COT 4600 Operating Systems Spring 2011

Dan C. Marinescu

Office: HEC 304

Office hours: Tu-Th 5:00 – 6:00 PM

Last time: Midterm

Today Solution of the midterm problems and midterm discussion

Next time Virtualization Locks

Lecture 14 – Tuesday, March 15, 2011

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Problem 1 Naming

Conceptual/abstract model: a description which retains the most important characteristics of the process/object in a given context. The model of an airplane wing The abstraction of an interpreter

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Example 1- abstract model of an interpreter – Figure 2.5

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Naming MODEL – (see Lecture 8)

Naming allows objects to refer to each other to locate another objects to determine the properties of other objects

Naming model a general scheme to resolve a name which can be applied for a spectrum of objects and implementations; examples

Searching for an address in phone book Compiling a program which uses a library Identifying the registers used during execution of a compiled program

Four components: Name space (the universe of names) Mapping function The universe of values Context

A function which has as input: a name and a context and produces a value

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Example 2: Abstract model of naming: Figure 2.10

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The role of the context for modular sharing Modular sharing: allows one module to use another module

developed independently, without the danger of name collision (page 117 of the text book)

All names encountered during the name resolution for module A are resolved using the context of A; in other words trhe context of module A points to all modules used by A (including A);

The context allows modular sharing; if both modules A and B use a function called W, but the two functions are different (call them WA and WB for convenience) then the context of A will point to the function WA and the context of B will point to WB

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Example: context used for modular sharing, Figure 3.5

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Problem 2 – UNIX file system

Lecture 8 Basic concepts related to file handling How UNIX users access a file – API Internal implementation of file operations for a generic system

How to map logical to physical organization UNIX

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B. The software layer: the file abstraction File: memory abstraction used by the application and OS layers

linear array of bits/bytes properties:

durable information will not be changed in time has a name allows access to individual bits/bytes has a cursor which defines the

current position in the file. The OS provides an API (Application Programming Interface)

supporting a range of file manipulation operations. A user must first OPEN a file before accessing it and CLOSE it after

it has finished with it. This strategy: allows different access rights (READ, WRITE, READ-WRITE) coordinate concurrent access to the file

Some file systems use OPEN and CLOSE to enforce before-or-after atomicity support all-or-nothing atomicity e.g., ensure that if the system

crashes before a CLOSE either all or none of WRITEs are carried out

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API for the Unix File SystemOPEN(name, flags, model) connect to a file Open an existing file called name, or Create a new file with permissions set to mode if flags is set. Set the file pointer (cursor) to 0. Return the file descriptor (fd).

CLOSE(fd) disconnect from a file Delete file descriptor fd.

READ(fd, buf,n) read from file Read n bytes from file fd into buf; start at the current cursor position and update the

file cursor (cursor = cursor + n).

WRITE(fd, buf,n) write to file Write n bytes to the file fd from buf; start at the current cursor position and update the

file cursor (cursor = cursor + n).

SEEK(fd, offset,whence) move cursor of file Set the cursor position of file fd to offset from the position specified by whence

(beginning, end, current position)

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API for the Unix File System (cont’d)

FSYNC(fd) make all changes to file fd durable.STAT(name) read metadata CHMOD, CHOWN change access mode/ownershipRENAME(from_name,to_name) change file nameLINK(name, link_name) create a hard linkUNLINK(name) remove name from directorySYMLINK(name, link_name) create a symbolic linkMKDIR(name) create directory nameRMDIR(name) delete directory nameCHDIR(name) change current directory to nameCHROOT Change the default root directory nameMOUNT(name,device) mount the file system name onto deviceUNMOUNT(name) unmount file system name

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Figure 2.20 and 2.22

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Generic OPEN and READ operations (Lecture 8)

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Logical and physical organization

Logical/physical records: a file consists of a set of logical records a persistent storage media (e.g., disk) is organized as a sequence of blocks/physical

records.

Hierarchical logical organization Records File Directory File system

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How to map logical to physical organization

Define a control structure which holds information about a file a directory, or a file system. For a file this information includes: Creation date Owner Size Access rights (read, write, execute) The physical location of the file

Store this meta information on the persistent storage (disk). In UNIX

call such a control structure – inode an inode describes a file, a directory, or a file system

The inode table stores the block where the inodes for directories are located

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How to access the inode for a file in UNIX

Given a path name the following steps are taken Go to Block0 to locate the physical bloc where the inode table is stored Go to the inode table to locate the inode for the root file system Search the inode of the root file system for the inode of the next directory in the

path for the file name. Reccursively search until you locate the inode of the directory containing the file. Identify the inode for the file.

