storage protocols

7/28/2019 Storage Protocols

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Storage Protocols

ATA, SATA, SCSI, FC, iSCSI


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ATA

Advanced Technology Interface

Designed for data transfer b/w motherboard andMass storage device

Up to 2 devices per bus

ATA is used in Storage

subsystem, not in SAN

Also referred as PATA (Parallel ATA)


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SATA

Serial ATA

Higher speed

Hot swapping

Thinner cables (7 lines instead of 40) Used ATA and ATAPI(AT Attachment Packet

Interface) command-set as legacy ATA devices

SATA is used in Storage Subsystem, not in SAN

More reliable


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SATA cable and connectors


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SCSI

Small Computer System Interface.

Defines commands, protocols, electrical andoptical interface.

Used to interface HDDs, Tape Drives, CDDrives, scanners, etc.

Current standard is SCSI-3.

Upto16 devices per bus.


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SCSI-3 Architecture Model (SAM)


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SAM Layers

SCSI Interconnect Layer-Electrical signaling methods, transfer modes etc.- Also called physical layer

SCSI Transport Protocol Layer- Rules for communication- Allows different devices to work together

SCSI Command Layer- Common commands for all devices- Devices specific SCSI commands

- SCSI commands can be transported over different media usingdifferent protocols. (FCP protocol over Fiber channel, iSCSI overEthernet, etc)

www.t10.org


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Some SCSI terms

SCSI Devices

- A device connected to a SCSI interface

- Ex. SCSI Hard disks, SCSI Tape Drives

SCSI Initiator/Client

- Is a device which sends out a SCSI command.

- Ex. SCSI Controller

SCSI Target

- Responds to SCSI commands

- Ex. SCSI Hard disk


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SCSI terms (Cont.)

Logical Unit (LU)

- Subset of the target device

- All SCSI commands are intended for the LU

- The Logical Units are numbered and are known as LUN(Logic Unit Number)

- What iss the size of LUN?

- LUN0 is mandatory and should support Report LUNs

command.- Context sensitive (So two computers that access the same

disk volume may know it by different LUNs)

- SCSI Device ID, universally unique.


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SCSI Operation

SCSI Operates in client server mode.

It allows devices distributed on a network.(SCSIDistributed Service Model)


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SCSI Commands

SCSI commands are vendor independent.

Independent of underlying transport protocols/interconnect.(FC,iSCSI, SAS)

Commands contains CDB (Command Descriptor Block),defines the operations to be performed.

Example SCSI Commands

- Inquiry command (Basic Information, eg. Device type)

- Test unit ready (To check whether unit is ready for operation)- Mode Sense (Current Information from mode pages. eg.)

- Read(6), Read(10)

- Write(6), Write(10)


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Advantages of SCSI

Single interface to hold wide range of devices.

More intelligent compared to IDE

- Task scheduling, Task Queuing

Most SCSI devices have own processing chip,less burden on CPU.

Provide backward compatibility

- Can connect SATA drives to SCSI sub system.


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Parallel SCSI

Also known as SCSI Parallel Interface (SPI).

SPI is an interconnect with parallel electricalbus.

In SCSI-1, speed was 8MB/sec.

Ultra SCSI 320, max speed, 320MB/sec


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Limitations of Parallel SCSI

Can support only max. 16 devices.

Parallel bus needs termination to avoidbouncing.

Timing issues due to parallel communication.

- Bigger the cable, bigger the issue.

Bulky cables (50 or 68 pins).


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Serial Attached SCSI (SAS)

Speed in the range of 3-6 Gbps.

No timing issues.

Can support up to 128 devices.

16K devices with expanders.

Thinner form factor connectors.

Thinner cable.


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SAS Devices

End Devices & Expanders

SAS Controllers (Initiator), SAS Disk (Target)

Each SAS device has a World Wide Name (WWN), also knownas SAS address.

Initiators, Expanders, and Targets together form a domain.

