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CS-430: Operating Systems Week 8 Dr. Jesús Borrego Lead Faculty, COS Regis University 1 scis.regis.edu ● [email protected] Topics • Chapter 16 – Virtual Machines • Chapter 17 – Distributed Systems • Final project presentation due this week ▫ 20 min. each, 3/hr, 5 minute break between each ▫ Provide presentation file before class • Final Exam – take home, due Monday, 12/16, midnight 2 Chapter 16 – Virtual Machines 3 Chapter 16: Virtual Machines • Overview • History • Benefits and Features • Building Blocks • Types of Virtual Machines and Their Implementations • Virtualization and Operating-System Components • Examples 4 Overview • Fundamental idea – abstract hardware of a single computer into several different execution environments ▫ Similar to layered approach ▫ But layer creates virtual system (virtual machine, or VM) on which operation systems or applications can run 5 Overview (Cont’d) • Several components ▫ Host – underlying hardware system ▫ Virtual machine manager (VMM) or hypervisor – creates and runs virtual machines by providing interface that is identical to the host (Except in the case of paravirtualization) ▫ Guest – process provided with virtual copy of the host Usually an operating system • Single physical machine can run multiple operating systems concurrently, each in its own virtual machine 6 System Models Non-virtual machine 7 Virtual machine Implementation of VMMs • Vary greatly, with options including: ▫ Type 0 hypervisors - Hardware-based solutions that provide support for virtual machine creation and management via firmware IBM LPARs and Oracle LDOMs are examples ▫ Type 1 hypervisors - Operating-system-like software built to provide virtualization Including VMware ESX, Joyent SmartOS, and Citrix XenServer 8 Implementation of VMMs (Cont’d) ▫ Type 1 hypervisors – Also includes generalpurpose operating systems that provide standard functions as well as VMM functions Including Microsoft Windows Server with HyperV and RedHat Linux with KVM ▫ Type 2 hypervisors - Applications that run on standard operating systems but provide VMM features to guest operating systems Includeing VMware Workstation and Fusion, Parallels Desktop, and Oracle VirtualBox 9 Implementation of VMMs (cont.) • Other variations include: ▫ Paravirtualization - Technique in which the guest operating system is modified to work in cooperation with the VMM to optimize performance ▫ Programming-environment virtualization - VMMs do not virtualize real hardware but instead create an optimized virtual system Used by Oracle Java and Microsoft.Net ▫ Emulators – Allow applications written for one hardware environment to run on a very different hardware environment, such as a different type of CPU ▫ Application containment - Not virtualization at all but rather provides virtualization-like features by segregating applications from the operating system, making them more secure, manageable Including Oracle Solaris Zones, BSD Jails, and IBM AIX WPARs • Much variation due to breadth, depth and importance of virtualization in modern computing 10 History • First appeared in IBM mainframes in 1972 • Allowed multiple users to share a batch-oriented system • Formal definition of virtualization helped move it beyond IBM 1.A VMM provides an environment for programs that is essentially identical to the original machine 2.Programs running within that environment show only minor performance decreases 3.The VMM is in complete control of system resources 11 History (Cont’d) • In late 1990s Intel CPUs fast enough for researchers to try virtualizing on general purpose PCs ▫ Xen and VMware created technologies, still used today ▫ Virtualization has expanded to many OSes, CPUs, VMMs 12 Benefits and Features • Host system protected from VMs, VMs protected from each other ▫ I.e. A virus less likely to spread ▫ Sharing is provided though via shared file system volume, network communication 13 Benefits and Features (Cont’d) • Freeze, suspend, running VM ▫ Then can move or copy somewhere else and resume ▫ Snapshot of a given state, able to restore back to that state Some VMMs allow multiple snapshots per VM ▫ Clone by creating copy and running both original and copy • Great for OS research, better system development efficiency • Run multiple, different OSes on a single machine ▫ Consolidation, app dev, … 14 Benefits and Features (cont.) • Templating – create an OS + application VM, provide it to customers, use it to create multiple instances of that combination • Live migration – move a running VM from one host to another! ▫ No interruption of user access • All those features taken together -> cloud computing ▫ Using APIs, programs tell cloud infrastructure (servers, networking, storage) to create new guests, VMs, virtual desktops 15 Building Blocks • Generally difficult to provide an exact duplicate of underlying machine • Especially if only dual-mode operation available on CPU • But getting easier over time as CPU features and support for VMM improves • Most VMMs implement virtual CPU (VCPU) to