1. What is RAID?
Redundant
Array of Independent Drives (or Disks), also known as Redundant Array of
Inexpensive Drives (or Disks) (RAID) is an important term for data storage
schemes that divide and/or replicate data among multiple hard drives. They
offer, depending on the scheme, increased data reliability and/or throughput.
RAID
is a way of storing the same data in different drives(thus, redundantly) on
multiple hard disks.
2. What are the advantages
of RAID?
Increased
redundancy
Increased
data availability
Higher
READ/Write performance in some RAID levels
Higher
Data throughput
better
reliability
*
Higher Data Security: Through the use of redundancy, most RAID levels
provide protection for the data stored on the array. This means that the data
on the array can withstand even the complete failure of one hard disk (or
sometimes more) without any data loss, and without requiring any data to be
restored from backup. This security
feature is a key benefit of RAID and probably the aspect that drives the
creation of more RAID arrays than any other. All RAID levels provide some
degree of data protection, depending on the exact implementation, except RAID
level 0.
* Fault Tolerance: RAID implementations
that include redundancy provide a much more reliable overall storage subsystem
than can be achieved by a single disk. This means there is a lower chance of
the storage subsystem as a whole failing due to hardware failures. (At the same
time though, the added hardware used in RAID means the chances of having a
hardware problem of some sort with an individual component, even if it doesn't
take down the storage subsystem, is increased; see this full discussion of RAID
reliability for more.)
* Improved Availability: Availability
refers to access to data. Good RAID systems improve availability both by
providing fault tolerance and by providing special features that allow for
recovery from hardware faults without disruption. See the discussion of RAID
reliability and also this discussion of advanced RAID features.
* Increased, Integrated Capacity: By
turning a number of smaller drives into a larger array, you add their capacity
together (though a percentage of total capacity is lost to overhead or
redundancy in most implementations). This facilitates applications that require
large amounts of contiguous disk space, and also makes disk space management
simpler. Let's suppose you need 300 GB of space for a large database.
Unfortunately, no hard disk manufacturer makes a drive nearly that large. You
could put five 72 GB drives into the system, but then you'd have to find some
way to split the database into five pieces, and you'd be stuck with trying to
remember what was were. Instead, you could set up a RAID 0 array containing
those five 72 GB hard disks; this will appear to the operating system as a
single, 360 GB hard disk! All RAID implementations provide this
"combining" benefit, though the ones that include redundancy of
course "waste" some of the space on that redundant information.
* Improved Performance: Last, but certainly
not least, RAID systems improve performance by allowing the controller to
exploit the capabilities of multiple hard disks to get around
performance-limiting mechanical issues that plague individual hard disks.
Different RAID implementations improve performance in different ways and to
different degrees, but all improve it in some way. See this full discussion of
RAID performance issues for more.
3. What are different
levels of RAID?
There
are many levels like
RAID
0,RAID 1,RAID 2,RAID 3,RAID 4,RAID 5,RAID 10,RAID 01,RAID 50,RAID 6
But
popular are RAID 0,RAID 1,RAID 5,RAID 10,RAID 01,RAID 50,RAID 6
generally
used are R0,R1,R5
4. Explain RAID0, RAID1,
RAID5 ?
RAID
0:
The
lowest designated level of RAID, level 0, is actually not a valid type of RAID.
It was given the designation of level 0 because it fails to provide any level
of redundancy for the data stored in the array. Thus, if one of the drives
fails, all the data is damaged.
RAID
0 uses a method called striping. Striping takes a single chunk of data like a
graphic image, and spreads that data across multiple drives. The advantage that
striping has is in improved performance. Twice the amount of data can be
written in a given time frame to the two drives compared to that same data
being written to a single drive.
RAID
1
RAID
version 1 was the first real implementation of RAID. It provides a simple form
of redundancy for data through a process called mirroring. This form typically
requires two individual drives of similar capacity. One drive is the active
drive and the secondary drive is the mirror. When data is written to the active
drive, the same data is written to the mirror drive.
RAID
5
This
is the most powerful form of RAID that can be found in a desktop computer system.
Typically it requires the form of a hardware controller card to manage the
array, but some desktop operating systems can create these via software. This
method uses a form of striping with parity to maintain data redundancy. A
minimum of three drives is required to build a RAID 5 array and they should be
identical drives for the best performance.
