Operating Systems

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Operating Systems

• RAID Redundant Array of Independent Disks

Thanks to Yoram Dahan
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Motivation -)
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What does RAID stand for?
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The Problem

• The Mean Time Between Failure (MTBF) of the
  array will be equal to the MTBF of an individual
  drive, divided by the number of drives in the
  array. Because of this, the MTBF of an array of
  drives would be too low for many application
  requirements.

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The Solution

• Disk arrays can be made fault-tolerant by
  redundantly storing information in various ways.
• Five types of array architectures, RAID-1 through
  RAID-5, were defined by the Berkeley paper, each
  providing disk fault-tolerance and each offering
  different trade-offs in features and performance.
• In addition to these five redundant array
  architectures, it has become popular to refer to
  a non-redundant array of disk drives as a RAID-0
  array.

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Data Striping

• Fundamental to RAID is "striping", a
  method of concatenating multiple drives into one
  logical storage unit.
• Striping involves partitioning each drive's
  storage space into strips which may be as small
  as one sector (512 bytes) or as large as several
  megabytes.

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Logical to physical data mapping for striping
Physical Disk 0
Physical Disk 1
Physical Disk 2
Physical Disk 3
strip 0
strip 1
strip 2
strip 3
stripe
strip 4
strip 5
strip 6
strip 7
strip 8
strip 9
strip 10
strip11
strip 12
strip 13
strip 14
strip 15
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RAID Idea

• Several improvements in disk-use techniques
  involve the use of multiple disks working
  cooperatively.
• Disk striping uses a group of disks as one
  storage unit.
• RAID schemes improve performance and improve the
  reliability of the storage system by storing
  redundant data.
• Mirroring or shadowing keeps duplicate of each
  disk.
• Block interleaved parity uses much less
  redundancy.

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RAID Common Characteristics

• A set of physical disk drives viewed by the OS as
  a single logical drive.
• Data are distributed across the array of disk
  drives.
• Redundant disk capacity is used to store parity
  information, which guarantees data recoverability
  in case of a disk failure.

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RAID Structure

• RAID provides reliability via redundancy.
• RAID is arranged into six different levels

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RAID Levels
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RAID 0
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RAID 0 (non-redundant)
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Data Mapping for RAID Level 0 Array
Physical Disk 0
Physical Disk 1
Physical Disk 2
Physical Disk 3
strip 0
strip 1
strip 2
strip 3
strip 4
strip 5
strip 6
strip 7
strip 8
strip 9
strip 10
strip11
strip 12
strip 13
strip 14
strip 15
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RAID 1
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RAID 1 (mirrored)
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RAID 2

• RAID-2
• RAID Level 2 uses Hamming error correction codes.
  
• Is intended for use with drives which do not have
  built-in error detection.
• All SCSI drives support built-in error detection,
  so this level is of little use when using SCSI
  drives.

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RAID 2
(Redundancy through Hamming code)
f2(b)
f1(b)
f0(b)
b2
b1
b0
b2
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RAID 3

• RAID-3
• RAID Level 3 stripes data at a byte level across
  several drives, with parity stored on one
  drive.It is otherwise similar to level 4.
• Byte-level striping requires hardware support for
  efficient use.

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RAID 3 (bit-interleaved parity)
P(b)
b2
b1
b0
b2
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RAID 4

• RAID-4
• RAID Level 4 stripes data at a block level across
  several drives, with parity stored on one drive.
• The parity information allows recovery from the
  failure of any single drive.
• The performance of a level 4 array is very good
  for reads (the same as level 0).
• Writes require that parity data be updated each
  time. This slows small random writes, but large
  writes or sequential writes are fairly fast.
• Because only one drive in the array stores
  redundant data, the cost per megabyte of a level
  4 array can be fairly low.

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RAID 4 (block-level parity)
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RAID 5

• RAID-5
• RAID Level 5 is similar to level 4, but
  distributes parity among the drives.
• Can speed small writes in multiprocessing
  systems, since the parity disk does not become a
  bottleneck.
• The performance for reads tends to be
  considerably lower than a level 4 array.
• The cost per megabyte is the same as for level
  4.

