iATA

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iATA

• Kulveer Singh
• Kulveer.singh_at_dit.dongseo.ac.kr

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ATA Concepts

• At the highest level, ATA is a family of
  interfaces for requesting services from I/O
  devices, including hard drives, tape drives, CD
  and DVD drives, printers, and scanners.
• In ATA parlance, an individual I/O device is
  called a "logical unit" (LU).
• ATAs main advantage is using a logical
  addressing of data-blocks instead of the physical
  addressing of cylinders, head and sector.
• This allow storage apps to be developed faster
  and to be reused when a new type of disk entered
  the market.

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ATA Concepts(cont.)

• ATA is a client-server architecture.
• Clients of a ATA interface are called "clients".
• clients issue ATA "commands" to request service
  from a logical unit( LU ).
• The "device server" on the logical unit accepts
  ATA commands and executes them.

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ATA Concepts(cont.)

• A ATA transport" maps the client-server ATA
  protocol to a specific interconnect.
• clients are one endpoint of a ATA transport. The
  "server" is the other endpoint.
• A server can have multiple Logical Units behind
  it.
• Each Logical Unit has an address within a server
  called a Logical Unit Number( LUN )

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ATA Concepts(cont.)

• Command Descriptor Blocks (CDB) is the data
  structure used to contain the command parameters
  that are to be handed by an client to a server.
• The CDB content and structure is defined by SAM
  and device-type specific ATA standards.

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Parallel ATA

• Traditional ATA coupled the protocol with the
  transport layer i.e. ATA commands were sent to
  ATA devices via the ATA-bus.
• a few years later, storage controllers in the
  Open-Systems arena have supported the ATA
  protocol, and have been attached to computers
  through the ATA parallel bus.
• The ATA parallel bus gave a very high bandwidth
  for bursting writes for a number of ATA devices
  connected through the bus.
• But, as things advanced in the storage world the
  Parallel ATA becomes a limiting factor.

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Parallel ATA Limitation

• Parallel ATA has very restrictive distance
  capabilities( a few meters only ).
• Parallel ATA also limits the number of the
  connected devices( 8-16 )
• And like any other parallel bus interconnect the
  ATA parallel bus, has bandwidth limitations when
  more than one device try to get hold of the bus.

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Parallel ATA Limitation

• While all those limitations might seem acceptable
  when considering a work-station or a small
  server, they are very restricting limitations
  when considering an enterprise Storage Network
  where a few hundreds of storage devices are
  shared between many computers.

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Fibre Channel

• The protocol was developed at the late 80 to be
  a very flexible, high-speed, scalable networking
  technology.
• Many protocols were remapped to FC including IP,
  ATM, ATA, Escon(Ficon)
• But, only the ATA mapping was widely adopted
• In the mid 90 it entered the storage world as a
  serial transport layer for ATA (fcp).

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FC Problems

• Price is too high( 1000 for a FC nic )
• Limited distance ( 10-50 KM )
• Interoperability Products of different FC
  vendors dont play together.
• Isolated from IP network, a special(and
  expensive) switch is needed to connect
• Another network to maintain( assuming the
  organization already have an IP network )
• Reinvents the wheel - a totally different set of
  protocols for communication, addressing,
  name-servers, managements

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iATA
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iATA IP Network

• The protocol will use the existing ATA
  architecture and commands, and the existing
  TCP/IP transport layer.
• Both these protocols are widely-deployed and
  well-understood. The thought is that using these
  mature protocols will entail a minimum of new
  invention, the most rapid possible adoption, and
  the greatest compatibility with Internet
  architecture, protocols, and equipment.
• The iATA protocol MUST NOT require modifications
  to the current IP and Ethernet infrastructure to
  support storage traffic over TCP.
• Nevertheless, the performance and security
  requirements ( not to mention the budget) of
  storage creates opportunities for improvement in
  security protocols and QoS implementations.

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iATA and ATA

• The iATA protocol MUST NOT require changes to the
  ATA-3 command sets and ATA client code except to
  reflect lengthier iATA server names and
  potentially lengthier timeouts.
• All ATA devices types SHOULD be supported, but
  iATA main interest are disk and tape controllers
  
• The iATA protocol MUST reliably transport ATA
  commands from the client to the server.

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ATA Envirnoment
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iATA Layers
ATA
iATA
Upper Functional Layers (e.g. SSL)
TCP
Lower Functional Layers (e.g. IPSec)
IP
LINK
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Request Packet
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Reply Packet
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Request/Reply Fields

• Features Parameters for the command
• Sector Count Number of sectors that will be used
  
• LBA Logical Block Address of the starting sector
  that will be used
• Device Device selection and additional bits are
  reserved for future usage.
• Command Command code
• Error Bits indicating different errors
• Status Bits indicating status of the action

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OS Layering
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ATA Commands

• Control Commands
• Identify Device
• Flush cache
• Set features
• Data Commands
• Read sectors
• Write Sectors

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Identify Device

• IDENTIFY-DEVICE This command is used to gather
  information from the device. Features ?eld in the
  request packet is used to specify the type of
  information requested. If the information cannot
  be returned within the Status ?eld of reply
  packet, a string for example, a data stream
  should be returned after the packet and the size
  of the data must be given in the Sector Count
  ?eld of the reply packet.

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Flush Cache

• FLUSH-CACHE This command is used to force the
  device to ?ush its cache. If there is any data
  that needs to be written physically, the device
  should perform the writing of the data. This
  cache is nothing to do with the bu?ering
  mechanism of iATA protocol. The cache is device
  speci?c and thus any device that does not have an
  inner cache must reply to this command by saying
  that it is successfully completed.

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Features

• SET-FEATURES This command is used to modify
  device parameters. Features ?eld in the request
  packet is used to specify the parameter that will
  be modi?ed. Similarly, Sector Count and LBA ?elds
  are used to specify the value of that parameter.

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Read Command

• READ-SECTORS This command is used to read one or
  more sectors from the device. After the reply
  packet, the device must send the data sector by
  sector as a stream. Size of the data stream can
  be calculated from the Sector Count ?eld.

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Write Command

• WRITE-SECTORS This command is used to write one
  or more sectors to the device. After the request
  packet, the host must send the data sector by
  sector as a stream. Size of the data stream can
  be calculated from the Sector Count ?eld.

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• Thanks