5. Whats the difference
between RAID0 & RAID1 ?
RAID
0+1
This
is a hybrid form of RAID that some manufacturers have implemented to try and
give the advantages of each of the two versions combined. Typically this can
only be done on a system with a minimum of 4 hard drives. It then combines the
methods of mirroring and striping to provide the performance and redundancy.
The first set of drives will be active and have the data striped across them
while the second set of drives will be a mirror of the data on the first two.
RAID
10 or 1+0
RAID
10 is effectively a similar version to RAID 0+1. Rather than striping data
between the disk sets and then mirroring them, the first two drives in the set
are a mirrored together. The second two drives form another set of disks that
is are mirror of one another but store striped data with the first pair. This
is a form of nested RAID setup. Drives 1 and 2 are a RAID 1 mirror and drives 3
and 4 are also a mirror. These two sets are then setup as stripped array.
6.
Whats the difference between RAID1 & RAID5 ?
RAID1
: Minimum 2 drives are required . Gives only 50% disk space.
RAID5
: Minimum 3 drives are required . Gives only (n-1)X Capacity where n is the no.
of disks, disk space.
7. Whats the difference
between RAID3 & RAID5 ?
RAID
3 and RAID 4: Striped Set (3 disk minimum) with Dedicated Parity, the parity
bits represent a memory location each, they have a value of 0 or 1, whether the
given memory location is empty or full, thus enhancing the speed of read and
write. : Provides improved performance and fault tolerance similar to RAID 5,
but with a dedicated parity disk rather than rotated parity stripes. The single
disk is a bottle-neck for writing since every write requires updating the
parity data. One minor benefit is the dedicated parity disk allows the parity
drive to fail and operation will continue without parity or performance
penalty.
RAID
5 does not have a dedicated parity drive but the parity is rotated across all
the drives hence the parity is distributed.
RAID
5: Striped Set (3 disk minimum) with Distributed Parity: Distributed parity
requires all but one drive to be present to operate; drive failure requires
replacement, but the array is not destroyed by a single drive failure. Upon
drive failure, any subsequent reads can be calculated from the distributed
parity such that the drive failure is masked from the end user. The array will
have data loss in the event of a second drive failure and is vulnerable until
the data that was on the failed drive is rebuilt onto a replacement drive.
8. Whats the difference
between RAID01 & RAID10 ?
RAID
0+1: Striped Set + Mirrored Set (4 disk minimum; Even number of disks) provides
fault tolerance and improved performance but increases complexity. Array
continues to operate with one failed drive. The key difference from RAID 1+0 is
that RAID 0+1 creates a second striped set to mirror a primary striped set, and
as a result can only sustain a maximum of a single disk loss, whereas 1+0 can
sustain multiple drive losses as long as no two drive loss comprise a single
pair.
RAID
1+0: Mirrored Set + Striped Set (4 disk minimum; Even number of disks) provides
fault tolerance and improved performance but increases complexity. Array
continues to operate with one or more failed drives. The key difference from
RAID 0+1 is that RAID 1+0 creates a striped set from a series of mirrored
drives.
9. How many minimum disk
drives are needed for R0,R1,R5,R10,R01 ?
R0:
Minimum 1
R1:
Minimum 2
R5:
Minimum 3
R10:
Minimum 4
R01:
Minimum 4
10.How
RAID 5 works and how parity is calculated ?
The
parity calculation is typically performed using a logical operation called
"exclusive OR" or "XOR". As you may know, the
"OR" logical operator is "true" (1) if either of its
operands is true, and false (0) if neither is true. The exclusive OR operator
is "true" if and only if one of its operands is true; it differs from
"OR" in that if both operands are true, "XOR" is false.
11.Other than RAID feature
what are the other features in Software Management Functionalities?
Hotspare
Raid
level migration (RLM)
SNMP
interaction/management
12.What is initialization ?
Intialization
is the process of preparing a drive for storage use. It erases all data on the
drive & makes way for new file system creation.
13.What is Check
consistency ?
Consistency
check or CC verifies correctness of data in logical drives. This is a feature
of some of the RAID hardware controller cards.
14.What is background
initialization?
This
is a Consistency check process forced when a new logical drive is created. This
is an automatic operation that starts 5 minutes after the new logical drive is
created.
15.What is a RAID array ?
RAID
array is a group of disks which are configured with RAID. That means they are
in a redundant setup to tolerate any disk failures.
