Free tutor on Basic principle NETWORK FILE SYSTEMS AND FILE SERVERS and Network Attached Storage (NAS)

Free tutor on Basic principle NETWORK FILE SYSTEMS AND FILE SERVERS Network Attached Storage (NAS)

Network file systems are the natural extension of local file systems. End users and applications can access directories and files that are physically located on a different computer – the file server – over a network file system (Section 4.2.1). File servers are so important in modern IT environments that preconfigured file servers, called Network Attached Storage (NAS), have emerged as a separate product category We highlight the performance bottlenecks of file servers and discuss the possibilities for the acceleration of network file systems. Finally, we introduce the Direct Access File System (DAFS), a new network file system that relies upon RDMA and VI instead of TCP/IP.

Basic principle

The metaphor of directories and files for the management of data is so easy to understand that it was for a long time the prevailing model for the access of data over networks. So-called network file systems give end users and applications access to data stored on a different computer (Figure 4.5).

The first widespread network file system was the Network File System (NFS) developed by Sun Microsystems, which is now the standard network file system on all Unix systems. Microsoft developed its own network file system – the Common Internet File System (CIFS) – for its Windows operating system and this is incompatible with NFS. Today, various software solutions exist that permit the exchange of data between Unix and Windows over a network file system.

With the aid of network file systems, end users and applications can work on a common data set from various computers. In order to do this on Unix computers the system administrator must link a file system exported from an NFS server into the local directory structure using the mount command. On Windows computers, any end user can do this himself using the Map Network Drive command. Then, both in Unix and in Windows, the fact that data is being accessed from a network file system, rather than a local file system, is completely hidden apart from performance differences. Long before theWorldWideWeb, the File Transfer Protocol (FTP) provided amechanism by means of which users could exchange files over the Internet. Even today, FTP servers remain an important means of distributing freely available software and freely available documents. Unlike network file systems, access to FTP servers is clearly visible to the end user. Users require a special FTP client with which they can copy back and forwards between the FTP server and their local computer. The Hyper Text Markup Language (HTML) and the Hyper Text Transfer Protocol (HTTP) radically changed the usage model of the Internet. In contrast to FTP, the data on the Internet is linked together by means of HTML documents. The user on the Internet no longer accesses individual files, instead he 'surfs' the World Wide Web (WWW). He views HTML documents on his browser that are sometimes statically available on a HTTP server in the form of files or today are increasingly dynamically

generated. Currently, graphic HTTP clients – the browsers – without exception have an integrated FTP client, with which they can easily 'download' files.

Network Attached Storage (NAS)

File servers are so important in current IT environments that they have developed into an independent product group in recent years. Network Attached Storage (NAS) is the name for preconfigured file servers. They consist of one or more internal servers, preconfigured disk capacity and usually a stripped-down or special operating system (Figure 4.6). NAS servers are usually connected via Ethernet to the LAN, where they provide their disk space as file servers. Web servers represent a further important field of application for NAS servers. By definition, the clients are located at the other end of the WAN so there is no alternative to communication over IP. Large NAS servers offer additional functions such as snapshots, remote mirroring and back-up over Fibre Channel SAN.

NAS servers were specially developed for file sharing. This has two advantages: since, by definition, the purpose of NAS servers is known, NAS operating systems can be significantly better optimized than generic operating systems. This means that NAS servers can operate more quickly than file servers on comparable hardware that are based upon a generic operating system. The second advantage of NAS is that NAS servers provide Plug&Play file systems,

i.e. connect – power up – use. In contrast to a generic operating system all functions can be removed that are not necessary for the file serving. NAS storage can therefore excel due to low installation and maintenance costs, which takes the pressure off system administrators. NAS servers are very scalable. For example the system administrator can attach a dedicated NAS server for every project or for every department. In this manner it is simple to expand large websites. E-ail file system full? No problem, I simply provide another NAS server for the next 10,000 users in my Ethernet. However, this approach can become a management nightmare if the storage requirement is very large, thus dozens of NAS servers are required. One disadvantage of NAS servers is the unclear upgrade path. For example, the internal server cannot simply be replaced by a more powerful server because this goes against the principle of the preconfigured file server. The upgrade options available in this situation are those offered by the manufacturer of the NAS server in question. Performance bottlenecks for more I/O-intensive applications such as databases, back-up, batch processes or multimedia applications represent a further important disadvantage of NAS servers. These are described in the following subsection.

