Transcript Single Level RAID - Salisbury University

RAID
REDUNDANT
ARRAY OF
INEXPENSIVE
DISCS
What is RAID?
 RAID is an acronym for Redundant
Array of Independent Drives (or Disks),
also known as Redundant Array of
Inexpensive Drives (or Disks)
 The various types of RAID are data
storage schemes that divide and/or
replicate data among multiple hard
drives
Why use RAID?
Improve Reliability
Improved Performance
Fault Tolerance
Improved Availability
Higher Data Security
Key Terms
 Mirroring - the copying of data to more than one
disk
 Striping - the splitting of data across more than
one disk
 Parity - a redundancy check that ensures that the
data is protected without having to have a full set
of duplicate drives.
 Duplexing - an extension of mirroring that is based
on the same principle as that technique expect it
goes one step further in that it also duplicates the
hardware that controls the two hard drives (or sets
of hard drives).
About Single Level
 There are 6 Levels of RAID
 Each level, in use on their own is
known as Single Level
 When two levels are combined for
usage (ex. 1+0, 0+1) they are
referred to as Multi Level (or
Nested)
RAID 0
 A striped set of at least two disks without
parity
 The data is broken down into blocks and
each block is written to a separate disk drive
 Best performance is achieved when data is
striped across multiple controllers with only
one drive per controller
Advantages of RAID 0
 I/O performance is greatly improved by
spreading the I/O load across many
channels and drives
 No parity calculation overhead is
involved
 Very simple design
 Easy to implement
Disadvantages of RAID 0
 Not a "True" RAID because it is NOT
fault-tolerant
 The failure of just one drive will result
in all data in an array being lost
 Should never be used in mission critical
environments
RAID 1
http://www.acnc.com/04_01_01.html
RAID 1
Advantages
 High data availability and high I/O rate
(small block size).
 Improves read performance - twice the read
transaction rate of single disks, same write
transaction rate as single disks
 100% redundancy of data means no rebuild is
necessary in case of a disk failure, just a copy
to the replacement disk
 Simplest RAID storage subsystem design –
easy to maintain
RAID 1
Disadvantages
 Expensive due to the extra capacity
required to duplicate data. Overhead
cost equals 100%, while usable storage
capacity is 50%.
 May not support hot swap of failed
disk when implemented with
software. Use hardware
implementation.
RAID 1
Uses
 Accounting
 Payroll
 Financial
 Any application requiring very high
availability
RAID 2
 Level 2 is the "black sheep" of the RAID family,
because it is the only RAID level that does not use
one or more of the "standard" techniques of
mirroring, striping and/or parity. RAID-2 is similar
to RAID-4, but stores error correcting code (ECC)
information instead of parity. Since all modern disk
drives incorporate ECC under the covers, this offers
little additional protection. RAID-2 can offer greater
data consistency if power is lost during a write,
however, battery backup and a clean shutdown can
offer the same benefits.
RAID 2
 Level 2 is the only RAID level that is not used
today. It is expensive and often requires
many drives. The controller required was
complex, specialized and expensive. Most of
all, level 2 was rendered obsolete by the use of
ECC within a hard disk; essentially, much of
what RAID 2 provides you now get for free
within each hard disk, with other RAID levels
providing protection above and beyond ECC.
 Due to its cost and complexity, level 2 never
really caught on.
RAID 3 & 4
A STRIPED SET WITH
A MINIMUM OF 3
DISKS AND
DEDICATED PARITY.
RAID 3
 uses byte-level striping with a dedicated parity
disk.
 is very rare in practice - one side-effect is that it
generally cannot service multiple requests
simultaneously
Disadvantages of RAID 3
 Transaction rate equal to that of a
single disk drive at best (if spindles
are synchronized)
 Controller design is fairly complex
 Very difficult and resource
intensive to do as a "software"
RAID
RAID 4
 uses block-level striping with dedicated parity
disk
 similar to RAID 3 except that it stripes at the
block level, rather than the byte level
 a minimum of 3 disks is required for a complete
RAID 4 configuration.
Disadvantages of RAID 4
 Complex controller design
 Worst Write transaction rate and Write
aggregate transfer rate
 Difficult and inefficient data rebuild in
the event of disk failure
 Block Read transfer rate equal to that of
a single disk
RAID 5
 Striped Set
 Minimum of 3 disks
 Spreads data over all drives
 If the drive goes down replacement is
required
Advantages of RAID 5
 Highest Read data transaction rate
 Medium Write data transaction
rate
 Low ratio of ECC (Parity) disks to
data disks means high efficiency
 Good aggregate transfer rate
Disadvantages of RAID 5
 Disk failure has a medium impact
on throughput
 Most complex controller design
 Difficult to rebuild in the event of a
disk failure
 Individual block data transfer rate
same as single disk
RAID 6
 Striped Set
 Minimum of 4 disks
 Very important when it comes to large size drives
and high availability systems
 Array will continue to operate if up to 2 drives fail
 Essentially an extension of RAID level 5 which
allows for additional fault tolerance by using a
second independent distributed parity scheme
(dual parity
Advantages of RAID 6
 Protects against multiple bad block failures while
non-degraded
 Protects against a single bad block failure while
operating in a degraded mode
 Perfect solution for mission critical applications
Disadvantages of RAID 6
 More complex controller design
 Controller overhead to compute parity addresses is
extremely high
 Write performance can be brought on par with RAID
Level 5 by using a custom ASIC for computing ReedSolomon parity
 Requires N+2 drives to implement because of dual
parity scheme