L5: Introduction to file systems (v4a)

Lesson 5 (a)File systems

Operating System Design

Bachelor in Informatics Engineering

ARCOS Group

Universidad Carlos III de Madrid

Alejandro Calderón Mateos

Goals

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1. To know the abstractions within a operating System.

2. To use monitoring and fine-tuning tools.

3. To search the best way for better solutions.


Recommended books

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1. Carretero 2007:

1. Chapter 9

1. Tanenbaum 2006(en):

1. Chap.5

2. Stallings 2005:

1. Three part

3. Silberschatz 2006:

1. Chap. 10, 11 & 12

Base Additional


Remember…

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1. To study the associated theory.� Better study the bibliography readings because the slides

are not enough.

� To add questions with answers, and proper justification.

2. To review what in class is introduced.� To do the practical Linux task step-by-step.

3. To practice the knowledge and capacities.� To do the practical tasks as soon as possible.

� To do as much exercises as possible.


Overview

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1. Introduction

2. Abstractions used

3. System software


Overview

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1. Introduction


3. System software

File systemDatabase

Storage devices

Applications


Introduction

� A photo editor (for example) has in memory its code and data.

� Each process works with its data (photo), and it might make new data (modified photo) as the result of this treatment.

7

Process

code

data

stack

code

data

stack

…

Sistemas Operativos - ARCOS

ARCOS @ UC3M


Introduction

� A photo editor (for example) has in memory its code and data.

� Each process works with its data (photo), and it might make new data (modified photo) as the result of this treatment.

� It can be several process in memory, and the operating system distributes and manages the memory.

8

Process

code

data

stack

code

data

stack

…

Operating System


ARCOS @ UC3M


Introduction

� The nowadays main memory is small, with word access and volatile.

� The data in main memory is not persistent (without electricity).

� The size only let us to store a fragment of data accessed by processor during a (small) period of time.

� It has random access to any word.

� Where data might be stored?

9

Process

code

data

stack

code

data

stack

…

Operating System


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Introduction

� Secondary memory has higher size, block access, and it is not volatile.

� Persistent data.

� To the process that create the data.

� Let us store more amount of data that main memory.

� Block based, it will need a block manager.

� Data is stored in secondary memory: hard disk, flash, etc.

10

Process

code

data

stack

code

data

stack

…

Operating System


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Introduction

� Part of the operating system is going to distributed and manages the S.M.

� The file system.

� The file system offers services for storing and retrieving data in a simple way.

� It hides the details of the S.M. using simple abstractions: files, directories, etc.

11

Process

code

data

stack

code

data

stack

…

Operating System

Res

erve

d by

O

S

…


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Introduction

� It is also possible that other applications manages the S.M.:

� Database Manager.

� The Operating System offers full access to the devices, the database manager manages it.

� It is possible a mix organization:

� File System plus Database manager

12

Process

code

data

stack

code

data

stack

…

Operating System

Res

erva

do

BB

DD

…


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Summary

� The SNIA propose an architecture for both:

� Storage Networking Industry Association

� http://www.snia.org

� The application get its data from storage devices by using file systems or database manager.

13

File systemDatabase

Storage devices

Applications


ARCOS @ UC3MSNIA: Storage Networking Industry Association (http://www.snia.org/)


Overview

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1. Introduction


3. System software

File SytemDatabase

Storage Devices

Devices

Partitions/Volumes

File Systems

Files + Directories

Applications


Introduction

� files, directories, file systems, volumes, and devices

File SytemDatabase

Storage Devices

Applications

Devices

Partitions/volumes

File Systems

Files + Directories

Physical visionLogic vision

Log

ical

vis

ion

Phy

sica

l vi

sion


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Introduction

� Be careful with expression “file system” that is used for both, naming the management software, and for the file system’s disk structures.


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File SytemDatabase

Storage Devices

Applications

Devices

Partitions/volumes

File Systems

Files + Directories


Devices

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File SytemDatabase

Storage Devices

Applications

Devices

Partitions/volumes

File Systems

Files + Directories


Devices

� Hard disk

� SSD (solid state)

� Optical system

� Etc.

