Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition, Chapter 16: Distributed System Structures Chapter 17: Distributed-File Systems

16.2 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Chapter Objectives To provide a high-level overview of distributed systems and the networks that interconnect them To discuss the general structure of distributed operating systems Distributed File Systems Naming and Transparency Remote File Access Stateful versus Stateless Service File Replication

16.3 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Motivation Distributed system is collection of loosely coupled processors interconnected by a communications network Processors variously called nodes, computers, machines, hosts Site is location of the processor Reasons for distributed systems Resource sharing  sharing and printing files at remote sites  processing information in a distributed database  using remote specialized hardware devices Computation speedup – load sharing Reliability – detect and recover from site failure, function transfer, reintegrate failed site Communication – message passing

16.4 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Types of Distributed Operating Systems Network Operating Systems Users are aware of multiplicity of machines. Distributed Operating Systems Users not aware of multiplicity of machines

16.5 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Network-Operating Systems Users are aware of multiplicity of machines. Access to resources of various machines is done explicitly Accomplished by: Remote logging into the appropriate remote machine (telnet, ssh) Remote Desktop (Microsoft Windows) Transferring data from remote machines to local machines, via the File Transfer Protocol (FTP) mechanism

16.6 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Network Operating System OR… networked systems Basically, the collection of elements needed to enable information exchange between people, systems, or people and systems Hardware  End points, routers, switches.. Software  Protocols, end applications.. Transmission media  Wires, air … Services  Reliability  Completeness of messages

16.7 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Network Conversations 7 Requester Replier End-to-end communication Network path Physical link path

16.8 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Communication Via ISO Network Model

16.9 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition The TCP/IP Protocol Layers

16.10 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Communication Protocol Physical layer – handles the mechanical and electrical details of the physical transmission of a bit stream Data-link layer – handles the frames, or fixed-length parts of packets, including any error detection and recovery that occurred in the physical layer Network layer – provides connections and routes packets in the communication network, including handling the address of outgoing packets, decoding the address of incoming packets, and maintaining routing information for proper response to changing load levels The communication network is partitioned into the following multiple layers:

16.11 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Communication Protocol (Cont.) Transport layer – responsible for low-level network access and for message transfer between clients, including partitioning messages into packets, maintaining packet order, controlling flow, and generating physical addresses Application layer – interacts directly with the users’ deals with file transfer, remote-login protocols and electronic mail, as well as schemas for distributed databases

16.12 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Network Topology Sites in the system can be physically connected in a variety of ways; they are compared with respect to the following criteria: Installation cost - How expensive is it to link the various sites in the system? Communication cost - How long does it take to send a message from site A to site B? Reliability - If a link or a site in the system fails, can the remaining sites still communicate with each other? The various topologies are depicted as graphs whose nodes correspond to sites An edge from node A to node B corresponds to a direct connection between the two sites The following six items depict various network topologies

16.13 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Network Topology

16.14 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Network Structure Local-Area Network (LAN) – designed to cover small geographical area. Multiaccess bus, ring, or star network Speed  10 – 100 megabits/second Broadcast is fast and cheap Nodes:  usually workstations and/or personal computers  a few (usually one or two) mainframes

16.15 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Depiction of typical LAN

16.16 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Network Types (Cont.) Wide-Area Network (WAN) – links geographically separated sites Point-to-point connections over long-haul lines (often leased from a phone company) Speed  – 45 megbits/second Broadcast usually requires multiple messages Nodes:  usually a high percentage of mainframes

16.17 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Communication Processors in a Wide-Area Network

16.18 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Communication Structure Naming and name resolution - How do two processes locate each other to communicate? Routing strategies - How are messages sent through the network? Connection strategies - How do two processes send a sequence of messages? Contention - The network is a shared resource, so how do we resolve conflicting demands for its use? The design of a communication network must address four basic issues:

16.19 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Naming and Name Resolution Name systems in the network IP (Internet Protocol) Address Address messages with the process-id Identify processes on remote systems by pair Domain name service (DNS) – specifies the naming structure of the hosts, as well as name to address resolution (Internet)

