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Silberschatz, Galvin and Gagne ©2011Operating System Concepts essentials – 8th Edition

Chapter 9: File-System Interface

9.2 Silberschatz, Galvin and Gagne ©2011Operating System Concepts Essentials – 8th Edition

File Concept

Uniform logical view of information storage (no matter the medium)

OS abstracts from physical properties into a logical storage unit, the file

Files mapped onto physical devices, usually nonvolatile

File is a collection of related information

Smallest allotment of nameable storage

Contiguous logical address space

Types:

Data

numeric

character

binary

Program

May be free form or rigidly formed (structured)


File Structure

None - sequence of words, bytes Simple record structure

Lines Fixed length Variable length

Complex Structures Formatted document Relocatable load file

Can simulate last two with first method by inserting appropriate control characters Who decides:

Operating system Program / programmer


File Attributes

Name – only information kept in human-readable form

Identifier – unique tag (number) identifies file within file system

Type – needed for systems that support different types

Location – pointer to file location on device

Size – current file size

Protection – controls who can do reading, writing, executing

Time, date, and user identification – data for protection, security, and usage monitoring

Information about files are kept in the directory structure, which is maintained on the disk

Typically file’s name and identifier

Identifier locates other file attributes

Attributes may be > 1KB

Directory structures may be > 1MB


File Operations

File is an abstract data type

Operations include the following (and usually more)

Create – find space, add entry to directory

Write – write data at current file position pointer location and update pointer

Read – read file contents at pointer location, update pointer

Reposition within file (seek) – change pointer location

Delete – free space and remove entry from directory

Truncate – delete data starting at pointer


Open Files and file data structure

Open(Fi) – allow process to access a file

Returns a file handle for system call reference to the file

Search the directory structure on disk for entry Fi, and move the content or cache some of entry to memory

Close(file handle) – end processes’ access to the file

Move the content of entry Fi in memory to directory structure on disk

Usually a global table containing process-independent open file information

Size

Access dates

Disk location of the file: cache of data access information

File-open count: counter of number of times a file is open

To allow removal of data from open-file table when last processes closes it

Per-process open file table contains pertinent info, plus pointer to entry in global open file table

Current file position pointer: pointer to next read/write location

Access rights: per-process access mode information

read, write, append


Open File Locking

Provided by some operating systems and file systems

Mediates access to a file

shared

exclusive

Mandatory or advisory:

Mandatory – access is denied depending on locks held and requested

Advisory – processes can find status of locks and decide what to do


File Types

Most operating systems recognize file types

Filename extension

I.e. resume.doc, server.java, readerthread.c

Most support them

Automatically open a type of file via a specific application (.doc)

Only execute files of a given extension (.exe, .com)

Run files of a given type via a scripting language (.bat)

Can get more advanced

If source code modified since executable compiled, if attempt made to execute, recompile and then execute (TOPS-20)

Mac OS encodes creating program’s name in file attributes

Double clicking on file passes the file name to appropriate application

Unix has magic number stored in file at first byte indicating file type


File Types – Name, Extension


Sequential-access File


Simulation of Sequential Access on Direct-access File


Example of Index and Relative Files


Disk Structure

Disk can be subdivided into partitions

Disks or partitions can be RAID protected against failure

Disk or partition can be used raw – without a file system, or formatted with a file system

Partitions also known as minidisks, slices

Entity containing file system known as a volume

Each volume containing file system also tracks that file system’s info in device directory or volume table of contents or directory)

Records information for all files on the volume

As well as general-purpose file systems there are many special-purpose file systems, frequently all within the same operating system or computer


A Typical File-system Organization


File System Types

Operating systems have multiple file system types

One or more general-purpose (for storing user files)

One or more special-purpose, i.e.