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Devices

Partitions/Volumes

File Systems

Directories

Files…


Devices

� To list the PCI devices:

� To list the USB devices:

19

acaldero@arcos:~/infodso/$ lspci00:00.0 Host bridge: Intel Corporation 82Q35 Express DRAM Controller (rev 02)00:01.0 PCI bridge: Intel Corporation 82Q35 Express PCI Express Root Port (rev 02)00:03.0 Communication controller: Intel Corporation 8 2Q35 Express MEI Controller (rev 02)

00:03.2 IDE interface: Intel Corporation 82Q35 Expre ss PT IDER Controller (rev 02)00:03.3 Serial controller: Intel Corporation 82Q35 E xpress Serial KT Controller (rev 02)...

acaldero@arcos:~/infodso/$ lsusbBus 001 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hubBus 002 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub...

Bus 008 Device 001: ID 1d6b:0001 Linux Foundation 1.1 root hubBus 003 Device 002: ID 1241:1166 Belkin MI-2150 Trust MouseBus 005 Device 002: ID 0c45:600d Microdia TwinkleCam U SB camera

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Disks

� To list the disks/partitions:

� To list the block devices (disks/partitions):

20

acaldero@arcos:~/infodso/$ cat /proc/partitionsmajor minor #blocks name

8 0 2930266584 sda8 1 19998720 sda18 2 512000000 sda2

...

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acaldero@arcos:~/infodso/$ lsblkNAME MAJ:MIN RM SIZE RO TYPE MOUNTPOINTsda 8:0 0 2,7T 0 disk ├─sda1 8:1 0 19,1G 0 part├─sda2 8:2 0 488,3G 0 part /├─sda3 8:3 0 2T 0 part /mnt/sda3└─sda4 8:4 0 196,7G 0 part


Devicesstackable services

� loopback device

� File as block device

� Example of working session:

1. [1] To make an empty initial file (~128 MB):dd if=/dev/zero of= /tmp/sf-01 bs=1M count=128

2. Assign the file to a loopback device:sudo losetup /dev/loop1 /tmp/sf-01

3. Working with the device… /dev/loop1

4. Unassign the device:losetup –d /dev/loop1

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Devices

…

Partitions/Volumes

File Systems

Directories

Files


Devicesstackable services

� md device

� Bonding devices as a device

� Example of working session :

1. [1] To make a mirror md device:mdadm --create --verbose /dev/md5 --level=1

--raid-devices=2 /dev/loop1 /dev/loop2

2. Working with the device:… /dev/md5

3. To stop the md device:mdadm --stop /dev/md5

4. To start the md device:

mdadm --assemble /dev/md5 /dev/loop1 /dev/loop2

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Dev.

…

Dev.

Partitions/Volumes

File Systems

Directories

Files


Partitions/volumes

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File SytemDatabase

Storage Devices

Applications

Devices

Partitions/volumes

File Systems

Files + Directories


� A partition is a portion of disk with its own identity, and can be used by

the operating system as an independent logical entity.

� A file system container (or data management system).

Partitions/volumes

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Part

ition

tabl

e Primary partition 1(active)

Primary partition 2(non-active)

First sector Last sector


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Partitions/volumes: attributes

� Typical attributes of a partition:� Kind of: primary, secondary, logic unit, bootable, etc.

� Size: partition start and end (or full size)

� Inner file system: linux, linux swap, vfat, etc.

� Identification: number of partition (order or UUID)

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Partitiontable Partition Partition


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Partitions/volumes

� To list the disks/partitions:

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acaldero@arcos:~/infodso/$ parted -l

Modelo: ATA ST3000DM001-1CH1 (scsi)Disco /dev/sda: 3001GBTamaño de sector (lógico/físico): 512B/4096BTabla de particiones. gpt

Numero Inicio Fin Tamaño Sistema de archivos Nombre Banderas1 1049kB 20,5GB 20,5GB bios_grub2 20,5GB 545GB 524GB ext43 545GB 2789GB 2245GB ntfs4 2789GB 3001GB 211GB ntfs msftdata

…

acaldero@arcos:~/infodso/$ blkid

/dev/sda4: LABEL="w10" UUID="0A568AE42B85AEDC" TYPE="ntfs" /dev/sda2: UUID="8de35b39-e3e8-43d8-a5b1-91cf76ca2e89" TYPE="ext4" /dev/sda3: UUID="2CB158D64D7CC0DD" TYPE="ntfs" …


Partitions/volumes traditional PC partition system

� The boot sectors contains the partition table.