represent state of CPU per guest as guest believes it to be • When guest context switched onto CPU by VMM, information from VCPU loaded and stored • Several techniques, as described in next slides 16 Building Block – Trap and Emulate • Dual mode CPU means guest executes in user mode ▫ Kernel runs in kernel mode ▫ Not safe to let guest kernel run in kernel mode too ▫ So VM needs two modes – virtual user mode and virtual kernel mode Both of which run in real user mode ▫ Actions in guest that usually cause switch to kernel mode must cause switch to virtual kernel mode 17 Trap-and-Emulate (cont.) • How does switch from virtual user mode to virtual kernel mode occur? ▫ Attempting a privileged instruction in user mode causes an error -> trap ▫ VMM gains control, analyzes error, executes operation as attempted by guest ▫ Returns control to guest in user mode ▫ Known as trap-and-emulate ▫ Most virtualization products use this at least in part 18 Trap-and-Emulate (cont.) • User mode code in guest runs at same speed as if not a guest • But kernel mode privilege mode code runs slower due to trap-and-emulate ▫ Especially a problem when multiple guests running, each needing trap-and-emulate • CPUs adding hardware support, mode CPU modes to improve virtualization performance 19 Trap-and-Emulate Virtualization Implementation 20 Building Block – Binary Translation • Some CPUs don’t have clean separation between privileged and nonprivileged instructions ▫ Earlier Intel x86 CPUs are among them Earliest Intel CPU designed for a calculator ▫ Backward compatibility means difficult to improve ▫ Consider Intel x86 popf instruction Loads CPU flags register from contents of the stack If CPU in privileged mode -> all flags replaced If CPU in user mode -> on some flags replaced No trap is generated 21 Binary Translation (cont.) Other similar problem instructions we will call special instructions Caused trap-and-emulate method considered impossible until 1998 Binary translation solves the problem Basics are simple, but implementation very complex 22 Binary Translation (cont.) 1. If guest VCPU is in user mode, guest can run instructions natively 2. If guest VCPU in kernel mode (guest believes it is in kernel mode) 1. VMM examines every instruction guest is about to execute by reading a few instructions ahead of program counter 2. Non-special-instructions run natively 3. Special instructions translated into new set of instructions that perform equivalent task (for example changing the flags in the VCPU) 23 Binary Translation (cont.) Implemented by translation of code within VMM Code reads native instructions dynamically from guest, on demand, generates native binary code that executes in place of original code Performance of this method would be poor without optimizations Products like VMware use caching Translate once, and when guest executes code containing special instruction cached translation used instead of translating again Testing showed booting Windows XP as guest caused 950,000 translations, at 3 microseconds each, or 3 second (5 %) slowdown over native 24 Binary Translation Virtualization Implementation 25 Nested Page Tables • Memory management another general challenge to VMM implementations • How can VMM keep page-table state for both guests believing they control the page tables and VMM that does control the tables? • Common method (for trap-and-emulate and binary translation) is nested page tables (NPTs) ▫ Each guest maintains page tables to translate virtual to physical addresses ▫ VMM maintains per guest NPTs to represent guest’s page-table state Just as VCPU stores guest CPU state ▫ When guest on CPU -> VMM makes that guest’s NPTs the active system page tables ▫ Guest tries to change page table -> VMM makes equivalent change to NPTs and its own page tables ▫ Can cause many more TLB misses -> much slower performance 26 Nested Page Tables 27 Types of Virtual Machines and Implementations Many variations as well as HW details Assume VMMs take advantage of HW features HW features can simplify implementation, improve performance Whatever the type, a VM has a lifecycle Created by VMM Resources assigned to it (number of cores, amount of memory, networking details, storage details) In type 0 hypervisor, resources usually dedicated Other types dedicate or share resources, or a mix When no longer needed, VM can be deleted, freeing resouces 28 Types of Virtual Machines and Implementations (Cont’d) Steps simpler, faster than with a physical machine install Can lead to virtual machine sprawl with lots of VMs, history and state difficult to track 29 Types of VMs – Type 0 Hypervisor • Old idea, under many names by HW manufacturers ▫ ▫ ▫ ▫ ▫ 30 “partitions”, “domains” A HW feature implemented by firmware OS need to nothing special, VMM is in firmware Smaller feature set than other types Each guest has dedicated HW Types of VMs – Type 0 Hypervisor • I/O a challenge as difficult to have enough devices, controllers to dedicate to each guest • Sometimes VMM implements a control partition running daemons that other guests communicate