tmpfs—“temporary” file system in volatile main memory, contents erased if the system reboots or crashes

objfs—a “virtual” file system (essentially an interface to the kernel that looks like a file system) that gives debuggers access to kernel symbols

ctfs— a virtual file system that maintains “contract” information to manage which processes start when the system boots and must continue to run during operation

lofs—a “loop back” file system that allows one file system to be accessed in place of another one

procfs—a virtual file system that presents information on all processes as a file system


Directory Overview and organization

Directory similar to symbol table translating file names to their directory entries

Can be organized in many ways

Organization needs to support operations including:

Search for a file or multiple files

Create a file

Delete a file

List a directory

Rename a file

Traverse the file system

Should have the features

Efficiency – locating a file quickly

Naming – convenient to users

Two users can have same name for different files

The same file can have several different names

Grouping – logical grouping of files by properties, (e.g., all Java programs, all games, …) or arbitrarily


Single-Level Directory

A single directory for all users

Naming problem

Grouping problem


Two-Level Directory

Separate directory for each user

Path name

Can have the same file name for different users

Efficient searching

No grouping capability


Added Directory Concepts

Many variations, but some components essential

Idea of current directory – default location for activities

Now need a path specification

If file is in current directory, just name it

If in another directory, must specify by more detailed name

Also need way to specify different filesystems

MS-DOS gives letter to each volume, “\” separates directory name from file name – C:\userb\test

VMS uses letter for volume and “[]” for directory specification – u:[sst.jdeck]login.com;1

Note the support for versions via the trailing number

Unix treats volume name as part of directory name - /u/pbg/test

Many Oses search a set of paths for command names

“ls” might search in current directory then in system directories


Tree-Structured Directories


Tree-Structured Directories (Cont.)

Most common

For example, allows users to can create directories within their directory

Directory can then contain files or other directories

Directory can be another file with defined formatting and attribute indicating its type

Separate system calls to manage directory actions

Absolute path is full specification of file local - /foo/bar/baz

Relative path is location relative to current directory - ../baz

Efficient searching

Search path

Grouping Capability

Current directory (working directory)

cd /spell/mail/prog

type list


Tree-Structured Directories (Cont)

Creating a new file is done in current directory

Delete a file

rm <file-name>

Creating a new subdirectory is done in current directory

mkdir <dir-name>

Example: if in current directory /mail

mkdir count

mail

prog copy prt exp count

Deleting “mail” deleting the entire subtree rooted by “mail”?

• Make users manually delete contents (and subcontents) first (MS-DOS)

• Provide an option to delete all contents (Unix)


Acyclic-Graph Directories

Have shared subdirectories and files


Acyclic-Graph Directories (Cont.)

Adds ability to directly share directories between users

But can now have multiple absolute paths to the same file

Two different names (aliasing)

If dict deletes list dangling pointer

Solutions:

Backpointers, so we can delete all pointersVariable size records a problem

Entry-hold-count solution

New directory entry type

Link – another name (pointer) to an existing file

Indirect pointer

Delete link separate from the files

Hard and symbolic

Resolve the link – follow pointer to locate the file


General Graph Directory


General Graph Directory (Cont.)

How do we guarantee no cycles?

Allow only links to file not subdirectories

Garbage collection

Every time a new link is added use a cycle detection algorithm to determine whether it is OK

Or just bypass links during directory traversal


File System Mounting

A file system must be mounted before it can be accessed

Privileged operation

First check for valid file system on volume

Kernel data structure to track mount points

Some systems have separate designation for mount point (i.e. “c:”)

Others integrate mounted file systems into existing directory naming system

In separate space (i.e. /volumes) or within current name space

A unmounted file system on /device/dsk (i.e., Fig. 11-11(b)) is mounted at a mount point

What if the mount point already has contents?

Configuration file or data structure to track default mounts

Used at reboot or to reset mounts

What if files are open on a device that is being unmounted?


