iSCSI Simulation Study of Storage System

Smita Vishwakarma and Sankalp Bagaria Computer Networks and Internet Engineering

Center for Development of Advanced Computing Kharghar, India – 400614

Email: {Sankalp, Smita} [AT] cdacmumbai [DOT] in

Abstract Demands for more storage in the industry has increased beyond that the traditional storage mechanisms can supply and thus, the concept of Storage Area Networks (SANs) has evolved. This has resulted in the design of iSCSI (Internet SCSI) protocols that have SCSI commands and allow data transfer to take place over the network. Fiber channel protocol was the first such protocol that used Gigabit per second links to carry commands over long distances. The iSCSI Protocol maps the SCSI block oriented storage data over TCP/IP and enables storage devices to be accessed over standard Ethernet based TCP/IP networks, With the development of Gigabit Ethernet and the iSCSI protocol, reduction in costs and a uniform network infrastructure can be achieved. Simulation offers a flexible way to study the iSCSI – based storage systems. We wish to study a storage area network, which uses iSCSI Protocol to exchange data between its various nodes. This study will help us in understanding how iSCSI works in a storage area network. We have used ns2 for our simulation work. We have simulated an iSCSI initiator and an iSCSI target, which communicate through iSCSI. iSCSI lies upon TCP Protocol. R/W requests are made by the iSCSI initiator and data

exchange takes place depending on these requests. The throughput is calculated and graph is plotted. 1. Introduction The iSCSI protocol has developed as a medium for conveying SCSI block-level access to storage data via an existing IP infrastructure. This can reduce the cost of storage data to a great extent, and also support the mirroring applications, the remote backup, etc. Because of relative maturity and easy availability of TCP, iSCSI protocol has also gained a lot of popularity. On the other side, the iSCSI-based storage is very different from a traditional one. A traditional storage system is usually physically restricted to a limited environment, e.g. in a data center. Transport protocols specially suited to the environment are used, e.g. Fibre Channel, parallel SCSI bus, etc. These characteristics tend to make the storage system more robust and achieve better performance. By studying the performance and comparing with the predictions made for the work-load, it is even possible to isolate bottlenecks, thereby further increasing the performance. On the other hand, in an iSCSI storage, the data transfer is not restricted to a small area. The initiator and the target can be quite far from each

Tenth International Conference on Computer Modeling and Simulation

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Tenth International Conference on Computer Modeling and Simulation

0-7695-3114-8/08 $25.00 © 2008 IEEEDOI 10.1109/UKSIM.2008.18

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Tenth International Conference on Computer Modeling and Simulation

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other. The networking technology in between the initiator and target can be diverse and heterogeneous, e.g. Ethernet, Optical DWDM, ATM, Wireless, satellite, etc. The network condition can be congested and dynamically changing. Packets may suffer from long delay or even loss and retransmission, etc. Thus, the iSCSI storage and retrieval is quite different from the traditional one. To build useful and robust storage systems based on iSCSI Protocol, we need to study the iSCSI protocol by varying its parameters and in different networking situations and its relation with underlying TCP Protocol. The performance measurement is the most direct and most common method to measure the characteristics of the iSCSI storage system. Real measurement can be very accurate. However, the measurement approach is often restricted by the physical equipments and settings and to a limited number of settings. Many times, the hardware or software of the equipment is not open. Thus an experimenter cannot adjust parameters, try an alternative algorithm, etc. to study the iSCSI related issues. We use the network simulator NS2 for our simulation purposes. NS2 is an event driven simulator very popular in the research of networking. It provides support for the simulation of TCP/UDP, multicast and routing protocols over wireless and wired networks, etc. We have made a simulation model for iSCSI-based storage system to study the characteristics of the iSCSI-based storage system. We also study the interactions between the TCP layer and the iSCSI to better support the iSCSI access.

This paper is organized as follows: Section 2 presents the iSCSI Protocol Data Transfer and its link with TCP. Section 3 discusses iSCSI with respect to NS2. Section 4 consists of calculations, results and graphs. Section 5 tells of other work done in this field. Finally this paper concludes in Section 6. 2. iSCSI Protocol The iSCSI protocol transfers the SCSI block oriented storage data over standard Ethernet based TCP/IP networks. iSCSI causes data transfer between the initiator(s) and target(s) over one or many connections. SCSI CDBs (Command Descriptor Blocks) are passed from SCSI layer to the iSCSI transport layer. The transport layer encapsulates CDB in an iSCSI PDU (Protocol Data Unit) and passes it to TCP. When received, the transport layer removes the CDB from PDU and forwards it to SCSI layer. TCP guarantees reliable and in-order delivery of data packets. TCP automatically sends requests of resending of data if an error like acknowledgement not received within time-out period or when loss of data occurs. And that is why, the iSCSI Protocol has been developed over TCP Protocol as storages requires reliable and in-order delivery of the packets. Even multiple connections may occur between the iSCSI initiator and iSCSI target. For instance, a Read Request is to be sent by the initiator to the target. A session is established between iSCSI initiator and iSCSI target. Then, connection is built within the session. The command PDU with Read operation

