it accessible to the operating system,
and load its information from backup, then use a redundant RAID
Group: RAID 5, RAID 3, RAID 1 mirrored pair, or RAID 1/0. Or bind
a RAID 0 Group or individual disk unit that you will later mirror
with software mirroring. If data availability is not critical, or disk