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RAID 5 (block-level distributed
parity)
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RAID 6 (dual redundancy)
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RAID (0 1) and (1 0)
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RAID Levels
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Summary (0)
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Summary (1)
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Summary (2)
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Summary (3)
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Summary (4)
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Summary (5)
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Hardware RAID

• The hardware based system manages the RAID
  subsystem independently from the host and
  presents to the host only a single disk per RAID
  array. This way the host doesn't have to be aware
  of the RAID subsystems(s).
• Two solutions
• Controller based hardware solution.
• External hardware solution (SCSI---SCSI RAID).

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RAID System (1)
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RAID System (2)
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The controller based hardware solution

• DPT's SCSI controllers are a good example for a
  controller based RAID solution.
• The intelligent controller manages the RAID
  subsystem independently from the host.
• The advantage over an external SCSI---SCSI RAID
  subsystem is that the controller is able to span
  the RAID subsystem over multiple SCSI channels
  and by this remove the limiting factor external
  RAID solutions have the transfer rate over the
  SCSI bus.

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The external hardware solution

• Solution SCSI---SCSI RAID
• An external RAID box moves all RAID handling
  "intelligence" into a controller that is sitting
  in the external disk subsystem.
• The whole subsystem is connected to the host via
  a normal SCSI controller and appears to the host
  as a single disk.

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Comparison of both solutions

• The external hardware solution has drawbacks
  compared to the controller based solution
• The single SCSI channel used in this solution
  creates a bottleneck.
• 4 SCSI drives can already completely flood a SCSI
  bus, since the average transfer size is around
  4KB and the command transfer overhead - which
  even in Ultra SCSI is still done asynchronously -
  takes most of the bus time.

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Software RAID

• RAID solution that is completely hardware
  independent.

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Examples for Software RAID

• The MD driver in the Linux kernel
• The MD driver in the Linux kernel is an example
  of a RAID solution that is completely hardware
  independent.
• Its application is limited, it only provides RAID
  level 0.

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Examples for Software RAID

• Adaptec RAID controllers
• They have no RAID functionality whatsoever on the
  controller.
• They depend on external drivers to provide all
  external RAID functionality.
• They are basically only multiple single AHA2940
  controllers which have been integrated on one
  card. Linux detects them as AHA2940 and treats
  them accordingly.
• Every OS needs its own special driver for this
  type of RAID solution, this is error prone and
  not very compatible

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Hardware vs. Software RAID

• Software-based arrays occupy host system memory,
  consume CPU cycles and are operating system
  dependent.
• Software-based arrays degrade overall server
  performance
• Unlike hardware-based arrays, the performance of
  a software-based array is directly dependent on
  server CPU performance and load.
• Software-based implementations commonly require a
  separate boot drive, which is NOT included in the
  array.

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Hardware vs. Software RAID

• Hardware arrays also do not occupy any host
  system memory.
• Not operating system dependent.
• Since the host CPU can execute user applications
  while the array adapter's processor
  simultaneously executes the array functions, the
  result is true hardware multi-tasking.
• Hardware arrays are also highly fault tolerant.

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DISK/TREND NEWS

• Enterprise storage is a changing landscape,
  butRAID sales continue up, topping 14 billion
  in 1999.

Source 1999 DISK/TREND Report
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Bibliography
http//www.raid-advisory.com/
http//www.sparcproductdirectory.com/raid2.html
http//www.storagesearch.com/products.html
http//www.uni-mainz.de/neuffer/scsi/what_is_raid
.html
http//www.disktrend.com/newsarry.htm
http//www.raidinc.com/RAID/xanmed_popup.html
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Disk Attachment

• Disks may be attached one of two ways
• Host attached via an I/O port.
• Network attached via a network connection.

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Network-Attached Storage
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Storage-Area Network