1. Investigating Serial Attached SCSI (SAS) over TCP (tSAS)
UCCS
Master’s Project
Deepti Reddy

2. Outline of the Talk Introduction to tSAS and related technology
Motivation and Challenges
tSAS Design
Performance Evaluation of a Mock Application
Lessons Learnt and Future Directions
Conclusion
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3. SCSI vs SAS SCSI (Small Computer System Interface)
The SCSI protocol is an application layer storage protocol.  
It's a standard for connecting peripherals to your computer via a standard hardware interface, which uses standard SCSI commands.
SAS (Serial Attached SCSI)
SAS is the successor of SCSI technology
Initially introduced at 3Gb/s in 2004
Currently, supports 6Gb/s
12Gbps by 2012
Significantly increased the available bandwidth offered by legacy SCSI storage systems.
Use of expanders increases the scalability over legacy SCSI
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4. SCSI vs SAS SCSI SAS Topology Parallel Bus Serial Bus
Reduces Noise (Better signal integrity)
Speed
3.2 Gbps
3 Gbps, 6Gbps, 12Gbps
Distance
1 to 12 meters
8 meters
Number of Targets
14 devices
128 expanders.
> 16,000 with cascaded expanders
Devices
SCSI only
SAS & SATA
Used widely in Data Centers today
Connectivity
Single Port
Dual Port
Fail-over/fail-back
Drive Form Factor
3.5”
2.5”
Cost
Low
Medium
Need to have font size >=18 for easy viewing
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5. iSCSI
iSCSI protocol describes a means of transporting of the SCSI packets over TCP/IP
The iSCSI specification is revised and updated by the Internet Engineering task Force (IEFT).
Work by S. Aiken, D. Grunwald, A. Pleszkun and J. Willeke shows the performance of a commercial iSCSI software implementation compared quite favorably with fibrechannel [7]
Make diagram as big as possible .
iSCSI protocol layering model
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6. Typical SAS Topology X X X X SAS CONTROLLER (HOST BUS ADAPTER) ~8m
Driver
SAS CONTROLLER
(HOST BUS ADAPTER)
~8m X X X
- DISK DRIVES X
- EXPANDERS
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7. Motivation & Challenges
Overcome the distance and scalability limitations of traditional Serial Attached SCSI (SAS) by using tSAS.
Like iSCSI, tSAS takes advantage of existing internet infrastructure, internet management facilities as well as addresses distance limitations
Provide research results for future industry specification for tSAS and iSCSI.
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8. Related Work
Michael Ko’s patent at IBM Research-Almaden on Serial Attached SCSI over Ethernet proposes a very similar solution to the tSAS solution provided in this project.
iSCSI specification (SCSI over TCP) itself is similar to a tSAS solution (SAS over TCP). The iSCSI solution can be heavily leveraged for a tSAS solution.
The Fibre Channel over TCP/IP specification also can be leveraged to design and implement a tSAS solution.
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9. tSAS Topology X - TCP link - SAS link X - DISK DRIVES - EXPANDERS tSAS
SCSI Driver
tSAS
HBA
SCSI Driver
tSAS CONTROLLER/HBA X
- TCP link
- DISK DRIVES
- SAS link X
- EXPANDERS
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10. tSAS Topology X - TCP link - SAS link X - DISK DRIVES - EXPANDERS tSAS
SCSI Driver
tSAS
HBA
SCSI Driver
tSAS CONTROLLER/HBA X
- TCP link
- DISK DRIVES
- SAS link X
- EXPANDERS
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11. Goals of the project Investigate how tSAS can be implemented
Design and develop a mock tSAS application
Write a client program using C/C++ that sends a set of tSAS commands to a server.
Write a server program using C/C++ that receives a set of tSAS commands, processes them and responds to the client with tSAS responses.
Evaluate the tSAS solution and compare with a mock iSCSI client/server application.
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12. Software and Hardware solutions for tSAS implementations
Software based tSAS solution
Cheaper than a hardware based solution
All tSAS processing is done by the processor and TCP/IP operations are also executed by the CPU
NIC is merely an interface to the network
TCP Offload Engine (TOE) solution
NICs with integrated TOEs have hardware built into the card that allows the TCP/IP processing to be done at the interface
Hardware based tSAS solution
In a hardware-based tSAS environment, the initiator and target machines contain a host bus adapter (HBA) that is responsible for both TCP/IP and tSAS processing
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13. tSAS Approach
The tSAS Request is initially sent by the tSAS Initiator to the tSAS Target over TCP.
The tSAS Target strips off the TCP header and sends the SAS frames using the SAS Initiator block on the tSAS Target to the SAS expander/drive.