� Primary partition, and secondary partition (with logic units)

� Old and limited:

� Only 4 partition (primaries + secondaries)

� In order to change the partition size, data has to be backup and restored

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First sector

Boo

tse

ctor

Primarypartition 1 (active)

Primarypartition 2

Fre

e sp

ace

Last sector

Extended partition

Logicalpartition

1

Logicalpartition

2


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Partitions/volumesvolumes-based system

� Logical volumes (~old partitions)over a volume group formed by physical volumes.

� Modern and flexible system:

� Without limit, dynamic change, multiple disk usage, etc.

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http://www.howtoforge.com/linux_lvm


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Partitions/volumesvolumes-based system

� To create a physical volume, a volume group, and a logical one:

1. # pvcreate /dev/sdb1

2. # vgcreate vol_infoso /dev/sdb1

3. # lvcreate –L100M –nweb vol_infoso

4. … /dev/vol_infoso/web

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File systems

30 ARCOS @ UC3M

File SytemDatabase

Storage Devices

Applications

Devices

Partitions/volumes

File Systems

Files + Directories


Sectors

� The block store device is divided into sectors, heads, and cylinders.

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Firstsector

Lastsector


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Blocks

� Block: logical group of disk sectors (2n sectors)

� It is the minimal transfer unit used by the O.S.

� To optimize the device I/O transfer efficiency.

� Users can define the block size to be used when the file system is created, otherwise the default selected by O.S. is used.

32

First block Last block

Firstsector

Lastsector


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Blocks

� Block: logical group of disk sectors (2n sectors)

� It is the minimal transfer unit used by the O.S.

� To optimize the device I/O transfer efficiency.

� Users can define the block size to be used when the file system is created, otherwise the default selected by O.S. is used.

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Block size

� The block size selection process tries to balance:

� Bandwith: large block size improve the bandwidth

� Space usage: lower block size improve the inner fragmentation (better space usage)

34

Tamaño de Bloque

ancho de banda

uso del disco

Anc

ho d

e B

anda

(K

byte

s/se

c)

Uso

del

Esp

acio

de

Dis

co

0

50

100

500

800

256 512 1K 2K 4K 8K 16K 32K

0

100 %

25

50

75

Sistemas operativos: una visión aplicada


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File systems

� A file system could manage the information inside the storage device in a structured format for the O.S:

� This format has the O.S. metainformation and user data.

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Metadata data


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File system: typical attributes

� Used space management: total number of blocks, block size, number of i-nodes, metadata size, etc.

� Free space management: free i-node/block identification.

� Entry management: for each entry (file or directory) a metadata space is reserved in order to describe it:

� General attributes: dates, permissions, owner identification, etc.

� Used block attributes: blocks used by this entry for storing its data.

� Root directory reference.

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Metadata

Boo

tblo

ck

Sup

er-b

lock

Res

ourc

eas

sign

men

t 000

001

002

003

004

i-nodes

data


File system

� To show the file system metadata:

37 ARCOS @ UC3M

acaldero@arcos:~/infodso/$ dumpe2fs /dev/sda2

dumpe2fs 1.42.9 (4-Feb-2014)Filesystem volume name: <none>Last mounted on: /Filesystem UUID: 8de35b39-e3e8-43d8-a5b1-91cf76ca2e89Filesystem magic number: 0xEF53Filesystem revision #: 1 (dynamic)Filesystem features: has_journal ext_attr resize_inode dir_index filetypeneeds_recovery extent flex_bg sparse_super large_file huge_file uninit_bgdir_nlink extra_isizeFilesystem flags: signed_directory_hashDefault mount options: user_xattr aclFilesystem state: cleanErrors behavior: ContinueFilesystem OS type: LinuxInode count: 32006144Block count: 128000000Reserved block count: 6400000Free blocks: 100051682…


File system: operations

� File system operations:� To create

� To mount

� To unmount

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Part

ition

tabl

e

Met

adat

a(s

wap

)First sector Last sector

data (swap) Metadata(ext2)

data (ext2)

� To write into disk the data structures needed for the management:

� # mkswap -c /dev/hda1 20800

� # mkfs.ext2 -c /dev/hda2 -b 8196 123100


File system: operations

� File system operations:� To create

� To mount

� To unmount

39

umount /usr

/

/lib /bin /usr

/

/d1 /d2 /d3

/d3/f2/d3/f1

/

/lib /bin /usr

/usr/d1 /usr/d3

/usr/d3/f2/usr/d3/f1

Volumen raiz(/dev/hd0)

