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Lectures
Lecture 34 Annotated
34 -
Outline
Intro
Files
Directories
Implementation
Layout
Addressing
Structures
Path Names
Directory Operations
File Naming
File Types
File Attributes
File Operations
Example Program
Announcements
Reading:
Intro
Files
Directories
Implementation
RAM is volatile
RAM is ill-suited for sharing
RAM is limited in size
Hard Disk Drive (HDD)
read block k
how do you find the information you want?
logical units of info created by processes
typically have a stem and extension/suffix
Regular files
hold user information
hidden files for file system structure
typically character or binary format
Directories
Soft links
Character Special files
Block Special files
abstraction for serial I/O
abstraction for disks
abstraction for disks
Create/Delete
files hidden from user
managed by OS
single-level no longer used
Specifies file location
in hierarchical system
Path contains names
between separators
hierarchical from Multics
"in-band": attributes stored directly in directory
e.g. USB FAT drives
e.g. inodes
"out-of-band": pointer to attributes in directory
Open/Close
Read
Write
Append
Seek
Get/Set Attributes
Rename
references to other files
length constraints:
special/restricted characters/names:
case sensitivity
e.g. stem.ext
extension sometimes used to determine how a
file should be opened
FAT: 8 stem chars, 3 ext chars
FAT/NTFS:
EXT4:
e.g. USBs can have different file systems than
OS, which can mess up transfer
EXT4: 255 chars
OS hides direct block accesses
how do you know which blocks are free?
how do you enforce access controls?
out of scope in this class
write block k
Solid State Drive (SSD/Flash)
Magnetic Tape (cheap)
Optical Drives (DVD/CD, largely obsolete)
High Capacity
Persistent
Concurrently Accessible
RAM is good at what it does, but has limitations:
1)
2)
3)
4)
MOS 4.1 - 4.2; 4.3.1 - 4.3.3; 4.3.7
PA 5 due Today.
Please use the Makefile to create the zip
submission.
Create your project groups on Canvas.
Please try to do this by Monday.
File Systems
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Windows: \usr\ast
UNIX: /usr/ast
Multics: >usr>ast
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1.
2.
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It is not good for long-term information storage.
Good long-term storage has 3 criteria:
Physical formats:
All are abstracted as "disk-like" block devices
Block devices represent a linear sequence of fixed-
size blocks with 2 basic operations:
Given a block device, several questions arise:
This is what the file system is for.
A file is a byte stream abstraction for disks.
OS file system is responsible for managing files and
providing a UI for interacting with files.
Different file systems have different limitations or
conventions on file names:
Information about the file (metadata)
Directories are persistent data structures that
organize and maintain information about files.
Typically stores file name and attributes
" * / : ? \ |
\0 / . ..
MOS Figure 4-5
Single-Level
Hierarchical
Can be absolute or relative
File systems typically have multiple abstraction
layers and components that are split between the
OS and the disk.
First we need to understand the file system
layout on disk.
Recall: disks can be divided into partitions with
independent file systems.
Create
also creates
`.` and `..`
Delete
Opendir
Closedir
Readdir
Rename
Link (add entry)
hard
soft
Unlink (remove entry)
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kernel has file subsystem for management
contains partition table
contains code that finds kernel loader in the
partition's file system and starts it
contains file system metadata
basically a mini OS (much more complex)
table of "Globally Unique Identifiers"
associated with each partition
faster, larger disks, more partitions
searches for GPT
logical/physical block size defined by device
disk represented as an array of logical blocks
represents the physical address of the block
kernel's block size can be different from logical
block size (usually integer multiple)
offset measured in bytes
relative to beginning of file data
maps to a block and offset within that block
kernel's file system uses its own block size
used to be "cylinder head sector"
low-level addressing you won't often see
e.g. file system type, number of blocks, etc.
kernel loader then finds and starts the kernel
partition table marks bootable partition
BIOS loads MBR and loads bootable partition's
partition boot record
file system data itself stored on disk
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e.g. on linux:
echo > foo ; ls -l foo ; du -h foo
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Applications
Logical File System
File Organization Module
Basic File System
I/O control
Devices
layered
file subsystem
device
driver
Operating System
Partition
Partition
Partition
Partition
MBR
Boot
Block
Superblock
Free Space
Mgmt.
Inodes
Root
Dir
Files &
Directories
Partition Table
entire disk
Master Boot Record (MBR)
Partition Boot Record (PBR / Boot Block)
Superblock
BIOS / MBR format
UEFI + GPT format
BIOS + MBR was slow and limited to 4 partitions
Minimal unit of allocation in a disk is a logical block
Logical Block Address is index of block on disk
OS accesses file data by file offset (abstraction)
Kernel works in terms of blocks as well
Unified Extensible Firmware Interface (UEFI)
GUID Partition Table (GPT)
MOS Figure 4-11
Disk (Block Device)
File Data
logical
block
kernel block
0
0
1
N
1
2
M
...
...
file offset