Note: this process is conducted when an OPEN file name is issued; then the control information from the inode is transferred to a control block in main memory.

A READ operation does not need to access the inode table. It

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Unix OPEN and READ

If the file exists then the OPEN consists of the following steps Locate the inode of the file using the procedure described earlier Check if the operation is allowed If the operation is allowed then transfer the information from the inode to the

open file table for the process

If the file does not exists then the OPEN creates the inode and adds the file to the table of open files.

The READ only accesses the table of open files in main memory Uses the file descriptor to locate the file entry in the table of open files Determines the position of the cursor Maps the cursor to a disk block Transfers the data a from the block where the cursor is pointing at Continues transferring blocks from the disk to the memory buffer until the entire

length of the data is covered.

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Problem 3 NFS (see Lecture 13)

Developed at Sun Microsystems in early to early 1980s. Application of the client-server paradigm. Objectives:

Design a shared file system to support collaborative work Simplify the management of a set of workstations

Facilitate the backups Uniform, administrative policies

Main design goals1. Compatibility with existing applications NFS should provide the same

semantics as a local UNIX file system

2. Ease of deployment NFS implementation should be easily ported to existing systems

3. Broad scope NSF clients should be able to run under a variety of operating systems

4. Efficiency the users of the systems should not notice a substantial performance degradation when accessing a remote file system relative to access to a local file system

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NFS clients and servers Should provide transparent access to remote file systems. It mounts a remote file system in the local name space it perform

a function analogous to the MOUNT UNIX call. The remote file system is specified as Host/Path

Host the host name of the host where the remote file system is located Path local path name on the remote host.

The NFS client sends to the NFS server an RPC with the file Path information and gets back from the server a file handle A 32 bit name that uniquely identifies the remote object.

The server encodes in the file handle: A file system identifier An inode number A generation number

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Implementation

Vnode – a structure in volatile memory which abstracts if a file or directory is local or remote. A file system call (Open, Read, Write, Close, etc.) is done through the vnode-layer. Example: To Open a file a client calls PATHNAME_TO_VNODE The file name is parsed and a LOOKUP is generated

if the directory is local and the file is found the local file system creates a vnode for the file

else the LOOKUP procedure implemented by the NFS client is invoked . The file

handle of the directory and the path name are passed as arguments The NFS client invokes the LOOKUP remote procedure on the sever via an RPC The NFS server extracts the file system id and the inode number and then calls

a LOOKUP in the vnode layer. The vnode layer on the server side does LOOKUP on the local file system

passing the path name as an argument. If the local file system on the server locates the file it creates a vnode for it and

returns the vnode to the NFS server. The NFS server sends a reply to the RPC containing the file handle of the vnode

and some metadata The NFS client creates a vnode containing the file handle

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Why file handles and not path names--------------------------------- Example 1 ------------------------------------------------ Program 1 on client 1 Program 2 on client 2

CHDIR (‘dir1’)

fd OPEN(“f”, READONLY)

RENAME(‘dir1’,’dir2)

RENAME(‘dir3’,’dir1’)

READ(fd,buf,n)

To follow the UNIX specification if both clients would be on the same system client1 would read from dir2.f. If the inode number allows the client 1 to follw the same semantics rather than read from dir1/f

----------------------------------- Example 2 -----------------------------------------------

fd OPEN(“file1”, READONLY)

UNLINK(“f”)

fd OPEN(“f”,CREATE)

READ(fd,buf,n)If the NFS server reuses the inode of the old file then the RPC from client 2 will read from the

new file created by client 1. The generation number allows the NSF server to distinguish between the old file opened by client 2 and the new one created by client 1.

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Read/Write coherence

Enforcing Read/Write coherence is non-trivial even for a local operations. For performance reasons a device driver may delay a Write operation issued by

Client1; caching could cause problems when Client2 tries to Read the file.

Possible solutions Close-to-Open consistency:

Client 1 executes the sequence : Open Write Close before Client 2 executes the sequence : Open Read Close Read/Write coherence is provided

Client 1 executes the sequence : Open Write before Client 2 executes the sequence : Open Read . Read/Write coherence may or may not be provided

Consistency for every operation: no caching

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NFS Close-to-Open semantics

A client stores with each block in its cache the timestamp of the block’s vnode at the time the client got the block from the NFS server.

When a user program opens a file the client sends a GETATTR request to get the timestamp of the latest modification for the file.

The client gets a new copy only if the file has been modified since it has last accessed it; else it uses the local copy.

To implement a Write a client updates only its copy in cache without a an RPC Write.

At Close time the client sends the cached copy to the server

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