The Panasas file system uses parallel and redundant access to object storage devices (OSDs), per-file RAID, distributed metadata management, consistent client caching, file locking services, and internal cluster management to provide a scalable, fault tolerant, high performance distributed file system. The clustered design of the storage system and the use of client-driven RAID provide scalable performance to many concurrent file system clients through parallel access to file data that is striped across OSD storage nodes. RAID recovery is performed in parallel by the cluster of metadata managers, and declustered data placement yields scalable RAID rebuild rates as the storage system grows larger. This paper presents performance measures of I/O, metadata, and recovery operations for storage clusters that range in size from 10 to 120 storage nodes, 1 to 12 metadata nodes, and with file system client counts ranging from 1 to 100 compute nodes. Production installations are as large as 500 storage nodes, 50 metadata managers, and 5000 clients.

/pdf/scalable-performance-of-the-panasas-parallel-file-system-2tywsljo75.pdf

Scalable performance of the Panasas parallel file system

Today's computational science demands have resulted in ever larger parallel computers, and storage systems have grown to match these demands. Parallel file systems used in this environment are increasingly specialized to extract the highest possible performance for large I/O operations, at the expense of other potential workloads. While some applications have adapted to I/O best practices and can obtain good performance on these systems, the natural I/O patterns of many applications result in generation of many small files. These applications are not well served by current parallel file systems at very large scale. This paper describes five techniques for optimizing small-file access in parallel file systems for very large scale systems. These five techniques are all implemented in a single parallel file system (PVFS) and then systematically assessed on two test platforms. A microbenchmark and the mdtest benchmark are used to evaluate the optimizations at an unprecedented scale. We observe as much as a 905% improvement in small-file create rates, 1,106% improvement in small-file stat rates, and 727% improvement in small-file removal rates, compared to a baseline PVFS configuration on a leadership computing platform using 16,384 cores.

/pdf/small-file-access-in-parallel-file-systems-2r4sn16t4w.pdf

Small-file access in parallel file systems

Remote Direct Memory Access (RDMA) is a mechanism whereby data is moved directly between the application memory of the local and remote computer. In bypassing the operating system, RDMA significantly reduces the CPU cost of large data transfers and eliminates intermediate copying across buffers, thereby making it very attractive for implementing distributed applications. With the advent of hardware implementations of RDMA over Ethernet (iWARP), its advantages have become even more obvious. In this paper we analyze the applicability of RDMA and identify hidden costs in the setup of its interactions that, if not handled carefully, remove any performance advantage, especially in hardware implementations. From an application point of view, the major difference to TCP/IP based communication is that the buffer management has to be done explicitly by the application. Without the proper optimizations, RDMA loses all its advantages. We discuss the problem in detail, analyze what applications can profit from RDMA, present a number of optimization strategies, and show through extensive performance experiments that these optimizations make a substantial difference in the overall performance of RDMA based applications.

Minimizing the Hidden Cost of RDMA

Large HPC installations typically make use of parallel file systems that adhere to POSIX I/O conventions, and that implement a separation of data and metadata in order to maintain high performance. File systems such as GPFS and Lustre have evolved to enable an increase in data bandwidth that is primarily achieved by adding more disk drives behind an increasing number of disk controllers. Improvements in metadata performance cannot be achieved by just deploying a large volume of hardware, as the defining characteristics are the number of simultaneous operations that can be carried out and the latency of those operations. For highly scalable applications using parallel I/O libraries, the speed of metadata operations, such as opening a file on thousands of processes, has the potential to become the major bottleneck to improved I/O performance. This Metadata Wall has the ability to grow such that metadata operations can take much longer than the subsequent data operations, even on systems with very large amounts of I/O data bandwidth. We present results showing the performance of metadata operations with standard disk equipment and with solid state storage hardware, and extrapolate whether we expect the evolution in hardware alone will be sufficient to limit the effects of this I/O Metadata Wall. We also report challenges in making the metadata I/O measurements and subsequent analysis for parallel file systems.

Parallel I/O and the metadata wall

Remote Direct Memory Access (RDMA)-based communication has enjoyed considerable growth since the introduction of the Virtual Interface Architecture (VIA) in the late 1990s. This growth has accelerated since the introduction of the Open Fabric Alliance’s (OFA) Verbs interface. The stability of this interface and its independence from the underlying physical network has facilitated significant growth of software applications that exploit the benefits of RDMA. Until recently, a user wishing to utilize RDMA could use either the InfiniBand Architecture, which required the use of the unique link and physical layers defined by the InfiniBand Architecture, or the RDDP protocols, designed to run over an IP network. Recent developments in Ethernet have expanded the available options for providing RDMA services to applications while using Ethernet link and physical layers. Specifically, there is a new proposal for adapting the InfiniBand Architecture transport to run over Converged Enhanced Ethernet (CEE). In this paper we describe this proposal and compare it to two existing methods for delivering RDMA services to an application using existing IP-based transports.