(a) Existing (b) Unmounted Partition


Mount Point


File Sharing

Sharing of files on multi-user systems is desirable

Sharing may be done through a protection scheme

On distributed systems, files may be shared across a network

Network File System (NFS) is a common distributed file-sharing method

File sharing – Multiple users: User IDs identify users, allowing permissions and protections to be per-user

Group IDs allow users to be in groups, permitting group access rights


File Sharing – Remote File Systems

Uses networking to allow file system access between systems

Manually via programs like FTP

Automatically, seamlessly using distributed file systems

Semi automatically via the world wide web

Using FTP under the covers

Client-server model allows clients to mount remote file systems from servers

Server can serve multiple clients

Client and user-on-client identification is insecure or complicated

NFS is standard UNIX client-server file sharing protocol

CIFS is standard Windows protocol

Standard operating system file calls are translated into remote calls

Distributed Information Systems (distributed naming services) such as LDAP, DNS, NIS, Active Directory implement unified access to information needed for remote computing

LDAP / Active Directory becoming industry standard -> Secure Single Sign-on

IP addresses can be spoofed

Protect remote access via firewalls

Open file request to remote server first checked for client-to-server permissions, then user-id checked for access permissions, then file handle returned

Client process then uses file handle as it would for a local file


Protection

File owner/creator should be able to manage controlled access:

What can be done

By whom

But never forget physical security

Types of access

Read

Write

Execute

Append

Delete

List

Others can include renaming, copying, editing, etc

System calls then check for valid rights before allowing operations

Another reason for open()

Many solutions proposed and implemented


Access Lists and Groups

Mode of access: read, write, execute Three classes of users

RWXa) owner access 7 1 1 1

RWXb) group access 6 1 1 0

RWXc) public access 1 0 0 1

Ask manager to create a group (unique name), say G, and add some users to the group. For a particular file (say game) or subdirectory, define an appropriate access.

owner group public

chmod 761 game

Attach a group to a file chgrp G game


A Sample UNIX Directory Listing


Chapter 10: File System Implementation


File-System Structure

File structure

Logical storage unit

Collection of related information

File system resides on secondary storage (disks)

Provided user interface to storage, mapping logical to physical

Provides efficient and convenient access to disk by allowing data to be stored, located retrieved easily

Disk provides in-place rewrite and random access

I/O transfers performed in blocks of sectors (usually 512 bytes)

File control block – storage structure consisting of information about a file

Device driver controls the physical device

File system organized into layers


Layered File System


File System Layers

Device drivers manage I/O devices at the I/O control layer

Given commands like “read drive1, cylinder 72, track 2, sector 10, into memory location 1060” outputs low-level hardware specific commands to hardware controller

Basic file system given command like “retrieve block 123” translates to device driver

Also manages memory buffers and caches (allocation, freeing, replacement)

Buffers hold data in transit

Caches hold frequently used data

File organization module understands files, logical address, and physical blocks

Translates logical block # to physical block #

Manages free space, disk allocation


File System Layers (Cont.)

Logical file system manages metadata information

Translates file name into file number, file handle, location by maintaining file control blocks (inodes in Unix)

Directory management

Protection

Layering useful for reducing complexity and redundancy, but adds overhead and can decrease performance

Logical layers can be implemented by any coding method according to OS designer

Many file systems, sometimes many within an operating system

Each with its own format (CD-ROM is ISO 9660; Unix has UFS, FFS; Windows has FAT, FAT32, NTFS as well as floppy, CD, DVD Blu-ray, Linux has more than 40 types, with extended file system ext2 and ext3 leading; plus distributed file systems, etc)

New ones still arriving – ZFS, GoogleFS, Oracle ASM, FUSE


File-System Implementation

We have system calls at the API level, but how do we implement their functions?