16.20 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Routing Strategies Fixed routing - A path from A to B is specified in advance; path changes only if a hardware failure disables it Since the shortest path is usually chosen, communication costs are minimized Ensures that messages will be delivered in the order in which they were sent Virtual circuit - A path from A to B is fixed for the duration of one session. Different sessions involving messages from A to B may have different paths Partial remedy to adapting to load changes Ensures that messages will be delivered in the order in which they were sent

16.21 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Routing Strategies (Cont.) Dynamic routing - The path used to send a message form site A to site B is chosen only when a message is sent Usually a site sends a message to another site on the link least used at that particular time Adapts to load changes by avoiding routing messages on heavily used path Messages may arrive out of order  This problem can be remedied by appending a sequence number to each message

16.22 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Connection Strategies Circuit switching - A permanent physical link is established for the duration of the communication (i.e., telephone system) Message switching - A temporary link is established for the duration of one message transfer (i.e., post-office mailing system) Packet switching - Messages of variable length are divided into fixed- length packets which are sent to the destination Each packet may take a different path through the network The packets must be reassembled into messages as they arrive Circuit switching requires setup time, but incurs less overhead for shipping each message, and may waste network bandwidth Message and packet switching require less setup time, but incur more overhead per message

16.23 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Contention CSMA/CD - Carrier sense with multiple access (CSMA); collision detection (CD) A site determines whether another message is currently being transmitted over that link. If two or more sites begin transmitting at exactly the same time, then they will register a CD and will stop transmitting When the system is very busy, many collisions may occur, and thus performance may be degraded CSMA/CD is used successfully in the Ethernet system, the most common network system Several sites may want to transmit information over a link simultaneously. Techniques to avoid repeated collisions include:

16.24 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Contention (Cont.) Token passing - A unique message type, known as a token, continuously circulates in the system (usually a ring structure) A site that wants to transmit information must wait until the token arrives When the site completes its round of message passing, it retransmits the token A token-passing scheme is used by some IBM and HP/Apollo systems Message slots - A number of fixed-length message slots continuously circulate in the system (usually a ring structure) Since a slot can contain only fixed-sized messages, a single logical message may have to be broken down into a number of smaller packets, each of which is sent in a separate slot This scheme has been adopted in the experimental Cambridge Digital Communication Ring

16.25 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Robustness Failure detection Reconfiguration

16.26 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Failure Detection Detecting hardware failure is difficult To detect a link failure, a handshaking protocol can be used Assume Site A and Site B have established a link At fixed intervals, each site will exchange an I-am-up message indicating that they are up and running If Site A does not receive a message within the fixed interval, it assumes either (a) the other site is not up or (b) the message was lost Site A can now send an Are-you-up? message to Site B If Site A does not receive a reply, it can repeat the message or try an alternate route to Site B

16.27 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Failure Detection (cont) If Site A does not ultimately receive a reply from Site B, it concludes some type of failure has occurred Types of failures: - Site B is down - The direct link between A and B is down - The alternate link from A to B is down - The message has been lost However, Site A cannot determine exactly why the failure has occurred

16.28 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Reconfiguration When Site A determines a failure has occurred, it must reconfigure the system: 1. If the link from A to B has failed, this must be broadcast to every site in the system 2. If a site has failed, every other site must also be notified indicating that the services offered by the failed site are no longer available When the link or the site becomes available again, this information must again be broadcast to all other sites

16.29 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Design Issues Transparency – the distributed system should appear as a conventional, centralized system to the user Fault tolerance – the distributed system should continue to function in the face of failure Scalability – as demands increase, the system should easily accept the addition of new resources to accommodate the increased demand Clusters – a collection of semi-autonomous machines that acts as a single system

Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition, End of Chapter 16

Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition, Chapter 17: Distributed-File Systems

16.32 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Distributed-Operating Systems Users not aware of multiplicity of machines Access to remote resources similar to access to local resources Accomplished by Data Migration – transfer data by transferring entire file, or transferring only those portions of the file necessary for the immediate task Computation Migration – transfer the computation, rather than the data, across the system Process Migration -execute an entire process, or parts of it, at different sites