The SAS expander/drive sends SAS frames to the tSAS Target.
Finally, the tSAS Target embeds the SAS frames received from the expander/drive over TCP and sends it to the tSAS Initiator.
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14. tSAS Message Format Ethernet Header IP Header TCP Header tSAS Header
Data
Ethernet Trailer
TCP Segment
IP Datagram
Ethernet Frame
The above shows how a legacy SAS header and data are embedded in an Ethernet frame.
iSCSI uses the same approach where the iSCSI header and data are encapsulated in an Ethernet frame.
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15. tSAS Approach SAS Expander tSAS Target tSAS Initiator
tSAS Request
Open Address Frame
Open Accept
SAS/SSP Request Frame
Xfer Rdy Frame
tSAS Xfer Rdy Frame .
Data Frame .
Data Frame
Data Frame
Data Frame
SSP Response Frame
tSAS Response Frame
The above diagram shows a typical tSAS (SSP Write ) Request & Response sequence diagram.
iSCSI uses a similar approach where the back-end between the iSCSI target and SCSI drives uses the legacy SCSI protocol.
tSAS/sreddy
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16. tSAS Approach Primitives
Primitives are special 8b/10b encoded characters that are used as frame delimiters, for out of band signaling, control sequencing in SAS
Most primitives can be conveniently ignored on the tSAS side except a few such as Broadcast primitives used to initiate discovery
SAS primitive can be encapsulated in an Ethernet frame
Discovery
Discovery in tSAS will be similar to SAS and will be accomplished by sending Serial management protocol (SMP) commands over TCP to the initiators and expanders downstream to learn the topology.
Will show a SAS Trace with READ commands to show the audience what primitives look like on the wire.
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17. tSAS Approach
SAS Primitives
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18. SMP Initiator on Expander
tSAS Approach
SMP Initiator on Expander
SMP Target on Expander
tSAS Client
SMP Request
Open Address Frame
Open Accept
SMP Request
SMP Response
Make the font bigger at least 18pt
Close
SMP Response
SMP Request & Response Sequence Diagram
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19. Test benches used for performance calculations
tSAS and iSCSI Mock Application
The client application and server application run on two different Windows machines connected using a NetGear ProSafe Gigabit Switch
Intel Core 2 Duo CPU
2.53 GHz, 3.45 GB RAM
Intel 82567LM Gigabit Network Adapter
Client makes Read/Write requests and the server processes and responds to requests made by the Client
Windows Machine A
(Running Client App)
Windows Machine B
(Running Server App)
NetGear
Switch
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20. Test benches used for performance calculations
Legacy SAS
A 6 Gbps SAS Host Bus Adapter in a PCIe slot of a Super Micro server
A 6 Gbps SAS target (Seagate SAS drive) connected to the Host Bus Adapter
IOMeter in Windows and VDBench in RHEL used to generate Read/Write IOs and measure performance
IOMeter and VDBench are IO generator tools used to measure IO performance of a storage system
A LeCroy SAS Protocol Analyzer placed between the HBA and the SAS Target
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21. Test benches used for performance calculations
Driver
SAS CONTROLLER
(HOST BUS ADAPTER)
SAS Protocol Analyzer X
SAS Protocol Analyzer
- SAS DISK DRIVE X
- EXPANDER
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22. Test benches used for performance calculations
Legacy iSCSI
Two windows machines/servers were used connected using a NetGear ProSafe Gigabit Switch
On one machine the StarWind iSCSi Initiator was running
On another machine, the KernSafe iSCSi target software was used to create a iSCSI target
IOMeter was used to send Read/Write requests from the iSCSI Initiator to the iSCSI Target and measure performance
Wireshark used to capture Network Traffic
iSCSI Initiator on Windows Machine A
iSCSI Target on Windows Machine B
NetGear
Switch
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23. tSAS vs iSCSI Mock Application at 10 Mbps
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24. tSAS vs iSCSI Mock Application at 100 Mbps
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25. tSAS vs iSCSI Mock Application at 1 Gbps
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26. tSAS 512K Read at 10 Mbps, 100 Mbps and 1 Gbps
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27. Lessons Learnt/Difficulties encountered
Using several iSCSI Target Softwares that were not licensed/purchased made it challenging since several features were either not supported or did not work on the unlicensed version.