On-disk and in-memory structures

Boot control block contains info needed by system to boot OS from that volume

Needed if volume contains OS, usually first block of volume

Volume control block (superblock, master file table) contains volume details

Total # of blocks, # of free blocks, block size, free block pointers or array

Directory structure organizes the files

Names and inode numbers, master file table

Per-file File Control Block (FCB) contains many details about the file

Inode number, permissions, size, dates

NFTS stores into in master file table using relational DB structures


A Typical File Control Block


In-Memory File System Structures

Mount table storing file system mounts, mount points, file system types

The following figure illustrates the necessary file system structures provided by the operating systems

Figure 12-3(a) refers to opening a file

Figure 12-3(b) refers to reading a file

Plus buffers hold data blocks from secondary storage

Open returns a file handle for subsequent use

Data from read eventually copied to specified user process memory address


In-Memory File System Structures


Directory Implementation

Linear list of file names with pointer to the data blocks

Simple to program

Time-consuming to execute

Linear search time

Could keep ordered alphabetically via linked list or use B+ tree

Hash Table – linear list with hash data structure

Decreases directory search time

Collisions – situations where two file names hash to the same location

Only good if entries are fixed size, or use chained-overflow method


Allocation Methods - Contiguous

An allocation method refers to how disk blocks are allocated for files:

Contiguous allocation – each file occupies set of contiguous blocks

Best performance in most cases

Simple – only starting location (block #) and length (number of blocks) are required

Problems include finding space for file, knowing file size, external fragmentation, need for compaction off-line (downtime) or on-line

Mapping from logical to physical

Q

LA/512

R

Block to be accessed = Q + starting address

Displacement into block = R


Contiguous Allocation of Disk Space


Extent-Based Systems

Many newer file systems (i.e., Veritas File System) use a modified contiguous allocation scheme

Extent-based file systems allocate disk blocks in extents

An extent is a contiguous block of disks

Extents are allocated for file allocation

A file consists of one or more extents


Allocation Methods - Linked

Linked allocation – each file a linked list of blocks

File ends at nil pointer

No external fragmentation

Each block contains pointer to next block

No compaction, external fragmentation

Free space management system called when new block needed

Improve efficiency by clustering blocks into groups but increases internal fragmentation

Reliability can be a problem

Locating a block can take many I/Os and disk seeks

FAT (File Allocation Table) variation

Beginning of volume has table, indexed by block number

Much like a linked list, but faster on disk and cacheable

New block allocation simple


Linked Allocation

Each file is a linked list of disk blocks: blocks may be scattered anywhere on the disk

pointerblock =


Linked Allocation

Mapping

Block to be accessed is the Qth block in the linked chain of blocks representing the file.Displacement into block = R + 1

LA/511

Q

R


Linked Allocation


File-Allocation Table


Allocation Methods - Indexed

Indexed allocation

Each file has its own index block(s) of pointers to its data blocks

Logical view

index table


Example of Indexed Allocation


Indexed Allocation (Cont.)

Need index table

Random access

Dynamic access without external fragmentation, but have overhead of index block

Mapping from logical to physical in a file of maximum size of 256K bytes and block size of 512 bytes. We need only 1 block for index table

LA/512

Q

R

Q = displacement into index tableR = displacement into block


Indexed Allocation – Mapping (Cont.)

Mapping from logical to physical in a file of unbounded length (block size of 512 words)

Linked scheme – Link blocks of index table (no limit on size)

LA / (512 x 511)Q1

R1

Q1 = block of index tableR1 is used as follows:

R1 / 512Q2

R2

Q2 = displacement into block of index tableR2 displacement into block of file:



Two-level index (4K blocks could store 1,024 four-byte pointers in outer index -> 1,048,567 data blocks and file size of up to 4GB)

LA / (512 x 512)Q1

R1

Q1 = displacement into outer-indexR1 is used as follows:

R1 / 512Q2

R2

Q2 = displacement into block of index tableR2 displacement into block of file:



outer-index

index table file


Performance

Best method depends on file access type

Contiguous great for sequential and random

Linked good for sequential, not random

Declare access type at creation -> select either contiguous or linked

Indexed more complex

Single block access could require 2 index block reads then data block read

Clustering can help improve throughput, reduce CPU overhead

Adding instructions to the execution path to save one disk I/O is reasonable

Intel Core i7 Extreme Edition 990x (2011) at 3.46Ghz = 159,000 MIPS

http://en.wikipedia.org/wiki/Instructions_per_second

Typical disk drive at 250 I/Os per second

159,000 MIPS / 250 = 630 million instructions during one disk I/O

Fast SSD drives provide 60,000 IOPS

159,000 MIPS / 60,000 = 2.65 millions instructions during one disk I/O


Free-Space Management

File system maintains free-space list to track available blocks/clusters

(Using term “block” for simplicity)

Bit vector or bit map (n blocks)

…

0 1 2 n-1

bit[i] = 1 block[i] free

0 block[i] occupied

Block number calculation

(number of bits per word) *(number of 0-value words) +offset of first 1 bit

CPUs have instructions to return offset within word of first “1” bit


Free-Space Management (Cont.)

Bit map requires extra space Example:

block size = 4KB = 212 bytes

disk size = 240 bytes (1 terabyte)

n = 240/212 = 228 bits (or 256 MB)

if clusters of 4 blocks -> 64MB of memory

Easy to get contiguous files

Linked list (free list) Cannot get contiguous space easily No waste of space No need to traverse the entire list (if # free blocks recorded)


Linked Free Space List on Disk


Free-Space Management (Cont.) Grouping

Modify linked list to store address of next n-1 free blocks in first free block, plus a pointer to next block that contains free-block-pointers (like this one)

Counting Because space is frequently contiguously used and freed, with contiguous-allocation allocation, extents, or

clustering Keep address of first free block and count of following free blocks Free space list then has entries containing addresses and counts

Space Maps Used in ZFS Divides device space into metaslab units and manages metaslabs

Given volume can contain hundreds of metaslabs Each metaslab has associated space map

Uses counting algorithm But records to log file rather than file system

Log of all block activity, in time order, in counting format Metaslab activity -> load space map into memory in balanced-tree structure, indexed by offset

Replay log into that structure Combine contiguous free blocks into single entry


Free-Space Management (Cont.)

Need to protect: Pointer to free list Bit map

Must be kept on disk Copy in memory and disk may differ Cannot allow for block[i] to have a situation where bit[i] = 1 in memory and bit[i] = 0 on disk

Solution: Set bit[i] = 1 in disk Allocate block[i] Set bit[i] = 1 in memory


Efficiency and Performance

Efficiency dependent on:

Disk allocation and directory algorithms

Types of data kept in file’s directory entry

Pre-allocation or as-needed allocation of metadata structures

Fixed-size or varying-size data structures

Performance

Keeping data and metadata close together

Buffer cache – separate section of main memory for frequently used blocks

Synchronous writes sometimes requested by apps or needed by OS

No buffering / caching – writes must hit disk before acknowledgement

Asynchronous writes more common, buffer-able, faster

Free-behind and read-ahead – techniques to optimize sequential access

Reads frequently slower than writes


Page Cache

A page cache caches pages rather than disk blocks using virtual memory techniques and addresses

Memory-mapped I/O uses a page cache

Routine I/O through the file system uses the buffer (disk) cache

This leads to the following figure


I/O Without a Unified Buffer Cache


Unified Buffer Cache

A unified buffer cache uses the same page cache to cache both memory-mapped pages and ordinary file system I/O to avoid double caching

But which caches get priority, and what replacement algorithms to use?