16.33 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Distributed-Operating Systems (Cont.) Process Migration – execute an entire process, or parts of it, at different sites Load balancing – distribute processes across network to even the workload Computation speedup – subprocesses can run concurrently on different sites Hardware preference – process execution may require specialized processor Software preference – required software may be available at only a particular site Data access – run process remotely, rather than transfer all data locally

16.34 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Distributed-File Systems

16.35 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Background Distributed file system (DFS) – a distributed implementation of the classical time-sharing model of a file system, where multiple users share files and storage resources A DFS manages set of dispersed storage devices Overall storage space managed by a DFS is composed of different, remotely located, smaller storage spaces

16.36 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition DFS Structure Service – software entity running on one or more machines and providing a particular type of function to a priori unknown clients Server – service software running on a single machine Client – process that can invoke a service using a set of operations that forms its client interface File can be replicated or stored at multiple sites

16.37 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition DFS Structure GOAL Client doesn’t know if the service or file is local or not!! A client interface for a file service is formed by a set of primitive file operations (create, delete, read, write) Client interface of a DFS should be transparent, i.e., not distinguish between local and remote files

16.38 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Naming and Transparency Naming – mapping between logical and physical objects Multilevel mapping – abstraction of a file that hides the details of how and where on the disk the file is actually stored A transparent DFS hides the location where in the network the file is stored For a file being replicated in several sites, the mapping returns a set of the locations of this file’s replicas; both the existence of multiple copies and their location are hidden

16.39 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Naming Structures Location transparency – file name does not reveal the file’s physical storage location Location independence – file name does not need to be changed when the file’s physical storage location changes

16.40 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Naming Schemes — Three Main Approaches Files named by combination of their host name and local name; guarantees a unique systemwide name Attach remote directories to local directories, giving the appearance of a coherent directory tree; only previously mounted remote directories can be accessed transparently Total integration of the component file systems A single global name structure spans all the files in the system If a server is unavailable, some arbitrary set of directories on different machines also becomes unavailable

16.41 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Remote File Access Remote-service mechanism is one transfer approach Data is returned for each request Caching Scheme: Reduce network traffic by retaining recently accessed disk blocks in a cache, so that repeated accesses to the same information can be handled locally Files identified with one master copy residing at the server machine, but copies of (parts of) the file are scattered in different caches Cache-consistency problem – keeping the cached copies consistent with the master file  Could be called network virtual memory

16.42 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Cache Update Policy Write-through – write data through to disk as soon as they are placed on any cache Reliable, but poor performance Delayed-write – modifications written to the cache and then written through to the server later Write accesses complete quickly; some data may be overwritten before they are written back, and so need never be written at all Poor reliability; unwritten data will be lost whenever a user machine crashes Variation – scan cache at regular intervals and flush blocks that have been modified since the last scan Variation – write-on-close, writes data back to the server when the file is closed  Best for files that are open for long periods and frequently modified

16.43 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Consistency Is locally cached copy of the data consistent with the master copy? Client-initiated approach Client initiates a validity check Server checks whether the local data are consistent with the master copy Server-initiated approach Server records, for each client, the (parts of) files it caches When server detects a potential inconsistency, it must react

16.44 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Comparing Caching and Remote Service Cashing Many remote accesses handled efficiently by the local cache; most remote accesses will be served as fast as local ones Servers are contracted only occasionally in caching (rather than for each access)  Reduces server load and network traffic  Enhances potential for scalability Remote Service Handles every remote access across the network; penalty in network traffic, server load, and performance Total network overhead in transmitting big chunks of data (caching) is lower than a series of responses to specific requests (remote-service)

16.45 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition File Service Stateful File Service The server tracks each file being accessed by each client Stateless file service The server simply provides blocks as they are requested by the client without knowledge of how those blocks are used.