Even using KernSafe iSCSI target, I was not able to create a iSCSI RamDisk Target or a iSCSI Hard disk target. I used a USB Flash drive as the iSCSI target
Subtracted the delay at the Flash Drive from the iSCSI IOMeter performance results
Delay of Flash Drive calculated by running IOMeter to the SCSI device on the target machine
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28. Lessons Learnt/Difficulties encountered
Jumbo frames
The NetGear ProSafe switch is a Gigabit switch that supports Jumbo Frames. However, I could not get jumbo frames to work since the Network Adapter on the Windows machines did not support Jumbo frames
One needs cabling and adapters as well that support Jumbo frames to use a switch that supports Jumbo frames
Understanding the SAS protocol and tools is very involved
Took substantial time to understand and learn the workings of SAS as well as the tools
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29. Future Work More data points
The tSAS mock application can be run using a faster switch with connection rates greater than 1 Gbps to get more data points (40G/100G Ethernet)
Piggybacking
Response frames are piggybacked with the last DATA frame or a DATA frame is piggy backed with a request frame. This would increase performance
Jumbo frames
Can be used to increase the amount of DATA that is passed from the initiator and target per Ethernet packet improving the performance results.
The Storage Associations can be motivated with similar work to fuel the conception of a tSAS specification
tSAS code can be implemented in a SAS HBA and performance can be measured using this direct implementation
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30. Conclusion tSAS is a viable solution
tSAS will be faster than a similar iSCSI implementation
Overhead in tSAS smaller than in iSCSI
Request, Response and Transfer Ready frames in tSAS smaller than Request, Response and Ready to Transfer frames in iSCSI
Back-end of tSAS will be legacy SAS
tSAS more visibly faster at larger transfer sizes than iSCSI
tSAS more reliable at smaller transfer sizes at 10 Mbps, 100 Mbps and 1 Gbps due to higher TCP retransmission rate at larger sizes
tSAS hardware solution would be the fastest vs a software solution
tSAS would perform better at smaller transfer sizes
tSAS will overcome the distance limitation of legacy SAS
For the 2nd bullet item, are you listing all the reasons? Or they are independent facts?
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31. References
[1] T10/1760-D Information Technology – Serial Attached SCSI – 2 (SAS-2),
T10, 18 April 2009,
Available from
[2] Harry Mason, Serial attached SCSI Establishes its Position in the Enterprise, LSI Corporation, available from
[3] J Satran, K Meth, C. Sapuntzakis, M. Chadalapka, E. Zeidner,
RFC 3720 Internet Small Computer Systems Interface (iSCSI) Specification
IETF, April 2004,
available from
[4] Cai, Y.; Fang, L.; Ratemo, R.; Liu, J.; Gross, K.; Kozma, M.;
A test case for 3Gbps serial attached SCSI (SAS)
Test Conference, Proceedings. ITC IEEE International, February
2006,
available from
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32. References [6] Kalmath Meth, Julian Satran,
[6] Kalmath Meth, Julian Satran,
Design of the iSCSI Protocol, Mass Storage Systems and Technologies, (MSST 2003). Proceedings. 20th
IEEE/11th NASA Goddard Conference on Mass Storage Systems and Technologies,
April 2003,
available from
[7] Stephen Aiken, Dirk Grunwald, Andrew R. Pleszkun, Jesse Willeke,
A Performance Analysis of the iSCSI Protocol
20th IEEE/11th NASA Goddard Conference on Mass Storage Systems and
Technologies, 2003,
available from
[8] M. Rajagopal, E. Rodriguez, R. Weber,
RFC 3821 Fibre Channel over TCP/IP (FCIP) standard,
IETF, July 2004,
available from
[9] BLi Bigang, Shu Jiwu, Zheng Weimin,
SCSI Target Simulator Based on FC and IP Protocols in TH-MSNS*
Department of Computer Science and Technology, Tsinghua University, Beijing
China, 2005
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33. References
[10] S. Chaitanya, K. Butler, A. Sivasubramaniam, P. McDaniel, M. Vilayannur,
  Design, Implementation and Evaluation of Security in iSCSI-based Network
Storage Systems,
StorageSS '06 Proceedings of the second ACM workshop on Storage security
and survivability,
October 2006,
available from
[11] 1Gbps and 10Gbps Ethernet
Server Scalability, NetApp,
available fromhttp://partners.netapp.com/go/techontap/matl/downloads/redhat-
neterion_10g.pdf
[12] Michael A. Ko, LAYERING SERIAL ATTACHED SMALL COMPUTER SYSTEM
INTERFACE (SAS) vOVER ETHERNET, United States Patent Application
, 09/18/2008
available from
[13] SCSI Primary Commands - 4 (SPC-4), Revision 31, T10, 2011/06/13,
available from
[14] Broadcom iSCSI HBA FAQ
available from
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