with for shared I/O • Can provide virtualization-within-virtualization (guest itself can be a VMM with guests ▫ Other types have difficulty doing this 31 Type 0 Hypervisor 32 Types of VMs – Type 1 Hypervisor • Commonly found in company datacenters ▫ In a sense becoming “datacenter operating systems” Datacenter managers control and manage OSes in new, sophisticated ways by controlling the Type 1 hypervisor Consolidation of multiple OSes and apps onto less HW Move guests between systems to balance performance Snapshots and cloning • Special purpose operating systems that run natively on HW ▫ Rather than providing system call interface, create run and manage guest OSes ▫ Can run on Type 0 hypervisors but not on other Type 1s ▫ Run in kernel mode ▫ Guests generally don’t know they are running in a VM ▫ Implement device drivers for host HW because no other component can ▫ Also provide other traditional OS services like CPU and memory management 33 Types of VMs – Type 1 Hypervisor (cont.) Another variation is a general purpose OS that also provides VMM functionality RedHat Enterprise Linux with KVM, Windows with Hyper-V, Oracle Solaris Perform normal duties as well as VMM duties Typically less feature rich than dedicated Type 1 hypervisors In many ways, treat guests OSes as just another process Albeit with special handling when guest tries to execute special instructions 34 Types of VMs – Type 2 Hypervisor • Less interesting from an OS perspective ▫ Very little OS involvement in virtualization ▫ VMM is simply another process, run and managed by host Even the host doesn’t know they are a VMM running guests ▫ Tend to have poorer overall performance because can’t take advantage of some HW features ▫ But also a benefit because require no changes to host OS Student could have Type 2 hypervisor on native host, run multiple guests, all on standard host OS such as Windows, Linux, MacOS 35 Types of VMs – Paravirtualization • Does not fit the definition of virtualization – VMM not presenting an exact duplication of underlying hardware ▫ But still useful! ▫ VMM provides services that guest must be modified to use ▫ Leads to increased performance ▫ Less needed as hardware support for VMs grows 36 Types of VMs – Paravirtualization (Cont’d) • Xen, leader in paravirtualized space, adds several techniques ▫ For example, clean and simple device abstractions Efficient I/O Good communication between guest and VMM about device I/O Each device has circular buffer shared by guest and VMM via shared memory 37 Xen I/O via Shared Circular Buffer 38 Types of VMs – Paravirtualization (cont.) Memory management does not include nested page tables Each guest has own read-only tables Guest uses hypercall (call to hypervisor) when page-table changes needed Paravirtualization allowed virtualization of older x86 CPUs (and others) without binary translation Guest had to be modified to use run on paravirtualized VMM But on modern CPUs Xen no longer requires guest modification -> no longer 39 paravirtualization Types of VMs – Programming Environment Virtualization Also not-really-virtualization but using same techniques, providing similar features Programming language is designed to run within custom-built virtualized environment For example Oracle Java has many features that depend on running in Java Virtual Machine (JVM) 40 Types of VMs – Programming Environment Virtualization In this case virtualization is defined as providing APIs that define a set of features made available to a language and programs written in that language to provide an improved execution environment JVM compiled to run on many systems (including some smart phones even) Programs written in Java run in the JVM no matter the underlying system Similar to interpreted languages 41 Types of VMs – Emulation • Another (older) way for running one operating system on a different operating system ▫ Virtualization requires underlying CPU to be same as guest was compiled for ▫ Emulation allows guest to run on different CPU • Necessary to translate all guest instructions from guest CPU to native CPU ▫ Emulation, not virtualization • Useful when host system has one architecture, guest compiled for other architecture ▫ Company replacing outdated servers with new servers containing different CPU architecture, but still want to run old applications • Performance challenge – order of magnitude slower than native code ▫ New machines faster than older machines so can reduce slowdown • Very popular – especially in gaming where old consoles emulated on new 42 Types of VMs – Application Containment • Some goals of virtualization are segregation of apps, performance and resource management, easy start, stop, move, and management of them • Can do those things without full-fledged virtualization ▫ If applications compiled for the host operating system, don’t need full virtualization to meet these goals 43 Types of VMs – Application Containment (Cont’d) • Oracle containers / zones for example create virtual layer between OS and apps ▫ Only one kernel running – host OS ▫ OS and devices are virtualized, providing resources within zone with impression that they are only processes on system ▫ Each zone has its own applications; networking stack, addresses, and ports; user accounts, etc ▫ CPU and memory resources divided between zones Zone can have its own scheduler to use those resources 44 Solaris 10 with Two Zones 45 Virtualization and Operating-System Components • Now look at operating system aspects of virtualization ▫ CPU scheduling, memory management, I/O, storage, and unique VM migration feature How do VMMs schedule CPU use when guests believe they have dedicated CPUs? How can memory management work when many guests require large amounts of memory? 