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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.