Volumen sin montar(/dev/hd1)

mount /dev/hd1 /usr

Volumen montado

Sistemas operativos: una visión aplicada


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File systems

� Great number of file systems

40http://en.wikipedia.org/wiki/List_of_file_systems

� For storage devices:– minix (Minix)

– ext2 (Linux)

– ext3 (Linux)

– ufs (BSD)

– fat (DOS)

– vfat (win 95)

– hpfs (OS/2)

– hfs (Mac OS)

– ntfs (winNT/2K/XP)

– ...

� Special ones:– procfs (/proc)

– devFS (/dev)

– umsdos(Unix over DOS)

– …

� Network file systems:

– NFS

– CODA

– SMBFS

– NCPFS (Novell)

– …


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Alternative file systemzfs file system (Sun now Oracle)

� It simplifies the usage of devices, volumes, and file systems by the integration of all into a single extend file system:� zpool create arcos /home/ocean/disk1

� zfs create arcos/alex

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Example: volumes + file system

� To use file as block devices:

1. dd if=/dev/zero of=/tmp/disk1 bs=1G count=1

2. losetup /dev/loop2 /tmp/disk1

� To create a physical volume, a volume group, and a logical one:

1. pvcreate /dev/loop2

2. vgcreate vol_dso /dev/loop2

3. lvcreate –L100M –ntest vol_dso

� To create a file sytem, and mount it in a directory:

1. mkfs –t ext3 /dev/vol_dso/test

2. mount /dev/vol_dso/test /mnt

3. df –mh

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Example: volumes + file system

� To change the file system size:

1. umount /mnt

2. fsck –f /dev/vol_dso/test

3. resize2fs /dev/vol_dso/test 50M

� To change the volume size:

1. lvreduce -L -50m /dev/vol_dso/test

2. mount /dev/vol_dso/test /mnt

3. df –mh

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Example: integrated file system (btrfs)

� To use file as block devices:





� To create a btrfs volume/filesytem and mount it:

1. mkfs.btrfs /dev/loop3

2. mount /dev/loop3 /btrfs

3. btrfs filesystem show /dev/loop3

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ARCOS @ UC3Mhttp://www.slideshare.net/anu-bhaskar/introducing-btrfs


Example: integrated file system (btrfs)

� To change the filesystem size:

1. btrfs filesystem resize -10m /btrfs/


� To add a new device space into the filesystem:

1. btrfs device add /dev/loop4 /btrfs

2. btrfs filesystem balance /btrfs/


� To create a volume (~a directory that can be mounted as root dir.):

1. cd /btrfs/

2. btrfs subvolume create vol_infodso

3. btrfs subvolume list /btrfs/

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ARCOS @ UC3Mhttp://www.funtoo.org/BTRFS_Fun


Files

46


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File SytemDatabase

Storage Devices

Applications

Devices

Partitions/volumes

File Systems

Files + Directories


Files

� Set of related informationthat has been defined by its creator.

� Different types of information:

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Files

� The content is represented as a sequence of bytes:

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R/W position


Files

� It is translated into binary, and stored as an array (or sequence) of bytes:

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R/W position


Files: interface

� Generic interface to access to information:� descriptor ← open (name, flags, mode)

� close (descriptor)

� read (descriptor, address, size)

� write (descriptor, address, size)

� lseek (descriptor, offset, reference_of_offset)

� ioctl (descriptor, request, address_of_parameters)

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R/W position


Files: POSIX interface

51

#include <unistd.h>#include <stdlib.h>#include <stdio.h>#include <string.h>#include <fcntl.h>

int main ( int argc, char *argv[] ){int fd1 ;char str1[10] ;int nb ;

fd1 = open ("/tmp/txt1",O_CREAT|O_RDWR, S_IRWXU);

if ( -1 == fd1) {perror("open:"); exit(-1);

}

strcpy(str1,"hola");nb = write (fd1,str1,strlen(str1));printf(" bytes escritos = %d\n",nb);

close (fd1);return (0) ;

}

write#include <unistd.h>#include <stdlib.h>#include <stdio.h>#include <string.h>#include <fcntl.h>

int main ( int argc, char *argv[] ){int fd1 ;char str1[10] ;int nb, i ;

fd1 = open ("/tmp/txt1", O_RDONLY);if ( -1 == fd1) {

perror("open:"); exit(-1);

}

i=0;do {

nb = read (fd1,&(str1[i]),1); i++;} while (nb != 0) ;str1[i] = '\0';printf(“%s\n",str1);

close (fd1);return (0);

}

read

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� File information:

Files

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� data

� Information that is storedby the file.