46 OS Component – CPU Scheduling • Even single-CPU systems act like multiprocessor ones when virtualized One or more virtual CPUs per guest • Generally VMM has one or more physical CPUs and number of threads to run on them ▫ Guests configured with certain number of VCPUs Can be adjusted throughout life of VM 47 OS Component – CPU Scheduling (cont.) • Cycle stealing by VMM and oversubscription of CPUs means guests don’t get CPU cycles they expect ▫ Consider timesharing scheduler in a guest trying to schedule 100ms time slices -> each may take 100ms, 1 second, or longer Poor response times for users of guest Time-of-day clocks incorrect ▫ Some VMMs provide application to run in each guest to fix time-of-day and provide other integration features 48 OS Component – Memory Management • Also suffers from oversubscription -> requires extra management efficiency from VMM • For example, VMware ESX guests have a configured amount of physical memory, then ESX uses 3 methods of memory management 1. Double-paging, in which the guest page table indicates a page is in a physical frame but the VMM moves some of those pages to backing store 2. Install a pseudo-device driver in each guest (it looks like a device driver to the guest kernel but really just adds kernel-mode code to the guest) Balloon memory manager communicates with VMM and is told to allocate or deallocate memory to decrease or increase physical memory use of guest, causing guest OS to free or have more memory available 3. Deduplication by VMM determining if same page loaded more than once, memory mapping the same page into multiple guests 49 OS Component – I/O • Easier for VMMs to integrate with guests because I/O has lots of variation ▫ Already somewhat segregated / flexible via device drivers ▫ VMM can provide new devices and device drivers • But overall I/O is complicated for VMMs ▫ Many short paths for I/O in standard OSes for improved performance ▫ Less hypervisor needs to do for I/O for guests, the better ▫ Possibilities include direct device access, DMA pass-through, direct interrupt delivery Again, HW support needed for these • Networking also complex as VMM and guests all need network access ▫ VMM can bridge guest to network (allowing direct access) ▫ And / or provide network address translation (NAT) NAT address local to machine on which guest is running, VMM provides address translation to guest to hide its address 50 OS Component – Storage Management • Both boot disk and general data access need be provided by VMM • Need to support potentially dozens of guests per VMM (so standard disk partitioning not sufficient) • Type 1 – storage guest root disks and config information within file system provided by VMM as a disk image 51 OS Component – Storage Management (Cont’d) • Type 2 – store as files in file system provided by host OS • Duplicate file -> create new guest • Move file to another system -> move guest • Physical-to-virtual (P-to-V) convert native disk blocks into VMM format • Virtual-to-physical (V-to-P) convert from virtual format to native or disk format • VMM also needs to provide access to network attached storage (just networking) and other disk images, disk partitions, disks, etc 52 OS Component – Live Migration • Taking advantage of VMM features leads to new functionality not found on general operating systems such as live migration • Running guest can be moved between systems, without interrupting user access to the guest or its apps • Very useful for resource management, maintenance downtime windows, etc 1. The source VMM establishes a connection with the target VMM 2. The target creates a new guest by creating a new VCPU, etc 3. The source sends all read-only guest memory pages to the target 4. The source sends all read-write pages to the target, marking them as clean 5. The source repeats step 4, as during that step some pages were probably modified by the guest and are now dirty 6. When cycle of steps 4 and 5 becomes very short, source VMM freezes guest, sends VCPU’s final state, sends other state details, sends final dirty pages, and tells target to start running the guest Once target acknowledges that guest running, source terminates guest 53 Live Migration of Guest Between Servers 54 Examples - VMware • VMware Workstation runs on x86, provides VMM for guests • Runs as application on other native, installed host operating system -> Type 2 • Lots of guests