Disk Redundancies
Any system has four primary resources: processor, memory, disk, and the network interface. Of
these, the disk is the slowest and most susceptible to failure. Because of this, administrators often
upgrade disk subsystems to improve their performance and redundancy.
Redundant array of inexpensive disks (RAID) subsystems provide fault tolerance for disks and
increase the system availability. Even if a disk fails, most RAID subsystems can tolerate the failure
and the system will continue to operate. RAID systems are becoming much more affordable as the
price of drives steadily falls and disk capacity steadily increases.
A redundant array of inexpensive disks (RAID) system would provide fault tolerance for disk drives and increase data availability if drives fail.
A cluster provides fault tolerance at the server level and ensures a service continues to operate even if a server fails. However, a cluster is more expensive than a RAID.
RAID-0
RAID-0 (striping) is somewhat of a misnomer because it doesn’t provide any redundancy or
fault tolerance. It includes two or more physical disks. Files stored on a RAID-0 array are spread
across each of the disks.
The benefit of a RAID-0 is increased read and write performance. Because a file is spread
across multiple physical disks, the different parts of the file can be read from or written to each of the
disks at the same time. If you have three 500 GB drives used in a RAID-0, you have 1500 GB (1.5
TB) of storage space.
RAID-1
RAID-1 (mirroring) uses two disks. Data written to one disk is also written to the other disk. If
one of the disks fails, the other disk still has all the data, so the system can continue to operate without any data loss. With this in mind, if you mirror all the drives in a system, you can actually lose half of the drives and continue to operate.
You can add an additional disk controller to a RAID-1 configuration to remove the disk
controller as a single point of failure. In other words, each of the disks also has its own disk
controller. Adding a second disk controller to a mirror is called disk duplexing.
If you have two 500 GB drives used in a RAID-1, you have 500 GB of storage space. The other
500 GB of storage space is dedicated to the fault-tolerant, mirrored volume.
RAID-2, RAID 3, and RAID-4 are rarely used.
RAID-5 and RAID-6
A RAID-5 is three or more disks that are striped together similar to RAID-0. However, the
equivalent of one drive includes parity information. This parity information is striped across each of
the drives in a RAID-5 and is used for fault tolerance. If one of the drives fails, the system can read
the information on the remaining drives and determine what the actual data should be. If two of the
drives fail in a RAID-5, the data is lost.
RAID-6 is an extension of RAID-5, and it includes an additional parity block. A huge benefit is
that the RAID-6 disk subsystem will continue to operate even if two disk drives fail. RAID-6
requires a minimum of four disks.
Remember this
RAID subsystems, such as RAID-1, RAID-5, and RAID-6, provide fault
tolerance and increased data availability. RAID-5 can survive the failure of
one disk. RAID-6 can survive the failure of two disks.
RAID-10
A RAID-10 configuration combines the features of mirroring (RAID-1) and striping (RAID-0).
RAID-10 is sometimes called RAID 1+0. A variation is RAID-01 or RAID 0+1 that also combines
the features of mirroring and striping but implements the drives a little differently.
Software Versus Hardware RAID
Hardware RAID configurations are significantly better than software RAID. In hardware RAID,
dedicated hardware manages the disks in the RAID, removing the load from the operating system. In
contrast, the operating system manages the disks in the RAID array in software RAID. Hardware
RAID systems provide better overall performance and often include extra features.
For example, a hardware RAID may include six physical disks using four in an active RAID-6
configuration and two as online spares. If one of the active disks in the RAID-6 fails, the RAID will
continue to operate because a RAID-6 can tolerate the failure.
However, a hardware RAID can logically take the failed disk out of the configuration, add one
of the online spares into the configuration, and rebuild the array. All of this happens without any
administrator intervention. Hardware RAID systems are often hot swappable, allowing
administrators to swap out the failed drive without powering the system down.
Source: Darril Gibson
Any system has four primary resources: processor, memory, disk, and the network interface. Of
these, the disk is the slowest and most susceptible to failure. Because of this, administrators often
upgrade disk subsystems to improve their performance and redundancy.
Redundant array of inexpensive disks (RAID) subsystems provide fault tolerance for disks and
increase the system availability. Even if a disk fails, most RAID subsystems can tolerate the failure
and the system will continue to operate. RAID systems are becoming much more affordable as the
price of drives steadily falls and disk capacity steadily increases.
A redundant array of inexpensive disks (RAID) system would provide fault tolerance for disk drives and increase data availability if drives fail.
A cluster provides fault tolerance at the server level and ensures a service continues to operate even if a server fails. However, a cluster is more expensive than a RAID.
RAID-0
RAID-0 (striping) is somewhat of a misnomer because it doesn’t provide any redundancy or
fault tolerance. It includes two or more physical disks. Files stored on a RAID-0 array are spread
across each of the disks.
The benefit of a RAID-0 is increased read and write performance. Because a file is spread
across multiple physical disks, the different parts of the file can be read from or written to each of the
disks at the same time. If you have three 500 GB drives used in a RAID-0, you have 1500 GB (1.5
TB) of storage space.
RAID-1
RAID-1 (mirroring) uses two disks. Data written to one disk is also written to the other disk. If
one of the disks fails, the other disk still has all the data, so the system can continue to operate without any data loss. With this in mind, if you mirror all the drives in a system, you can actually lose half of the drives and continue to operate.
You can add an additional disk controller to a RAID-1 configuration to remove the disk
controller as a single point of failure. In other words, each of the disks also has its own disk
controller. Adding a second disk controller to a mirror is called disk duplexing.
If you have two 500 GB drives used in a RAID-1, you have 500 GB of storage space. The other
500 GB of storage space is dedicated to the fault-tolerant, mirrored volume.
RAID-2, RAID 3, and RAID-4 are rarely used.
RAID-5 and RAID-6
A RAID-5 is three or more disks that are striped together similar to RAID-0. However, the
equivalent of one drive includes parity information. This parity information is striped across each of
the drives in a RAID-5 and is used for fault tolerance. If one of the drives fails, the system can read
the information on the remaining drives and determine what the actual data should be. If two of the
drives fail in a RAID-5, the data is lost.
RAID-6 is an extension of RAID-5, and it includes an additional parity block. A huge benefit is
that the RAID-6 disk subsystem will continue to operate even if two disk drives fail. RAID-6
requires a minimum of four disks.
Remember this
RAID subsystems, such as RAID-1, RAID-5, and RAID-6, provide fault
tolerance and increased data availability. RAID-5 can survive the failure of
one disk. RAID-6 can survive the failure of two disks.
RAID-10
A RAID-10 configuration combines the features of mirroring (RAID-1) and striping (RAID-0).
RAID-10 is sometimes called RAID 1+0. A variation is RAID-01 or RAID 0+1 that also combines
the features of mirroring and striping but implements the drives a little differently.
Software Versus Hardware RAID
Hardware RAID configurations are significantly better than software RAID. In hardware RAID,
dedicated hardware manages the disks in the RAID, removing the load from the operating system. In
contrast, the operating system manages the disks in the RAID array in software RAID. Hardware
RAID systems provide better overall performance and often include extra features.
For example, a hardware RAID may include six physical disks using four in an active RAID-6
configuration and two as online spares. If one of the active disks in the RAID-6 fails, the RAID will
continue to operate because a RAID-6 can tolerate the failure.
However, a hardware RAID can logically take the failed disk out of the configuration, add one
of the online spares into the configuration, and rebuild the array. All of this happens without any
administrator intervention. Hardware RAID systems are often hot swappable, allowing
administrators to swap out the failed drive without powering the system down.
Source: Darril Gibson
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