� Metadata

� Information aboutthe file.

� Several attributes



Files

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� data


� Metadata


� Several attributes



Files

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� data


� Metadata


� Several attributes:

� Name and type

� Disk blocks, and size

� Protection

� Dates (creation, …)

� Etc.


Directories

55


ARCOS @ UC3M

File SytemDatabase

Storage Devices

Applications

Devices

Partitions/volumes

File Systems

Files + Directories


Directories

� Data structure that let us to group a number of files by using some user criteria.

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Directories

� To organize and to provide information about the structure of the file systems:

57

/

foto.jpg x.exe nota.txt

alex ana

/

boot lib home

alex ana

/

boot lib home users

� One level� 1 dir with n files

� 1 file with 1 dir.

� Tree-based� 1 dir with n entries

� 1 entry with 1 dir.

� Acyclic graph� 1 dir. with n entries

� 1 entry with n dir.

pic.jpg x.exe


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Directories

� To organize and to provide information about the structure of the file systems:

58

/

foto.jpg x.exe nota.txt

alex ana

/

boot lib home

alex ana

/

boot lib home users

� One level� 1 dir with n files

� 1 file with 1 dir.

� Tree-based� 1 dir with n entries

� 1 entry with 1 dir.

� Acyclic graph� 1 dir. with n entries

� 1 entry with n dir.

pic.jpg x.exe


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It is needed to track the referencesto the entry (to remove it):• Linked list• Reference counter


Directories

� Hierarchy names for the identification.

� Kinds of directory naming:

� Absolute path: name specification relative to the root directory. (/ in LINUX, \ in Windows)

� Relative path: name specification relative to one directory (other than root).

� Example: (being in /users/) alex/correo.txt

� Relative ones to the working/actual directory: based on the current directory where the user is (working directory)

� Special directories:

� Current directory or working directory: .

� Parent directory: ..

� User home directory: $HOME

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alex ana

/

boot lib home users


Directories: interface

� Generic interface to work with directories:� mkdir (name,mode)

� rmdir (name)

� chdir (name)

� getcwd (name, name_size)

� descriptor ← opendir (name)

� closedir (descriptor)

� entry_struct ← readdir (descriptor)

� rewindir (descriptor)

� unlink (name)

� rename (old_name, new_name)

60


alex ana

/

boot lib home users

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Directories: POSIX interface

61

#include <unistd.h>#include <sys/types.h>#include <dirent.h>#include <stdio.h>

int main ( int argc, char *argv[] ){DIR *dir1 ;struct dirent *dp ;char nombre[256] ; int ret ;

ret = chdir ("/tmp/") ;if (ret < 0) exit(-1) ;

getcwd (nombre, 256);printf("%s\n",nombre);

dir1 = opendir (nombre);if (NULL == dir1) exit(-1) ;while ( (dp = readdir (dir1)) != NULL) {

printf(“%/%s\n",nombre,dp->d_name);}closedir (dir1);

return (0) ;}

listing of /tmp Sistemas Operativos - ARCOS

To change the working directory

To print the current working directory

To open a directory working session

To read the directory entries and to print thename of each entry

To close the directory work session

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� Directory information:

� data

� “special file” that contains the entries list.

� Metadata

� Information about the directory.

� Different attributes about the directory

Directories

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files | directory


� Directory information:

� data

� “special file” that contains the entries list.

� Metadata

� Information about the directory.

� Different attributes about the directory:

Directories

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files | directory

o Name and type

o Disk blocks, and size

o Protection

o Dates (creation, …)

o Etc.