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is sent to the target by the initiator. The target receives the command PDU and calculates how many data PDUs are to be sent. The data PDUs are then sent via the TCP layer. On receiving the data PDUs, the initiator sends the response PDU. 3. Description of Simulation of iSCSI Protocol in NS2 It serves used NS2 for simulating iSCSI – based storage system. We have done our Simulation with FullTcp Agent. We chose FullTcp as our Agent because this agent supports two way communications. In our Simulation we have created two nodes named as iSCSIInitiator and iSCSITarget; these nodes are attached to the TCP Agent (FullTcp) and linked with the duplex connection. Implementation: Our implementation of the iSCSI protocol in ns-2 supports the following classes: iSCSIInitiator: This class is derived from the Application class. This class uses parameters like MaxRecvDataSegmentlength, DataSegmentLength, MaxConnections, and DataPDUInOrder. ISCSIInitiator class simulates the behavior of the storage area network iSCSI Initiator nodes. It includes the iSCSIInitiatorSession Class object and iSCSIInitiatorTimer Class object as elements. iSCSITarget: This class has also been derived from the Application class. This class includes the functions specific to the iSCSI Target operation. It also includes the iSCSITargetTimer class

object and iSCSITargetSession class object as its elements. This class contains the iSCSI Parameters such as: FirstBurstLength, MaxRecvDataSegmentLength, and InitialR2T. iSCSIInitiatorSession: This class maintain the session between the connections. This class can support more than one session within a connection. iSCSITargetSession: This class support the session at the Target side. iSCSIInitiatorTimer: For the Timer Handling we are using the iSCSIInitiatorTimerHandler class. The functions such as delay and calculation of Round Trip Time are supported by this class for the iSCSIInitiator operations. iSCSITargetTimer: This class perform functions specific to the Timer Handling operations for the Target operation. The functions such as delay and calculation of Round Trip Time are supported by this class for the iSCSITarget operations. Our implementation of iSCSI – based storage system in NS2 supports following features:

• One session between the initiator and the target

• Maintains sequence number of Text Command PDUs

• Random Generation of Read/Write Request Data Size

• Any combination of the immediate, unsolicited and solicited data in a write request

• Arbitrary values of the MaxRecvDataSegmentLength, FirstBurstLength

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4. Performance Analysis Response time is the difference of the time at which all the data of a request has been received and time at which the request was sent. Throughput is the data requested divided by the response time. From the graphs, we see that both the response time and the throughput increases with the increase in the requested data – size. Response time increases because more data is being sent for the increased data – size. Throughput increases as size of data requested increases because the assigned bandwidth is better utilized. Throughput also increases as PDU size increases because effective overhead decreases.

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5. Related Work There has been much simulation work on iSCSI Protocol for storage area networks i.e. Paper [1] describes how various TCP protocols will fare when used along with iSCSI Protocol. Paper [2] describes a useful model for simulating iSCSI in NS2. Our paper shows the performance characteristics of the iSCSI over TCP/IP as simulated in NS2. Further work can consist of studying the effect of other parameters like FirstburstLength, MaxBurstLength, and MaxRecvDataSegment Length etc on response time and throughput. Disk Simulation can be used to study response of disk, when used with iSCSI.

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6. Conclusion This paper describes the partial implementation of iSCSI within ns-2. It provides the details of parameters, their effect on response time and throughput. Average throughput is calculated for randomly generated R/W PDUs with uniform distribution. The larger the PDU size, the larger the throughput. Both the response time and the throughput increases with the size of the data PDU. References: [1] Girish Motwani and K. Gopinath: E valuation of Advanced TCP Stacks in the iSCSI Environment using Simulation Model in the Proceedings of the 22nd

IEEE/ 13th NASA Golden Conference on Mass Storage Systems and Technologies (MSST 05) [2] Yingping Lu, Farrukh Noman, David H.C. Du: Simulation Study of iSCSI-based Storage System in http://www.dtc.umn.edu/publications/reports/2005_06.pdf [3] “The ns Manual (formerly ns Notes and Documentation) collaboration between researcher at UC Berkley, LBL, USC/ISI, and Xerox PARC Editor Kevin Fall and Kannan Varadhan [4] iSCSI-The Universal Storage Connection by John L. Hufferd, published by Addison-Wesley

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