RAID
RAID is a technology that is used to
increase the performance and/or reliability of data storage. The abbreviation
stands for Redundant Array of Inexpensive Disks. A RAID system consists
of two or more disks working in parallel. These disks can be hard discs but
there is a trend to also use the technology for solid state drives. There are
different RAID levels, each optimized for a specific situation. These are not
standardized by an industry group or standardisation committee. This explains
why companies sometimes come up with their own unique numbers and
implementations.
The software to perform the
RAID-functionality and control the hard disks can either be located on a
separate controller card (a hardware RAID controller) or it can simply be a
driver. Some versions of Windows, such as Windows Server 2003, as well as Mac
OS X include software RAID functionality. Hardware RAID controllers cost more than
pure software but they also offer better performance.
RAID-systems can be based with an
number of interfaces, including SCSI, IDE, SATA or FC (fibre channel.) There
are systems that use SATA disks internally but that have a FireWire or
SCSI-interface for the host system.
Sometimes disks in a RAID system are
defined as JBOD, which stands for ‘Just a Bunch Of Disks’. This means
that those disks do not use a specific RAID level and acts as stand-alone
disks. This is often done for drives that contain swap files or spooling data.
Below is an overview of the most
popular RAID levels:
RAID
level 0 – Striping
In a RAID 0 system data are split up
in blocks that get written across all the drives in the array. By using
multiple disks (at least 2) at the same time, this offers superior I/O
performance. This performance can be enhanced further by using multiple
controllers, ideally one controller per disk.
Advantages
- RAID 0 offers great performance, both in read and write
operations. There is no overhead caused by parity controls.
- All storage capacity is used, there is no disk
overhead.
- The technology is easy to implement.
Disadvantages
RAID 0 is not fault-tolerant. If one
disk fails, all data in the RAID 0 array are lost. It should not be used on
mission-critical systems.
Ideal
use
RAID 0 is ideal for non-critical
storage of data that have to be read/written at a high speed, such as on a
Photoshop image retouching station.
RAID
level 1 – Mirroring
Data are stored twice by writing
them to both the data disk (or set of data disks) and a mirror disk (or set of
disks) . If a disk fails, the controller uses either the data drive or the
mirror drive for data recovery and continues operation. You need at least 2
disks for a RAID 1 array.
RAID 1 systems are often combined
with RAID 0 to improve performance. Such a system is sometimes referred to by
the combined number: a RAID 10 system.
Advantages
- RAID 1 offers excellent read speed and a write-speed
that is comparable to that of a single disk.
- In case a disk fails, data do not have to be rebuild,
they just have to be copied to the replacement disk.
- RAID 1 is a very simple technology.
Disadvantages
- The main disadvantage is that the effective storage
capacity is only half of the total disk capacity because all data get
written twice.
- Software RAID 1 solutions do not always allow a hot
swap of a failed disk (meaning it cannot be replaced while the server
keeps running). Ideally a hardware controller is used.
Ideal
use
RAID-1 is ideal for mission critical
storage, for instance for accounting systems. It is also suitable for small
servers in which only two disks will be used.
RAID
level 3
On RAID 3 systems, data blocks are
subdivided (striped) and written in parallel on two or more drives. An
additional drive stores parity information. You need at least 3 disks for a
RAID 3 array.
Since parity is used, a RAID 3
stripe set can withstand a single disk failure without losing data or access to
data.
Advantages
- RAID-3 provides high throughput (both read and write)
for large data transfers.
- Disk failures do not significantly slow down
throughput.
Disadvantages
- This technology is fairly complex and too resource
intensive to be done in software.
- Performance is slower for random, small I/O operations.
Ideal
use
RAID 3 is not that common in
prepress.
RAID
level 5
RAID 5 is the most common secure
RAID level. It is similar to RAID-3 except that data are transferred to disks
by independent read and write operations (not in parallel). The data chunks that
are written are also larger. Instead of a dedicated parity disk, parity
information is spread across all the drives. You need at least 3 disks for a
RAID 5 array.
A RAID 5 array can withstand a single disk failure without losing data or
access to data. Although RAID 5 can be achieved in software, a hardware
controller is recommended. Often extra cache memory is used on these
controllers to improve the write performance.
Advantages
Read data transactions are very fast
while write data transaction are somewhat slower (due to the parity that has to
be calculated).
Disadvantages
- Disk failures have an effect on throughput, although
this is still acceptable.
- Like RAID 3, this is complex technology.
Ideal
use
RAID 5 is a good all-round system
that combines efficient storage with excellent security and decent performance.
It is ideal for file and application servers.
RAID
level 10 – Combining RAID 0 & RAID 1
RAID 10 combines the advantages (and
disadvantages) of RAID 0 and RAID 1 in one single system. It provides security
by mirroring all data on a secondary set of disks (disk 3 and 4 in the drawing
below) while using striping across each set of disks to speed up data
transfers.
.
RAID
is no substitute for back-up!
All RAID levels except RAID 0 offer
protection from a single drive failure. A RAID 6 system even survives 2 disks dying
simultaneously. For complete security you do still need to back-up the data
from a RAID system.
- That back-up will come in handy if all drives fail
simultaneously because of a power spike.
- It is a safeguard if the storage system gets stolen.
- Back-ups can be kept off-site at a different location.
This can come in handy if a natural disaster or fire destroys your
workplace.
- The most important reason to back-up multiple
generations of data is user error. If someone accidentally deletes some
important data and this goes unnoticed for several hours, days or weeks, a
good set of back-ups ensure you can still retrieve those files.