Overview

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1. Introduction


3. System softwareBenchmarks

Consistency

Reliability

File SytemDatabase

Storage Devices

Devices

Partitions/Volumes

File Systems

Files + Directories

Applications


System software

65 ARCOS @ UC3M


Benchmarks

Consistency

Reliability

File SytemDatabase

Storage Devices

Devices

Partitions/Volumes

File Systems

Files + Directories

Applications


Benchmarking

� Sometimes it is needed to know the best file system for a certain work (with some specific workload).

� Solution:� To use software (benchmarks) that let us to measure the performace

of the file system.

� There are designed to measure several aspects: latency, bandwith, number of files handled per unit of time, etc.

� Two aspects to be evaluated:

� Metadata management.

� Data management.

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Benchmarkingdata

67 http://www.phoronix.com/scan.php?page=article&item=ubuntu_1204_fs&num=1 ARCOS @ UC3M

� Examples:

� iozone, postmark, etc.


BenchmarkingMetadata

68 ARCOS @ UC3M

� Examples:

� fdtree, mdtest, etc.

http://linuxgazette.net/122/piszcz.html

http://www.linux-mag.com/id/7682/


Increase performance

� To Defrag:� To reorder the file blocks in order to place them in order

of creation, or in the order that are loaded into memory the last time, etc.

� E.g.: defrag in Windows

� E.g.: defragfs in Linux

69 ARCOS @ UC3M


Consistency in the file system

� Software failures might trigger inconsistency in the data (and metadata) stored.

� Solution:� To design some tools to check the file system in order to detect the

inconsistency, and to repair the found errors.

� Two aspect to be checked:

� To check that the physical structure of the file system is correct.

� To check that the logical structureof the file system is correct.

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Consistency in the file systemphysical structure

� Controller logic: � To test/stress the disk-controller

� E.g.: S.M.A.R.T.

71

� Disk surface:

� Disk block are read and written oneby one to check disk surfaceproblems

� E.g.: if what is read is different to whatis written then error on surface


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Consistency in the file systemlogical structure

� Disk structures: � To review disk metadata in order to check that are valid for partition,

directories, and file stored in.

� E.g.: fsck in Linux or scandisk in Windows

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Reliability in a file system

� Hardware and software failures may cause fail in a volume(information may be lost after failure)

� Solutions:

� To detect it:

� E.g.: Controler that deal with failing sectors

� Redundancy:

� Redundant Array of Independent/Inexpensive Disks

� Backup:

� Full Backup, incremental or diferential

� On disk, tapes, or remote backup

ARCOS @ UC3M73




� Solutions:

� To detect it:


� Redundancy:


� Backup:



ARCOS @ UC3M74


Redundancywith replication

ARCOS @ UC3M75 Sistemas operativos: una visión aplicada

A

B

B

A


Redundancywith parity

ARCOS @ UC3M76 Sistemas operativos: una visión aplicada

b0Parity 012b1 b2

Disk 0 Disk 1 Disk 2 Disk 3

b3 …




� Solutions:

� To detect it:


� Redundancy:


� Backup:



ARCOS @ UC3M77


BackupWhere?

� Place:

� Far from the main system to protect

� Protection against the water, fire, etc.

� Fireproof cabinets

� Media:

� Hard disk

� Adv.: capacity and price, Dis.: somehow weak

� Tape

� Adv.: capacity and price, Dis.: slow

ARCOS @ UC3M78


BackupHow?

� Full backup:A copy of all file system information.

� Differential backup:It has all files (and directories) that has been updated since the last full backup.

� Incremental backup: It has all the files (and directories) that has been updated since the last full or differential backup.

ARCOS @ UC3M79


BackupWhen?

� Off-line:

� The backup is done while the system is not used (e.g.: at midnight).

� On-line:

� The backup is done while the system is been used.

� It is needed techniques to avoid consistency problems:

� Snapshotsread-only copy of the file system

� Copy-on-writeupdates after the snapshot are done in a copy of the original block

ARCOS @ UC3M80


BackupApple Time Machine, Microsoft vault, etc.

ARCOS @ UC3M81

http://www.reghardware.com/2007/11/08/review_leopard_pt2/page2.html

http://www.genbeta.com/sistemas-operativos/primeras-imagenes-de-history-vault-el-time-machine-de-windows-8

Lesson 5 (a)File systems

Operating System Design

Bachelor in Informatics Engineering

ARCOS Group

Universidad Carlos III de Madrid

L5: Introduction to file systems (v4a)

Education

Transcript of L5: Introduction to file systems (v4a)