possible, including Windows, Linux, etc all runnable concurrently (as resources allow) • Virtualization layer abstracts underlying HW, providing guest with is own virtual CPUs, memory, disk drives, network interfaces, etc • Physical disks can be provided to guests, or virtual physical disks (just files within host file system) 55 VMware Workstation Architecture 56 Examples – Java Virtual Machine Example of programming-environment virtualization Very popular language / application environment invented by Sun Microsystems in 1995 Write once, run anywhere Includes language specification (Java), API library, Java virtual machine (JVM) Java objects specified by class construct, Java program is one or more objects 57 Examples – Java Virtual Machine (Cont’d) Each Java object compiled into architecture-neutral bytecode output (.class) which JVM class loader loads JVM compiled per architecture, reads bytecode and executes Includes garbage collection to reclaim memory no longer in use Made faster by just-in-time (JIT) compiler that turns bytecodes into native code and caches them 58 The Java Virtual Machine 59 Chapter 17 – Distributed Systems 60 Overview 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 Generally a server has a resource a client node at a different site wants to use 61 Reasons for Distributed Systems • 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 or job migration ▫ Reliability – detect and recover from site failure, function transfer, reintegrate failed site 62 Reasons for Distributed Systems (Cont’d) ▫ Communication – message passing All higher-level functions of a standalone system can be expanded to encompass a distributed system ▫ Computers can be downsized, more flexibility, better user interfaces and easier maintenance by moving from large system to multiple smaller systems performing distributed computing 63 Types of Distributed Operating Systems • Network Operating Systems • Distributed Operating Systems 64 Network-Operating Systems • Users are aware of multiplicity of machines • Access to resources of various machines is done explicitly 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 • Users must change paradigms – establish a session, give network-based commands ▫ More difficult for users 65 Distributed-Operating Systems • Users not aware of multiplicity of machines ▫ Access to remote resources similar to access to local resources • 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 ▫ Via remote procedure calls (RPCs) ▫ or via messaging system 66 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 • Consider the World Wide Web 67 Network Structure • Local-Area Network (LAN) – designed to cover small geographical area ▫ Multiple topologies like star or ring ▫ Speeds from 1Mb per second (Appletalk, bluetooth) to 40 Gbps for fastest Ethernet over twisted pair copper or optical fibre ▫ Consists of multiple computers (mainframes through mobile devices), peripherals (printers, storage arrays), routers (specialized network communication processors) providing access to other networks 68 Network Structure (Cont’d) ▫ Ethernet most common way to construct LANs Multiaccess bus-based Defined by standard IEEE 802.3 ▫ Wireless spectrum (WiFi) increasingly used for networking I.e. IEEE 802.11g standard implemented at 54 Mbps 69 Local-area Network 70 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) Implemented via connection processors known as routers ▫ Internet WAN enables hosts world wide to communicate Hosts differ in all dimensions but WAN allows communications 71 Network Types (Cont.) ▫ Speeds T1 link is 1.544 Megabits per second T3 is 28 x T1s = 45 Mbps OC-12 is 622 Mbps ▫ WANs and LANs interconnect, similar to cell phone network: Cell phones use radio waves to cell towers Towers connect to other towers and hubs 72 Communication Processors in a Wide-Area Network 73 Communication Structure The design of a communication network must address four basic issues: • 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? 74 Naming and Name Resolution • Name systems in the network • Address messages with the process-id • Identify processes on remote systems by <host-name, identifier> pair • Domain name system (DNS) – specifies the naming structure of the hosts, as well as name to address resolution (Internet) 75 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 ▫ Fixed routing cannot adapt to load changes ▫ Ensures that messages will be delivered in the order in which they were sent 76 Routing Strategies (Cont’d) • Virtual routing- 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 77 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 ▫ Most complex to set up 78 Routing Strategies (Cont.) • Tradeoffs mean all methods are used ▫ UNIX provides ability to mix fixed and dynamic ▫ Hosts may have fixed routes and gateways connecting networks together may have dynamic routes 79 Routing Strategies (Cont.) • Router is communications processor responsible for routing messages • Must have at least 2 network connections • Maybe special purpose or just function running on host • Checks