Investigating Serial Attached SCSI (SAS) over TCP (tSAS)

UCCS

Master’s Project

Deepti Reddy

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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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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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SCSI vs SAS

SCSI SAS

Topology Parallel Bus Serial BusReduces 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 & SATAUsed widely in Data Centers today

Connectivity Single Port Dual PortFail-over/fail-back

Drive Form Factor 3.5” 2.5”

Cost Low Medium11/3/2011tSAS/sreddy

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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]

iSCSI protocol layering model11/3/2011 tSAS/sreddy

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Typical SAS Topology

X X X

X

- DISK DRIVES

- EXPANDERS

DriverSAS CONTROLLER(HOST BUS ADAPTER)

~8m

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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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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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tSAS Topology

X

X

- DISK DRIVES

- EXPANDERS

SCSI DrivertSAS CONTROLLER/HBA

- TCP link- SAS link

SCSI DrivertSAS

HBA

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X

X

- DISK DRIVES

- EXPANDERS

SCSI DrivertSAS CONTROLLER/HBA

- TCP link- SAS link

tSASHBA

SCSI Driver

tSAS Topology

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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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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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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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tSAS Message Format

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.

Ethernet Header

IP Header

TCP Header

tSAS Header

tSASData

Ethernet Trailer

TCP Segment

IP Datagram

Ethernet Frame

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tSAS Approach

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 Request

tSAS Initiator

tSAS Target

SAS Expander

Open Address Frame

Open Accept

SAS/SSP Request Frame

Xfer Rdy Frame

.

.

Data FrameData Frame Data Frame

tSAS Xfer Rdy Frame

.

.

Data Frame

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SSP Response

Frame

tSAS Response

Frame

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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.

tSAS Approach

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tSAS Approach

SAS Primitives

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SMP Request

tSAS Client

SMP Initiator

on Expander

SMP Target on Expander

Open Address Frame

Open Accept

SMP Request

SMP Response

CloseSMP

Response

tSAS Approach

SMP Request & Response Sequence Diagram

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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

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Windows Machine A(Running

Client App)

Windows Machine B(Running

Server App)

NetGearSwitch

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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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X

X

- SAS DISK DRIVE- EXPANDER

DriverSAS CONTROLLER(HOST BUS ADAPTER)

SAS Protocol Analyzer

SAS Protocol Analyzer

Test benches used for performance calculations

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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

NetGearSwitch

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tSAS vs iSCSI Mock Application at 10 Mbps

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0 200 400 600 800 1000 12000

500

1000

1500

2000

2500

iSCSI vs tSAS READ Completion Time at 10 Mbps

tSAS READ Completion timeiSCSI READ Completion Time

Transfer Size (Kilobytes)

Time (Milliseconds)

0 200 400 600 800 1000 1200 1400 1600 1800 2000 22000

2000

4000

6000

8000

tSAS vs iSCSI Write 10Mbps

tSAS Write 10MBpsiSCSI Write 10MBps

Transfer Size (Kilobytes)

Time (milliseconds)

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tSAS vs iSCSI Mock Application at 100 Mbps

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0 200 400 600 800 1000 1200 1400 1600 1800 2000 22000

100

200

300

400

500

600

700

tSAS vs iSCSI Read 100 Mbps

tSAS Read 100 MbpsiSCSI Read 100 Mbps

Transfer Length (Kilobytes)

Time(Milliseconds)

0 200 400 600 800 1000 1200 1400 1600 1800 2000 22000

200

400

600

800

1000

tSAS vs iSCSi Write 100 Mbps

tSAS Write 100 MbpsiSCSI Write 100 Mbps

Transfer Length (KB)

Time(Milliseconds)

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tSAS vs iSCSI Mock Application at 1 Gbps

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0 200 400 600 800 1000 1200 1400 1600 1800 2000 22000

10

20

30

40

50

tSAS vs iSCSI Read 1 Gbps

tSAS Read 1 GbpsiSCSi Read 1 Gbps

Transfer Length (KB)

Time(Milliseconds)

0 200 400 600 800 1000 1200 1400 1600 1800 2000 22000

1020304050607080

tSAS vs iSCSI Write 1 Gbps

tSAS Write 1 GbpsiSCSI Write 1 Gbps

Transfer Size (KB)

Time(Milliseconds)

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tSAS 512K Read at 10 Mbps, 100 Mbps and 1 Gbps

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0 200 400 600 800 1000 12000

200

400

600

800

1000

1200

Time for READ Completion with transfer size of 512K at 10 Mbps, 100Mbps and 1Gbps

Time for READ Compeltion

Connection Rate (Mbps)

Time (Milliseconds)

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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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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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Lessons Learnt/Difficulties encountered

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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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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

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References [1] T10/1760-D Information Technology – Serial Attached SCSI – 2 (SAS-2), T10, 18 April 2009, Available from http://www.t10.org/drafts.htm#SCSI3_SAS [2] Harry Mason, Serial attached SCSI Establishes its Position in the Enterprise, LSI Corporation, available from http://www.scsita.org/aboutscsi/sas/6GbpsSAS.pdf [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 http://www.ietf.org/rfc/rfc3720.txt [4] Cai, Y.; Fang, L.; Ratemo, R.; Liu, J.; Gross, K.; Kozma, M.; A test case for 3Gbps serial attached SCSI (SAS) Test Conference, 2005. Proceedings. ITC 2005. IEEE International, February 2006, available from http://ieeexplore.ieee.org/xpls/abs_all.jsp?arnumber=1584027  

 

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References [6] Kalmath Meth, Julian Satran, Design of the iSCSI Protocol,Mass Storage Systems and Technologies, 2003. (MSST 2003). Proceedings. 20th IEEE/11th NASA Goddard Conference on Mass Storage Systems and Technologies, April 2003, available from http://ieeexplore.ieee.org/xpls/abs_all.jsp?arnumber=1194848&tag=1  [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 http://www.storageconference.org/2003/papers/20-Aikens-Performance.pdf  [8] M. Rajagopal, E. Rodriguez, R. Weber, RFC 3821 Fibre Channel over TCP/IP (FCIP) standard, IETF, July 2004, available from http://tools.ietf.org/html/rfc3821

[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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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 http://portal.acm.org/citation.cfm?id=1179564

[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 20080228897, 09/18/2008 available from http://www.faqs.org/patents/app/20080228897

[13] SCSI Primary Commands - 4 (SPC-4), Revision 31, T10, 2011/06/13, available from http://www.t10.org/members/w_spc4.htm

[14] Broadcom iSCSI HBA FAQ available from http://www.broadcom.com/collateral/faq/iSCSI-FAQ100-R.pdf

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