Download Chapter 10-11 File Systems

Chapter 10-11
File Systems
file offset pointer
per-process
current-file-position pointer
Mac OS/X files
resource fork
o data type
o creator type
data fork
Unix files
magic number
o executable file
o batch file – shell script
file extensions – provides hint but is not controlling as to use
sequence of 8-bit bytes
Text files – ascii
Executable files – relocatable object code – tables or relocatable addresses
Internal File Structure – Disk Systems
sector size  block size
disk I/O is done in units of blocks
blocks are all the same size
disks have no external fragmentation
blocks may have internal fragmentation
packing data into blocks -- may eliminate internal (block) fragmentation
but may make data more difficult to locate
packing data into bytes – unrelated to current topic, e.g., see binary coded decimals
Disk System Partitions -- segregate portions of the disk system for
File Systems
o Directory Structures
o Files -- Data Storage
Swap Space
Raw Disk Space
Remark:
A file system may span multiple disks as long
as they, the disks, are included in one partition
File System Structure
I/O transfers of data between memory and disk – blocks
block contains some number of sectors
sector size consists of some multiple of 32 bytes, normally 512 bytes
frame, page and frames are normally sized such blocks and frames contain some
multiple of one another and thus there is no inter-media fragmentation
Levels
0. I/O Control -- device drivers
Device Driver
physical block
information -drive, cylinder,
track, sector
(translates)
specific bit patterns
written to specified
memory locations or
registers
Device
Controller
disk
1. Basic File System
Manages
Memory Buffers
& Data Caches
Transfer Data Block
(in-use)
Transfer Data Block
(in-reserve)
file-system data,
directory data
Transmits
2. File-Organization Module
possibly used by
multiple file systems
Logical Block Information
Physical Block Information
File Allocation Information
Free-Space Manager – tracks unallocated blocks
3. Logical File System
File Name with path
Manages
Meta-Data
Directory Structure
File Structure
File-Control Block (FCB), i.e., UNIX inode
ownership,
permissions,
location of file contents,
last modification date
protection
security
UNIX
Unix File System (UFS)
derived from the Berkeley
Fast File System (FFS)
Windows XP
FAT, FAT32,
NTFS (Windows NT File System)
Linux
Extended File System ext2, ext3
supports 40+ other file systems
partition == volume == file system storage space
On-Disk Structures
Boot Control Block
UFS – boot block; NTFS – partition boot sector
o one per partition
o information necessary to boot operating system from the partition
o may be empty, i.e., there is no operating system on the volume
o normally it is the first block of a volume
Volume Control Block UFS – superblock; NTFS – stored as part of the Master File Table
o one per partition
o number of blocks in partition
o block size
o free-block count
o free-block pointers
o free-FCB (inode) count
o FCB (inode) pointers
Directory Structure
o one per file system
o UFS – file names & associated inode numbers
o NTFS – stored as part of the Master File Table
File Control Block (FCB), i.e., Unix inode structure
o one per file
o details about the file
o unique identifier (Unix inode) number  directory entry (Unix inode number)
o NTFS – stored as part of the Master File Table (relational database, one row per file)
In-Memory Structures
Mount Table
o contains information about each mounted volume
Directory-Structure Cache
o holds directory information about recently accessed directories
System-Wide Open-File Table
o contains a copy of the FCB, i.e., inode, of each open file
Per-Process Open-File Table
o contains a pointer to the appropriate entry in the System-Wide Open-File Table
I/O Memory Buffers
o hold file-system blocks while they are being read from or written to disk
File Creation
application process requests the creation of a new file
logical file system allocates a new FCB, i.e., inode structure, the FCB is either
o allocated from free memory
or
o selected from a pre-allocated set of free-FCB, i.e., free-inodes
appropriate directory is updated with the new file name and FCB, i.e., inode
Directories
Unix – directories are treated as files containing special data
Windows – directories differently from files;
they require a separate set of systems calls to create, manipulate, etc
Operations
open( )
o search System-Wide Open-File Table to see if file is currently in use
 if it is, create a Per-Process Open-File table entry pointing to the existing
System-Wide Open-File Table
 if it is not, search the directory for the file name; once found, place the FCB
in the System-Wide Open-File Table
o make an entry, i.e., Unix file descriptor, Windows file handle in the Per-Process
Open-File Table, with pointers to the entry in the System-Wide Open-File Table
and other fields which include a pointer to the current location in the file and
the access mode in which the file is open
o increment the open cont in the System-Wide Open-File Table
o returns a pointer to the appropriate entry in the Per-Process Open-File Table
o all subsequent operations are performed with this pointer
P0 Per-Process Open-File Table
fp0
P5
offset pointer
status information
System-Wide Open-File Table
(Unix UFS Directory System)
data location
access mode information
creation/modification date
file size
open count (files currently accessing)
File Control
Block, i.e.,
Unix inode
P0 and P1
File Control
Block, i.e.,
Unix inode
P17
fp1
P1 Per-Process Open-File Table
offset pointer
status information
File Control
Block, i.e.,
Unix inode
process closes file  Per-Process Open-File Table entry is removed; open count
decremented
all processes close file  copy in-memory directory information to disk and
System-Wide Open-File Table is removed from memory
Unix i-node System
inode 34
Unix Directory
filename
proj1
midterm
notes6July
inode “number”
34
65
108
data
data
data
data
data
Unix (UFS)
System-Wide Open-File Table holds inodes for
files, directories, devices, and network connections
inode numbering system is only unique within a given file system
across file systems, the use of a pointer is required
Mounting File Systems
Boot Block – series of sequential blocks containing a memory image of a
program, call the boot loader, that locates and mounts the root partition; the
partition contains the kernel; the boot loader locates, loads, and starts the kernel
executing
In-memory mount table – external file systems must be mounted on devices, the
mount table records the mount points, types of file systems mounted, and an
access path to the desired file system
Unix – the in-memory mount table contains a pointer to the superblock of the file
system on that device
Virtual File System
inode object -- vnodes
file object
superblock object
dentry object