its tables to determine where destination host is, where to send messages ▫ Static routing – table only changed manually ▫ Dynamic routing – table changed via routing protocol 80 Routing Strategies (Cont.) • Routing managed by intelligent software more intelligently than routing protocols ▫ OpenFlow is device-independent, allowing developers to introduce network efficiencies by decoupling data-routing decisions from underlying network devices • Messages vary in length – simplified design breaks them into packets (or frames, or datagrams) • Connectionless message is just one packet ▫ Otherwise need a connection to get a multi-packet message from source to destination 81 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) 82 Connection Strategies (Cont’d) • 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 83 Communication Protocol The communication network is partitioned into the following multiple layers: • Layer 1: Physical layer – handles the mechanical and electrical details of the physical transmission of a bit stream • Layer 2: Data-link layer – handles the frames, or fixed-length parts of packets, including any error detection and recovery that occurred in the physical layer 84 Communication Protocol (Cont’d) • Layer 3: 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 85 Communication Protocol (Cont.) • Layer 4: 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 • Layer 5: Session layer – implements sessions, or process-to-process communications protocols 86 Communication Protocol (Cont.) • Layer 6: Presentation layer – resolves the differences in formats among the various sites in the network, including character conversions, and half duplex/full duplex (echoing) • Layer 7: Application layer – interacts directly with the users, deals with file transfer, remote-login protocols and electronic mail, as well as schemas for distributed databases 87 Communication Via ISO Network Model 88 The ISO Protocol Layer 89 The ISO Network Message 90 The TCP/IP Protocol Layers 91 Example: TCP/IP • The transmission of a network packet between hosts on an Ethernet network • Every host has a unique IP address and a corresponding Ethernet Media Access Control (MAC) address • Communication requires both addresses • Domain Name Service (DNS) IP addresses 92 can be used to acquire Example: TCP/IP (Cont’d) • Address Resolution Protocol (ARP) is used to map MAC addresses to IP addresses ▫ Broadcast to all other systems on the Ethernet network • If the hosts are on the same network, ARP can be used ▫ If the hosts are on different networks, the sending host will send the packet to a router which routes the packet to the destination network 93 An Ethernet Packet 94 Distributed File System • 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 • There is usually a correspondence between constituent storage spaces and sets of files • Challenges include: ▫ Naming and Transparency 95 ▫ Remote File Access 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 • A client interface for a file service is formed by a set of primitive file operations (create, delete, read, write) 96 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 97 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 98 Naming Schemes — Three Main Approaches • Files named by combination of their host name and local name; guarantees a unique system-wide name • Attach remote directories to local directories, giving the appearance of a coherent directory tree; only previously mounted remote directories can be accessed transparently 99 Naming Schemes — Three Main Approaches (Cont’d) • 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 • In practice most DFSs use static, locationtransparent mapping for user-level names ▫ Some support file migration ▫ Hadoop supports file migration but without following POSIX standards 100 Remote File Access • Remote-service mechanism is one transfer approach • Reduce network traffic by retaining recently accessed disk blocks in a cache, so that repeated accesses to the same information can be handled locally 101 Remote File Access (Cont’d) ▫ If needed data not already cached, a copy of data is brought from the server to the user ▫ Accesses are performed on the cached copy ▫ 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 102 Cache Location – Disk vs. Main Memory • Advantages of disk caches ▫ More reliable ▫ Cached data kept on disk are still there during recovery and don’t need to be fetched again • Advantages of main-memory caches: ▫ ▫ ▫ ▫ 103 Permit workstations to be diskless Data can be accessed more quickly Performance speedup in bigger memories Server caches (used to speed up disk I/O) are in main memory regardless of where user caches are located; using main-memory caches on the user machine permits a single caching mechanism for servers and users 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 (write-back) – 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 104 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 105 Oral Presentations 106 Questions! • Email to [email protected] 107