16.46 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Stateful File Service Mechanism Client opens a file Server fetches information about the file from its disk, stores it in its memory, and gives the client a connection identifier unique to the client and the open file Identifier is used for subsequent accesses until the session ends Server must reclaim the main-memory space used by clients who are no longer active Increased performance Fewer disk accesses Stateful server knows if a file was opened for sequential access and can thus read ahead the next blocks

16.47 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Stateless File Server Avoids state information by making each request self-contained Each request identifies the file and position in the file No need to establish and terminate a connection by open and close operations

16.48 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Distinctions Between Stateful & Stateless Service Failure Recovery A stateful server loses all its volatile state in a crash  Restore state by recovery protocol based on a dialog with clients, or abort operations that were underway when the crash occurred  Server needs to be aware of client failures in order to reclaim space allocated to record the state of crashed client processes (orphan detection and elimination) With stateless server, the effects of server failure sand recovery are almost unnoticeable  A newly reincarnated server can respond to a self- contained request without any difficulty

16.49 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Distinctions (Cont.) Penalties for using the robust stateless service: longer request messages slower request processing additional constraints imposed on DFS design Some environments require stateful service A server employing server-initiated cache validation cannot provide stateless service, since it maintains a record of which files are cached by which clients

16.50 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition File Replication Replicas of the same file reside on independent machines Improves availability and can shorten service time Redundancy Issues: Naming scheme maps a replicated file name to a particular replica Existence of replicas should be invisible to higher levels Replicas must be distinguished from one another by different lower-level names Updates – replicas of a file denote the same logical entity, and thus an update to any replica must be reflected on all other replicas

16.51 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition An Example: AFS A distributed computing environment (Andrew) under development since 1983 at Carnegie-Mellon University, purchased by IBM and released as Transarc DFS, now open sourced as OpenAFS AFS tries to solve complex issues such as uniform name space, location-independent file sharing, client-side caching (with cache consistency), secure authentication (via Kerberos) Also includes server-side caching (via replicas), high availability Can span 5,000 workstations

16.52 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition ANDREW (Cont.) Clients are presented with a partitioned space of file names: a local name space and a shared name space Dedicated servers, called Vice, present the shared name space to the clients as an homogeneous, identical, and location transparent file hierarchy The local name space is the root file system of a workstation, from which the shared name space descends Workstations run the Virtue protocol to communicate with Vice, and are required to have local disks where they store their local name space Servers collectively are responsible for the storage and management of the shared name space

16.53 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition ANDREW (Cont.) Clients and servers are structured in clusters interconnected by a backbone LAN A cluster consists of a collection of workstations and a cluster server and is connected to the backbone by a router A key mechanism selected for remote file operations is whole file caching Opening a file causes it to be cached, in its entirety, on the local disk

16.54 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition ANDREW Shared Name Space Andrew’s volumes are small component units associated with the files of a single client A fid identifies a Vice file or directory - A fid is 96 bits long and has three equal-length components: volume number vnode number – index into an array containing the inodes of files in a single volume uniquifier – allows reuse of vnode numbers, thereby keeping certain data structures, compact Fids are location transparent; therefore, file movements from server to server do not invalidate cached directory contents Location information is kept on a volume basis, and the information is replicated on each server

16.55 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition ANDREW File Operations Andrew caches entire files form servers A client workstation interacts with Vice servers only during opening and closing of files Venus – caches files from Vice when they are opened, and stores modified copies of files back when they are closed Reading and writing bytes of a file are done by the kernel without Venus intervention on the cached copy Venus caches contents of directories and symbolic links, for path- name translation Exceptions to the caching policy are modifications to directories that are made directly on the server responsibility for that directory

16.56 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition ANDREW Implementation Client processes are interfaced to a UNIX kernel with the usual set of system calls Venus carries out path-name translation component by component The UNIX file system is used as a low-level storage system for both servers and clients The client cache is a local directory on the workstation’s disk Both Venus and server processes access UNIX files directly by their inodes to avoid the expensive path name-to-inode translation routine

16.57 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition ANDREW Implementation (Cont.) Venus manages two separate caches: one for status one for data LRU algorithm used to keep each of them bounded in size The status cache is kept in virtual memory to allow rapid servicing of stat() (file status returning) system calls The data cache is resident on the local disk, but the UNIX I/O buffering mechanism does some caching of the disk blocks in memory that are transparent to Venus

Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition, End of Chapter 17

16.59 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Fig