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

UCCS

Master’s Project

Deepti Reddy

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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 BusSpeed 3.2 Gbps 3 Gbps, 6Gbps,

12Gbps

Distance 1 to 12 meters 8 metersNumber of Targets 14 devices 128 expanders.

> 16,000 with cascaded expanders

Devices SCSI only SAS & SATAConnectivity Single Port Dual PortDrive Form Factor 3.5” 2.5”Cost Low Medium

Comparing SCSI and SAS

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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 model

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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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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 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

eSAS Header

eSASData

Ethernet Trailer

TCP SegmentIP Datagram

Ethernet Frame

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tSAS Approach

The above diagram shows a typical tSAS (SSP Read ) 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 Request Frame

Data Frame..

Data Frame

tSAS Response

Response Frame

Data Frame..

Data 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

− 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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SMP Request

ClientSMP Initiator

Port on Expander

SMP Target Port on Target Expander

Open Address Frame

Open AcceptSMP 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/servers connected using a NetGear ProSafe Gigabit Switch

− Client makes Read/Write requests and the server processes and responds to requests made by the Client

Legacy SAS− A 6 Gbps Sas Host Bus Adapter in a PCIe slot of a Super Micro

server− A 6 Gbps SAS target connected to the Host Bus Adapter− IOMeter in Windows and VDBench in RHEL used to generate

Read/Write IOs and measure performance− A SAS analyzer placed between the HBA and the SAS Target

Legacy iSCSI− Two windows machines/servers were used− On one machine the StarWind iSCSi Initiator was used− 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

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tSAS vs iSCSI Mock Application at 10 Mbps

0 200 400 600 800 1000 12000

5001000150020002500

tSAS vs iSCSI READ 10 MbpsX-axis – Transfer Size (Bytes)Y-axis – Time in milliseconds

tSAS READ Completion timeiSCSI READ Completion Time

0 500 1000 1500 2000 25000

10002000300040005000600070008000

tSAS vs iSCSI Write 10MbpsX-axis – Transfer Size (Bytes)Y-axis – Time in milliseconds

tSAS Write 10MBpsiSCSI Write 10MBps

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tSAS vs iSCSI Mock Application at 100 Mbps

0 500 1000 1500 2000 25000

100200300400500600700

tSAS vs iSCSI READ 100 MbpsX-axis – Transfer Size (Bytes)Y-axis – Time in milliseconds

tSAS Read 100 MbpsiSCSI Read 100 Mbps

0 500 1000 1500 2000 25000

200400600800

1000

tSAS vs iSCSI READ 100 MbpsX-axis – Transfer Size (Bytes)Y-axis – Time in milliseconds

tSAS Write 100 MbpsiSCSI Write 100 Mbps

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tSAS vs iSCSI Mock Application at 1 Gbps

0 500 1000 1500 2000 25000

1020304050

tSAS vs iSCSI READ 1 GbpsX-axis – Transfer Size (Bytes)Y-axis – Time in milliseconds

tSAS Read 1 GbpsiSCSi Read 1 Gbps

0 500 1000 1500 2000 25000

20406080

tSAS vs iSCSI READ 1 GbpsX-axis – Transfer Size (Bytes)Y-axis – Time in milliseconds

tSAS Write 1 GbpsiSCSI Write 1 Gbps

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

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

X-axis – Transfer Size (Bytes)Y-axis – Time in milliseconds

Time for READ Compeltion

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Comparing tSAS mock application to legacy iSCSI and legacy SAS

tSAS results are extrapolated to 100 Gbps and so are the legacy iSCSI results to compare tSAS with legacy iSCSI

− Assuming 100 Gbps connection rate performs 43.348 times better than 1 Gbps connection rates based on the 1 Gbps and 10 Gbps server scalability paper by NetApp where it is concluded that 10 Gbps performs 4.3348 times better than 1 Gbps.

− These comparison results can’t be considered to be a fair comparison as the tSAS mock application is not a full fledged tSAS implementation

tSAS results are compared to legacy SAS− Legacy SAS performance results between the HBA and the SAS

drive observed using the SAS analyzer for various transfer sizes under 2 MB show that Read/Write completions are < 1 ms. tSAS performs much slower than this at 100 Gbps

− tSAS is compared to legacy SAS without including the delay at drive

− These comparison results can’t be considered to be a fair comparison as the legacy SAS application is HBA based while the tSAS application is S/W based and is not a full fledged application

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Comparing tSAS mock application to legacy iSCSI

0 500 1000 1500 2000 25000

5

10

15

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Write performance tSAS at 100 Gbps vs legacy iSCSI at 100 Gbps using IOMeter without delay at

the driveTransfer Sizes (Bytes) vs Time (Milliseconds)

Write performance extrapo-lated tSAS 100 GbpsLegacy iSCSI Write timings using IOMeter without de-lay at the drive extrapo-lated 100 Gbps

0 500 1000 1500 2000 250002468

1012141618

Read performance tSAS at 100 Gbps vs legacy iSCSI at 100 Gbps using IOMeter without delay at the drive

Transfer Sizes (Bytes) vs Time (Milliseconds)

Read performance extrapo-lated tSAS 100 GbpsLegacy iSCSI Read timings using IOMeter without delay at the drive extrapolated 100 Gbps

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Comparing tSAS Mock application to legacy SASX-Axis – Transfer Length (Bytes), Y-Axis – Time in

Milliseconds

0 500 1000 1500 2000 250002468

10121416

tSAS 100 Gbps Performance vs Legacy SSP Read tim-ings without delay at the drive

Read extrapolated tSAS 100 GbpsLegacy SAS(SSP) Read Tim-ings using IOMeter without Delay at the driveLegacy SAS(SSP) Read Tim-ings using VDBench without Delay at the Drive

0 500 1000 1500 2000 25000

2

4

6

8

10

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tSAS 100 Gbps Performance vs Legacy SSP Write timings without delay at the drive

Write extrapolated tSAS 100 GbpsLegacy SAS(SSP) Write Timings using IOMeter without Delay at the driveLegacy SAS(SSP) Write Timings using VDBench without Delay at the Drive

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Similar Work

Michael Ko’s patent 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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Conclusion tSAS is a viable solution tSAS will be faster than a similar iSCSI implementation

− Frame sizes in tSAS smaller than in iSCSI− Back-end of tSAS will be legacy SAS− tSAS should be visibly faster at larger transfer 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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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

Piggybacking− Response frames are piggybacked with the last DATA

frame or a DATA frame is piggy backed with a request frame

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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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