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File Systems - I
Indranil GuptaLecture 2210/17/
CS423UG Operating Systems
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CS 423UG Operating Systems,
Indranil Gupta
Content
File systems basic concepts
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CS 423UG Operating Systems,
Indranil Gupta
Question for You
FastOS : the newest, hottest and fastest OS to beinvented yet– it relies on cheap hardware (CPU, I/Odevices, memory), it has close to 100% CPUutilization, it has close to 100% I/O device utilization,and it is very easy to understand.
The only catch is – it doesn’t support the concept ofa “file” or a “directory”.
Now, you’re asked to do your MP on a FastOSmachine.
How would you type up your CS 423UG HW onFastOS?
CS 423UG Operating Systems,
Indranil Gupta
What’s the problem with Files?
Physical reality
Physical sector isunit of storage
Block oriented
No protection amongusers of the system
Data might becorrupted if machinecrashes
File system model
File is a unit of storage
File is a sequence ofbytes
Users protected fromeach other
Robust to machinefailures
CS 423UG Operating Systems,
Indranil Gupta
File System Requirements
Users must be able to:
create, modify, and delete files at will.
read, write, and modify file contents withoutworrying about blocking, buffering, etc. (in otherwords, buffering etc. should be
transparent
to the
user).
see a logical view of their own group of fileswithout concern for how they are stored. shareeach other's files with proper authorization.
transfer information between files.
refer to files by symbolic names.
retrieve backup copies of files lost throughaccident or malicious destruction.
CS 423UG Operating Systems,
Indranil Gupta
File Concepts
File
=basic unit of storage as seen as one
entity by the user
Directory
= a collection of files and/or other
directories, seen as one entity by the user
Partitions
=a group of directories accessed
as one entity
OS abstracts from the physical properties ofits storage device to define a logical storageunit, called file. Files are mapped by the OSonto physical devices.
CS 423UG Operating Systems,
Indranil Gupta
File Attributes
Name:
symbolic file name, human-readable form. Many OS’s
support two part file names (filename.filetype, e.g., foo.txt)
Type:
Regular files - user information, regular files are generally eitherbinary (e.g., .exe) or ASCII (e.g., all .cpp, all .txt)
Directories - system files for maintaining the structure of the filesystem
Character special files - related to input/output and used to modelserial I/O devices such as terminals, printers, and networks
Block special files - used to model disks
Location:
(1) pointer to a device and (2) pointer to the location
of the file on that device
Size:
current size and maximal possible size
Protection:
Access-control information, who can read, who can
write
Time, date, user identification
: creation time, last modification
time, last use time
CS 423UG Operating Systems,
Indranil Gupta
File Operations
Create a file
creat() or fopen()
Write a file
implicitly advances pointer
fwrite()
Read a file
implicitly advances pointer
fread()
Reposition within a file
lseek()
Delete a file
del or rm
Append to a file
cat
Copy or move a file
cp or mv
CS 423UG Operating Systems,
Indranil Gupta
File Structures
Byte sequence
Unstructured sequence of bytes
Record sequence
Fixed or variable length
Read or write a number of records
Tree
Records with keys
Read, insert, delete a record (typically using B-tree)
CS 423UG Operating Systems,
Indranil Gupta
File Structures Today
Sequence of bytes
Simplest to implement in kernel
Easy to manipulate in other forms
Little performance loss
More complicated structures
Hardware assistance fell out of favor
Special-purpose hardware slower, costly
CS 423UG Operating Systems,
Indranil Gupta
File Types
ASCII – plain text
A Unix executable file
header: magic number (Unix:compiled Java files have 0xCAFEBABE),
sizes, entry point, flags
Text (code)
Data
relocation bits
symbol table
Anything else in the system that needs a “name”,e.g., devices (/dev/*)
CS 423UG Operating Systems,
Indranil Gupta
So What Makes File systems Hard?
Files grow and shrink in pieces
Little
a priori
knowledge of this dynamism
Several orders of magnitude in file sizes –smallest files are 0B large, largest are a fewGB’s-TeraBytes’s (that’s 1024 GB’s)
Need to overcome/hide/mask diskperformance behavior
Desire for efficiency
Coping with failure (of devices, or by users)
CS 423UG Operating Systems,
Indranil Gupta
Reading And Writing
So, you’ve opened a file…What happens when you…
read 10 bytes from a file?
write 10 bytes into an existing file?
write 4096 bytes into a file?
Problems:
Is this a valid read within the file?
Where is this data located on disk?
Can we speed up the data access (through diskcache)
CS 423UG Operating Systems,
Indranil Gupta
Reading A Block
PCB Open
file table
Metadata
read( fd, userBuf, size )
Logical
physical
read( device, phyBlock, size )
Get physical block to sysBuf
copy to userBuf
Disk device driver
Buffer cache
CS 423UG Operating Systems,
Indranil Gupta
Reminders
MP2 due today
HW4 out today, due Nov 2
Reading for this lecture: Section 6.
Reading for next lecture: Sections 6.2 and6.