I/O Using a Unified Buffer Cache


Recovery

Consistency checking – compares data in directory structure with data blocks on disk, and tries to fix inconsistencies

Can be slow and sometimes fails

Use system programs to back up data from disk to another storage device (magnetic tape, other magnetic disk, optical)

Recover lost file or disk by restoring data from backup


Log Structured File Systems

Log structured (or journaling) file systems record each metadata update to the file system as a transaction

All transactions are written to a log

A transaction is considered committed once it is written to the log (sequentially)

Sometimes to a separate device or section of disk

However, the file system may not yet be updated

The transactions in the log are asynchronously written to the file system structures

When the file system structures are modified, the transaction is removed from the log

If the file system crashes, all remaining transactions in the log must still be performed

Faster recovery from crash, removes chance of inconsistency of metadata


The Sun Network File System (NFS)

An implementation and a specification of a software system for accessing remote files across LANs (or WANs)

The implementation is part of the Solaris and SunOS operating systems running on Sun workstations using an unreliable datagram protocol (UDP/IP protocol and Ethernet


NFS (Cont.)

Interconnected workstations viewed as a set of independent machines with independent file systems, which allows sharing among these file systems in a transparent manner A remote directory is mounted over a local file system directory

The mounted directory looks like an integral subtree of the local file system, replacing the subtree descending from the local directory

Specification of the remote directory for the mount operation is nontransparent; the host name of the remote directory has to be provided Files in the remote directory can then be accessed in a transparent

manner Subject to access-rights accreditation, potentially any file system (or

directory within a file system), can be mounted remotely on top of any local directory


NFS (Cont.)

NFS is designed to operate in a heterogeneous environment of different machines, operating systems, and network architectures; the NFS specifications independent of these media

This independence is achieved through the use of RPC primitives built on top of an External Data Representation (XDR) protocol used between two implementation-independent interfaces

The NFS specification distinguishes between the services provided by a mount mechanism and the actual remote-file-access services


Three Independent File Systems


Mounting in NFS

Mounts Cascading mounts


NFS Mount Protocol

Establishes initial logical connection between server and client

Mount operation includes name of remote directory to be mounted and name of server machine storing it

Mount request is mapped to corresponding RPC and forwarded to mount server running on server machine

Export list – specifies local file systems that server exports for mounting, along with names of machines that are permitted to mount them

Following a mount request that conforms to its export list, the server returns a file handle—a key for further accesses

File handle – a file-system identifier, and an inode number to identify the mounted directory within the exported file system

The mount operation changes only the user’s view and does not affect the server side


NFS Protocol

Provides a set of remote procedure calls for remote file operations. The procedures support the following operations:

searching for a file within a directory reading a set of directory entries manipulating links and directories accessing file attributes reading and writing files

NFS servers are stateless; each request has to provide a full set of arguments (NFS V4 is just coming available – very different, stateful)

Modified data must be committed to the server’s disk before results are returned to the client (lose advantages of caching)

The NFS protocol does not provide concurrency-control mechanisms


Three Major Layers of NFS Architecture

UNIX file-system interface (based on the open, read, write, and close calls, and file descriptors)

Virtual File System (VFS) layer – distinguishes local files from remote ones, and local files are further distinguished according to their file-system types

The VFS activates file-system-specific operations to handle local requests according to their file-system types

Calls the NFS protocol procedures for remote requests

NFS service layer – bottom layer of the architecture

Implements the NFS protocol


Schematic View of NFS Architecture


NFS Path-Name Translation

Performed by breaking the path into component names and performing a separate NFS lookup call for every pair of component name and directory vnode

To make lookup faster, a directory name lookup cache on the client’s side holds the vnodes for remote directory names


NFS Remote Operations

Nearly one-to-one correspondence between regular UNIX system calls and the NFS protocol RPCs (except opening and closing files)

NFS adheres to the remote-service paradigm, but employs buffering and caching techniques for the sake of performance

File-blocks cache – when a file is opened, the kernel checks with the remote server whether to fetch or revalidate the cached attributes

Cached file blocks are used only if the corresponding cached attributes are up to date

File-attribute cache – the attribute cache is updated whenever new attributes arrive from the server

Clients do not free delayed-write blocks until the server confirms that the data have been written to disk


End of Chapter 10

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