BASICES I/O TECHNIQUES AND THE PHYSICAL I/O PATH FROM THE CPUTO THE STORAGE SYSTEM

Computers generate, process and delete data. However, they can only store data for very short periods. Therefore, computers move data to storage devices such as tape libraries and the disk subsystems discussed in the previous chapter for long-term storage and fetch it back from these storage media for further processing. So-called I/O techniques realize the data exchange between computers and storage devices. This chapter describes I/O techniques that are currently in use or that the authors believe will very probably be used in the coming years.This chapter first considers the I/O path from the CPU to the storage system (Sect-ion 3.1). An important technique for the realization of the I/O path is SCSI (Section 3.2).To be precise, SCSI defines a medium (SCSI cable) and a communication protocol (SCSI protocol). The idea of Fibre Channel SAN is to replace the SCSI cable by a network that is realized using Fibre Channel technology: servers and storage devices exchange data as before using SCSI commands, but the data is transmitted via the Fibre Channel network instead of via the SCSI cable (Sections 3.3, 3.4). An alternative to Fibre Channel SAN is IP storage. Like Fibre Channel, IP storage connects several servers and storage devices via a network on which data exchange takes place using the SCSI protocol. In contrast to Fibre Channel, however, the devices are connected by TCP/IP and Ethernet (Section 3.5). With InfiniBand, networks move even closer to the CPU. The objective of InfiniBand is to replace the PCI bus in the computer by a serial network (Section 3.6). These new transfer technologies (Fibre Channel, Gigabit Ethernet and in particular InfiniBand) form the basis for lightweight communication connections such as Virtual Interfaces (VI) and Remote Direct Memory Access (RDMA), which permit the fast and efficient exchange of data (Section 3.7). Finally we present an emerging protocol family which combinesthese lightweight communication connections with TCP/IP. Namely we present RDMAover TCP, the Socket Direct Protocol (SDP) and the iSCSI Extension for RDMA (iSER)(Section 3.8).

THE PHYSICAL I/O PATH FROM THE CPUTO THE STORAGE SYSTEM

In the computer, one or more CPUs process data that is stored in the CPU cache or in themain memory (Random Access Memory, RAM). CPU cache and main memory are very fast; however, they cannot store data after the power has been switched off. Furthermore, main memory is expensive in comparison to disk and tape storage. Therefore, the data is moved from the main memory to the storage devices such as disk subsystems and tape libraries via system bus, host bus and I/O bus (Figure 3.1). Although storage devices are slower than CPU cache and main memory, they compensate for this by being cheaper and Figure 3.1

The physical I/O path from the CPU to the storage system consists of system bus, host I/O bus and I/O bus.More recent technologies such as InfiniBand, Fibre Channel and iSCSI replace individual buses with a serial network. For historic reasons the corresponding connections are still called host I/O bus or I/O bus

THE PHYSICAL I/O PATH FROM THE CPU TO THE STORAGE SYSTEM 51

by their ability to store data even when the power is switched off. Incidentally, the sameI/O path also exists within a disk subsystem between the connection ports and the disksubsystem controller and between the controller and the internal hard disk (Figure 3.2).At the heart of the computer, the system bus ensures the rapid transfer of data between CPUs and main memory. The system bus must be timed at a very high frequency so that

it can supply the CPU with data sufficiently quickly. It is realized in the form of printed conductors on the main circuit board. Due to physical properties, high system speeds require short printed conductors. Therefore, the system bus is kept as short as possible and thus connects only CPUs and main memory.

In modern computers as many tasks as possible are moved to special processors such as graphics processors in order to free up the CPU for the processing of the application. These cannot be connected to the system bus due to the physical limitations mentioned above. Therefore, most computer architectures realize a second bus, the so-called host I/O bus. So-called bridge communication chips provide the connection between system bus and

host I/O bus. Peripheral Component Interconnect (PCI) is currently the most widespread technology for the realization of host I/O buses. InfiniBand is an emerging technology that will very probably replace the parallel PCI bus by a serial network (Section 3.6). Device drivers are responsible for the control of and communication with peripheral devices of all types. The device drivers for storage devices are partially realized in the form of software that is processed by the CPU. However, part of the device driver for the communication with storage devices is almost always realized by firmware that is processed by special processors (Application Specific Integrated Circuits, ASICs). These ASICs are currently partially integrated into the main circuit board, such as on-board SCSI controllers, or connected to the main board via add-on cards (PCI cards). These add-on cards are usually called network cards (Network Interface Controller, NIC) or simply controllers. Storage devices are connected to the server via the host bus adapter (HBA) or via the on-board controller. The communication connection between controller and peripheral device is called the I/O bus. The most important technologies for I/O buses are currently SCSI (Small Computer

System Interface) and Fibre Channel. SCSI defines a parallel bus that can connect up to 16 servers and storage devices with one another. Fibre Channel, on the other hand, defines different topologies for storage networks that can connect several millions of servers and storage devices. As an alternative to Fibre Channel, the industry is currently experimenting with different options for the realization of storage networks by means of

TCP/IP and Ethernet (IP storage). It is worth noting that all new technologies continue to use the SCSI protocol for device communication. The Virtual Interface Architecture (VIA) is a further I/O protocol. The VIA permits rapid and CPU-saving data exchange between two processes that run on two different servers or storage devices. In contrast to the I/O techniques discussed previously the Virtual Interface Architecture defines only a protocol. As a medium it requires the exis-tence of a powerful and low-error communication path, which is realized, for example, by means of Fibre Channel, Gigabit Ethernet or InfiniBand. VIA could become an important technology for storage networks and server clusters. There are numerous other I/O bus technologies on the market that will not be discussedfurther in this book, for example, Serial Storage Architecture (SSA), IEEE 1394 (Apple's Fire wire, Sony's i.Link), High-Performance Parallel Interface (HIPPI), Advanced Tech-nology Attachment (ATA)/Integrated Drive Electronics (IDE), Serial ATA (SATA) and Universal Serial Bus (USB). All have in common that they are either used by veryfew manufacturers or are not powerful enough for the connection of servers and storage devices. Some of these technologies can form small storage networks. However, none is anywhere near as flexible and scalable as the Fibre Channel and IP Storage technologiesdescribed in this book.