Remote Direct Memory Access over the Converged Enhanced Ethernet Fabric: Evaluating the Options

The term iWarp indicates a set of published protocol specifications that provide remote read and write access to user applications, without operating system intervention or intermediate data copies. The iWarp protocol provides for higher bandwidth and lower latency transfers over existing, widely deployed TCP/IP networks. While hardware implementations of iWarp are starting to emerge, there is a need for software implementations to enable offload on servers as a transition mechanism, for protocol testing, and for future

Design and Implementation of the iWarp Protocol in Software.

As storage systems evolve, the block-based design of today's disks is becoming inadequate. As an alternative, object-based storage devices (OSDs) offer a view where the disk manages data layout and keeps track of various attributes about data objects. By moving functionality that is traditionally the responsibility of the host OS to the disk, it is possible to improve overall performance and simplify management of a storage system. The capabilities of OSDs will also permit performance improvements in parallel file systems, such as further decoupling metadata operations and thus reducing metadata server bottlenecks. In this work we present an implementation of the Parallel Virtual File System (PVFS) integrated with a software emulator of an OSD and describe an infrastructure for client access. Even with the overhead of emulation, performance is comparable to a traditional server-fronted implementation, demonstrating that serverless parallel file systems using OSDs are an achievable goal.

/pdf/integrating-parallel-file-systems-with-object-based-storage-sp8ig66rk5.pdf

Integrating parallel file systems with object-based storage devices

Zero-copy, RDMA, and protocol offload are three very important characteristics of high performance interconnects. Previous networks that made use of these techniques were built upon proprietary, and often expensive, hardware. With the introduction of iWarp, it is now possible to achieve all three over existing low-cost TCP/IP networks.iWarp is a step in the right direction, but currently requires an expensive RNIC to enable zero-copy, RDMA, and protocol offload. While the hardware is expensive at present, given that iWarp is based on a commodity interconnect, prices will surely fall. In the meantime only the most critical of servers will likely make use of iWarp, but in order to take advantage of the RNIC both sides must be so equipped.It is for this reason that we have implemented the iWarp protocol in software. This allows a server equipped with an RNIC to exploit its advantages even if the client does not have an RNIC. While throughput and latency do not improve by doing this, the server with the RNIC does experience a dramatic reduction in system load. This means that the server is much more scalable, and can handle many more clients than would otherwise be possible with the usual sockets/TCP/IP protocol stack.

/pdf/iwarp-protocol-kernel-space-software-implementation-1s05q5ae49.pdf

iWarp protocol kernel space software implementation

Distributed file systems that use multiple servers to store data in parallel are becoming commonplace. Much work has already gone into such systems to maximize data throughput. However, metadata management has historically been treated as an afterthought. In previous work we focused on improving metadata management techniques by placing file metadata along with data on object-based storage devices (OSDs). However, we did not investigate directory operations. This work looks at the possibility of designing directory structures directly on OSDs, without the need for intervening servers. In particular, the need for atomicity is a fundamental requirement that we explore in depth. Through performance results of benchmarks and applications we show the feasibility of using OSDs directly for metadata, including directory operations.

An OSD-based approach to managing directory operations in parallel file systems

Network speeds are increasing well beyond the capabilities of today's CPUs to efficiently handle the traffic. This bottleneck at the CPU causes the processor to spend more of its time handling communication and less time on actual processing. As network speeds reach 10 Gb/s and more, the CPU simply can not keep up with the data. Various methods have been proposed to solve this problem. High performance interconnects, such as Infiniband, have been developed that rely on RDMA and protocol offload in order to achieve higher throughput and lower latency. In this paper we evaluate the feasibility of a similar approach which, unlike existing high performance interconnects, requires no special infrastructure. RDMA over Ethernet, otherwise known as iWARP, facilitates the zero copy exchange of data over ordinary local area networks. Since it is based on TCP, iWARP enables RDMA in the wide area network as well. This paper provides a look into the performance of one of the earliest commodity implementations of this emerging technology, the Ammasso 1100 RNIC

/pdf/a-performance-analysis-of-the-ammasso-rdma-enabled-ethernet-12vaj1va6r.pdf

A Performance Analysis of the Ammasso RDMA Enabled Ethernet Adapter and its iWARP API

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