Expanders connects an initiators to multiple targets.

- Edge expanders can connect up to 128 targets

- Fanout expanders can connect up to 128 Edge expanders

SMP (Serial Management Protocol), manages expanders.


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Benefits of SAS

Hight speed (3Gbps, 6Gbps), can go up to12Gbps.

More devices can be connected

Interoperability with SATA

SAS and SATA uses same physicalconnectors.

Thinner cables.


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Fiber Channel

A serial I/O interconnect and support multipleprotocols (FCP, TCP/IP, FICON etc)

Standardizes by T11 committee

Currently in 2Gbps and 4Gbps (8Gbps and16Gbps in roadmap)

Can be implemented using optical fiber cables

or copper wires


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FC Topologies

Point to point (FC-P2P)

Two devices connected to each other

Simple topology

Limited connectivity


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FC-AL

All devices connected in loop

Loop is reinitialized when adevice is added or removed.

Failure of one port will affect all

Allows up to 126 nodes

FC-AL given way to FC

Switches

Now used mainly inside JBODsto connect disks


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FC-Switched Fabric

Switch based implementation are also calledFabric

All devices connected to FC switch


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FC-SW Advantage

Switches can be connected to one another

Easily scalable

Devices can be added or removed without

disruption

Port failures can be localizes

Preferred over P2P and AL


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FC Protocol Layers


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Identifying devices in a fabric

Each port is identified by a WWN (World WideName)

WWN is universally unique

It is an 8byte number

Expressed hex format

21:00:00:e0:8b:08:a5:44

IEEE assigns Organizationally Unique Identifier


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FC Flow Control

Buffer to Buffer flow control

Happens b/w all intermediate devices/switches

During login both ports communicate their receive

capability(Buffer to Buffer Credit) each other.

End to End flow control

The two end devices will communicate their receivecapability (End to End Credit) each other.


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Fabric Services

Login server (port 0xFFFFFE)

Responds with devices assigned domain and area values

Name server (port 0xFFFFFC)

Maintains a directory of all connected devices

Synchronized frequently b/w switches

Fabric/Switch controller (port 0xFFFFFD)

Notifies the devices about fabric topology/state changes

when ever any happens Managing server (port 0xFFFFFA)

Accessed by fabric management applications

Time server (port 0xFFFFFB)


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Zoning in FC

Reduced interference

Higher security

Two types

Port based (based on port address)

Name based (based on WWN)


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Zoning in FC


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FC - Strength

High speed

Combines benefits of channel and networktechnologies

Can support millions of devices (8 million)

New standards like FCoE

FC - Weakness High cost

Maximum distance 10km


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iSCSI

Internet SCSI

Provides block oriented storage over TCP/IP n/w

Will work with existing SCSI devices using

existing TCP/IP networks

No special cabling like in FC


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iSCSI topology


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Initiator & Target

Initiator sends command over IP n/w

s/w initiators (device drivers) or hard wareinitiator (iSCSI HBAs)

Target does not initiate any connection,responds to requests


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iSCSI Names

Every iSCSI device will have universally unique nodename

A node will have an iSCSI address too. This is notglobally unique, but should be unique with in a domain.

:

eg. 10.40.1.2:3260

diskarrays.xyz.com


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iSCSIFeatures

Device Discovery

Multipathing

Load balancing

Failover iSCSI Redirect

iSCSI network BOOT

Current speed 1Gbps

With 10Gig Ethernet, speed can go up to10Gbps


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iSCSI Advantages

Less cost

Easy to install, can be integrated into existingEthernet n/w

No distance limitation

Good for applications like remote backup


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iSCSI vs FC

FC has high cost and distance limitation, henceiSCSI is catching up popularity in SMBs

FCoE is an FC modification to support FC over

internet, but FCoE does not uses TCP, insteadused FC itself as transport protocol. Hence needchange in the existing Ethernet spec. to supportit.

Coming years will decide which protocol willdominate over the other.

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