16.Whats the difference
between a JBOD & a RAID array ?
Just
A Bunch Of Disks (JBOD) - hard disks that aren't configured in a RAID configuration.
They are just disks piled or connected in one single enclosure.
RAID
is having the advantage of bearing a disk failure & still give data
availability.
17.When JBOD is preferred
over RAID array ?
When
there is no need for redundancy & when it is ok if there is some hard disk
failure or data unavilability in such scenarios JBOD is prefered over RAID
because JBOD is inexpensive storage solution. It is also easy to setup &
start using compared to RAID.
18.What is a hot spare ?
Hot
spare is an extra,unused disk drive that is part of the disk subsystem. It is
usually in standby mode ready for service if a drive fails. Whenever there is a
drive failure this hotspare kicksin & takes over that failed drive's role.
19.What is a Logical drive
or Virtual drive ?
The
partitioning or division of a large hard drive into smaller units. A single,
large Physical Drive can be partitioned into two or more smaller Logical
Drives.
20.What is rebuilding of
array ?
Whenever
there is a disk failure in the RAID array the array goes to DOWNGRADED STATE.
SO when we plug out the failed drive & insert a new functioning drive the
RAID configured array starts regenerating the data to the newer drive. This
process is called rebuilding.
21.What you do when a drive
in an array fails, how you bring it back to optimal online mode ?
We
swap out failed drive & plugin new functioning drive & wait for the
rebuilding process to complete. We make sure rebuild process happens without
any error. Once that completes array is back to optimal online state.
22.What are the different
states an array can be in and explain each state?
Online
Downgraded
Offline
Rebuilding
23.Explain
Online,Offline,Degraded states of an array ?
Online
- when all drives are working fine
Downgraded
- Whenever there is a drive failure but still the array is functioning fine
Offline
- Array or whole data storage is down
Rebuilding
- Storage access is there but since a new drive has been inserted in place of a
failed drive data is being written to new drive which might slow down the
performance of the whole RAID array.
24.What is the difference
between a global hotspare & a dedicated hotspare ?
Global
hotpsare is available for the any array in
the whole enclosure or Storage subsystem.
If
there is an enclosure having 10 drives & we have 3 drives in RAID5(1st
array) , 3 more drives in second RAID5(2nd array) & 2 more drives in RAID 1
config.We can specify in RAID config utility whether a Dedicated hotspare is
assigned for 1st RAID5 array. If there is a drive failure in 2nd or 3rd array
this dedicated hotspare will not be involved there. But if the array for which
this is dedicated has any drive failure this dedicated hotspare takes over .
25.How RAID is configured
through BIOS ?
If
we have a Hardware RAID controller card it gives an option while machine
booting to enter into RAID BIOS utility. Here we have options which give us
options to create RAID using a semi-GUI(DOS based GUI) interface.
26.HoW RAID is configured
in OS level?
Once
we install device drivers & also RAID config or management utility using
that we can configure RAID in OS level.
27.What is the difference
between a software RAID & hardware RAID ?
In
order for RAID to function, there needs to be software either through the
operating system or via dedicated hardware to properly handle the flow of data
from the computer system to the drive array. This is particularly important
when it comes to RAID 5 due to the large amount of computing required to
generate the parity calculations.
In
the case of software implementations, CPU cycles are taken away from the
general computing environment to perform the necessary tasks for the RAID
interface. Software implementations are very low cost monetarily because all
that is necessary to implement one is the hard drives. The problem with
software RAID implementations is the performance drop of the system. In
general, this performance hit can be anywhere from 5% or even greater depending
upon the processor, memory, drives used and the level of RAID implemented. Most
people do not use software RAID anymore due to the decreasing costs of hardware
RAID controllers over the years.
Hardware
RAID has the advantage of dedicated circuitry to handle all the RAID drive
array calculations outside of the processor. This provides excellent
performance for the storage array. The drawbacks to hardware RAID have been the
costs. In the case of RAID 0/1 controllers, those costs have become so low that
many chipset and motherboard manufacturers are including these capabilities on
the motherboards. The real costs rest with RAID 5 hardware that require more
circuitry for added computing ability.
28.Which is best RAID level
for performance and which is best for redundancy?
RAID
0 for performance
RAID
5 or RAID 6 better for redundancy(availibility)
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