Technical information

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Which is the Right RAID Level?
The answer, of course, is it depends on the application. RAID-3 offers high transfer rates and is ideal for
large blocks of data where speed is of importance. Computer Aided Design, Graphics, Scientific Computing,
Image and Multimedia applications are all good examples.
RAID-5 arrays offer high I/O transaction rates for small transfers and is ideal for on-line transaction
processing (OLTP) applications, such as for banks, insurance companies, hospitals, and some office
environments. These systems typically perform large numbers of concurrent requests, each of which makes
a small number of disk accesses. A minimum of 3 disks is required for RAID-5 and up to 1 disk in 8 will be
redundant. e.g. 18GB useable disk space requires 3 x 9GB disks, 63GB useable disk space requires 8 x
9GB disk drives.
It is important to understand that RAID is a performance / resilience trade off. Few users seem to understand
this and it is common place to find systems where the entire database workload has been implemented on a
single RAID 5 filesystem and then wonder why the system is slow. On RAID 5, write performance can be 2 to
3 times slower than RAID 0 or 1. On a well tuned database system, the performance of the system is often
governed by the speed of the database logs which consist primarily of sequential writes. Hence, it is not
generally a good idea to put a complete database system on a single RAID 5 filesystem.
Benefits and Limitations of most popular RAID Levels
RAID
Level
Benefits Limitations
0
Maximum data capacity. High read and write
performance.
No fault tolerance or redundancy.
1
100% data redundancy. Usable capacity reduced by 50%. Requires 2
disk drives.
3
Optimised for applications which transfer large
files sequentially. Low cost fault tolerance.
Not suitable for smaller random data access.
Requires a minimum of 3 drives.
5
Advantages of striping with low cost fault
tolerance. Balanced performance and cost for
smaller random data access.
Slower write performance than RAID 0 or 1.
Requires a minimum of 3 drives.
RAID Controllers for
team
server
Two PCI RAID controllers are offered on Fujitsu
team
server
systems.
The Mylex AcceleRAID 250 RAID Adapter with 8MB cache (max 128MB) (SV521033) is available on
C800
i
and supports RAID levels 0, 1, 3, 5, 10, 30, 50, 0+1 and JBOD. The Mylex eXtremeRAID 1100, 2 channel
controller with 32MB SDRAM cache (max 64MB), with battery (SV521036) is available on
C800
i
(not for
internal disks)
, G800
i
, L800
i
and T800
i
and supports RAID levels 0, 1, 3, 5, 10, 30, 50, 0+1 and JBOD.
Software RAID
Software-based RAID implementations are either operating system-based, or they are application programs
that run on the server. Array operations and management functions are controlled by the array software
running on the host CPU.
UnixWare 7 provides RAID levels 0, 1, 5 & 10 through an additional software package called Online Data
Manager. NT Server provides RAID levels 0, 1 & 5 (RAID 5 is referred to as striped set with parity) as part of
the operating system. IntranetWare provides RAID levels 0 & 1 as part of the operating system.
Just like any other application, software-based arrays occupy host system memory, consume CPU cycles and
are operating system dependent. By contending with other applications that are running concurrently for host
CPU cycles and memory, software-based arrays degrade overall server performance. Also, unlike hardware-
based arrays, the performance of a software-based array is directly dependent on server CPU performance
and load.