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Modeling and Performance Evaluation of iSCSI SANs over TCP/IP MANs and WANs

Modeling and Performance Evaluation of iSCSI SANs over TCP/IP MANs and WANs. C. M. Gauger, M. Köhn, S. Gunreben, D. Sass, S. G. Perez Institute of Communication Networks and Computer Engineering Universität Stuttgart {gauger, koehn, gunreben, sass}@ikr.uni-stuttgart.de. Objectives and Motivation.

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Modeling and Performance Evaluation of iSCSI SANs over TCP/IP MANs and WANs

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  1. Modeling and Performance Evaluation of iSCSI SANs over TCP/IP MANs and WANs C. M. Gauger, M. Köhn, S. Gunreben, D. Sass, S. G. PerezInstitute of Communication Networks and Computer EngineeringUniversität Stuttgart{gauger, koehn, gunreben, sass}@ikr.uni-stuttgart.de

  2. Objectives and Motivation • SANs gain importance as network application due to • Storage consolidation, IT virtualization • Business continuity and disaster recovery • Terrorist attacks, post 9/11 regulations • Natural catastrophes, c.f. earthquake risc in Bay Area SAN extensions over larger distances (approx. > 200km) • SANs evolve from dedicated equipment and networks to standard equipment and network services • Architectures • FiberChannel (FC): mostly local, expensive equipment, own addressing, own isolated high-QoS networks • FCIP, iFCP: FC extensions over IP networks (tunneling) • iSCSI: native IP protocols, addressing, TCP/IP networks Q: Requirements towards MAN/WAN networks?  Impact of distance and network QoS on iSCSI performance

  3. iSCSI SAN extension scenario Server with local primary storage Secondary storage TCP/IP network, e.g. enterprise VPN iSCSI initiator iSCSI target SCSI requests TCP/IP network, e.g. enterprise VPN

  4. iSCSI • Standardized by IETF in RFC 3720, 3980 • Lightweight protocol • Encapsulates and transports SCSI requests • Relies on underlying TCP/IP network for failure-free transport • Simple block-based flow control • To be implemented in software • Hosts most likely to be in Ethernet (GbEth) LANs • Inexpensive NICs • NIC can employ TCP-offload engine (TOE) to speed-up TCP processing and offload hosts

  5. Modeling and Evaluation scenario • Traffic models • Empirical I/O traffic model (IBM) • Modified empirical and generic I/O traffic models • Network model • Packet loss probability (10-2, 10-3, 10-6) • Round-trip time (initiator-target distance: 0...1000km) • TCP/IP network abstraction model • Bandwidth determined by • Packet loss probability • RTT • TCP window sizes • Access bandwidth • Models by Padhye et al. and Mathis et al. • All details, c.f. report

  6. Modeling • Storage service: asynchronous write across n/w • iSCSI scenarios • Single, isolated request Twrite=f(RTT, p, S, Tproc, ...) • Dynamic request traffic without interleaving • Dynamic request traffic with interleaving

  7. Impact of RTT and QoS • Throughput limited by TCP: both RTT and losses • Short requests, throughput limited by iSCSI • Larger requests • Given RTT: better QoS can even double throughput • Given throughput: better QoS can increase distance significantly • QoS: few differences below p=10-4

  8. Impct of processing delay • Decreasing Tproc increases throughput • For small requests up to factor of 4 • For large requests smaller gains • Few benefit for lossy networks

  9. Results for dynamic traffic – no interleaving • Under dynamic traffic queueing occurs • Queueing delay increase with RTT (c.f. Twrite) • Significant additional delay strongly depending on traffic model • For loss <10-3 few impact of QoS

  10. Results for dynamic traffic – with interleaving • With interleaving of requests in an iSCSI session transfer of several requests can be parallelized • Queueing delay significantly reduced with Twrite as RTT now only limits TCP but not iSCSI any more • For large requests, QoS becomes dominant for given RTT

  11. Impact of offered load • Service time of the request queueing system is not only determined by the transmission time of requests but by processing and transfer overheadQ: when is stability limit reached for the request queues? • Great improvement with interleaving • Strong impact of processing delay • Remaining link capacity can be used by parallel iSCSI session Load=1 with respect to 1Gbps link Tproc=0ms with interleaving shows max. TCP throughput for this scenario

  12. Conclusions • Modelling and performance evaluation of principal iSCSI behavior over MAN/WAN networks • Single request model • Throughput limited by TCP: both RTT and losses • QoS: few differences below p=10-4 • Dynamic request traffic • Significant impact of traffic model • Interleaving greatly improves performance for larger distances • QoS more dominant for larger requests with interleaving • Detailed models and results available in report (inline object below) or on NOBEL server, WP2 Madrid, also submitted for publication

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