1. No category

Project: File System
Textbook: pages 501-506
Lubomir Bic
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Assignment
• Design and implement a simple file system using ldisk (a
file to emulate a physical disk)
• Overall organization:
user
driver
• Input
cr foo
op foo
wr 1 y 10
sk 1 0
rd 1 3
File
System
I/O System
(ldisk)
• Output
foo created
foo opened 1
10 bytes written
position is 0
yyy
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I/O System
• I/O system presents disk as a linear sequence of blocks:
– ldisk[L][B]
• L is the number of logical blocks on ldisk
• B is the block length (in bytes)
– implement as byte array
• I/O system interface:
read_block(int i, char *p)
write_block(int i, char *p)
– each command reads or writes an entire block (B bytes)
– memory area (*p) is also a byte array
• use type casting or conversion to r/w integers
• FS can access ldisk using only these functions (no direct
access to ldisk is allowed)
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File System -- User Interface
•
•
•
•
•
•
•
•
•
•
create(symbolic_file_name)
destroy(symbolic_file_name)
open(symbolic_file_name): return OFT index
close(index)
read(index, mem_area, count): return bytes read
write(index, mem_area, count): return #bytes written
lseek(index, pos)
directory: return list of files
init(file.txt): restore ldisk from file.txt or create new (if no file)
save(file.txt): save ldisk to file.txt
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Organization of the file system
• directory
– single flat list of all files
– implemented as one regular file
• use regular file operations: read, write, lseek
– organized as unsorted array of fixed-size slots
– each slot contains:
• symbolic name (4 bytes max)
• index of descriptor (int)
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Organization of the file system
• file descriptors
– kept in dedicated k disk blocks (ldisk[1..k])
– each contains: length (bytes), disk map
– disk map: fixed list of max 3 disk blocks
– descriptor 0 is reserved for directory
• free storage management
– bit map
– kept in dedicated disk block 0
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Organization of the file system
ldisk[0]...[k]
file descriptors
…
bit map
ldisk[k+1] ...
Data blocks …
Descriptor for FS directory
Each descriptor
Block numbers
length
File length in bytes
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Create a file
• cr abc
– find a free file descriptor
– find a free directory entry
– fill both entries
0
bit map
len
i
…
free
…
bit map
len
…
free
0
… abc i
…
…
…
…
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Destroy a file
•
•
•
•
•
search directory to find file descriptor
remove directory entry
update bit map (if file was not empty)
free file descriptor
return status
bit map
len
…
0
… abc i
…
…
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Open a file
OFT:
current position
r/w buffer
index
...

j:
...
•
•
•
•
•
•
...
j: block 0
...
0
i
search directory to find index of file descriptor (i)
allocate a free OFT entry (reuse deleted entries)
fill in current position (0) and file descriptor index (i)
read block 0 of file into the r/w buffer (read-ahead)
return OFT index (j) (or return error)
consider adding a file length field (to simplify checking)
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Close a file
•
•
•
•
write buffer to disk
update file length in descriptor
free OFT entry
return status
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Read an (open) file
• compute position in the r/w buffer
• copy from buffer to memory until
1. desired count or end of file is reached:
• update current position, return status
2. end of buffer is reached
• write the buffer to disk
• read the next block
• continue copying
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Write a file
• compute position in the r/w buffer
• copy from memory into buffer until
1. desired count or end of file is reached:
• update current pos, return status
2. end of buffer is reached
• if block does not exist yet (file is expanding):
– allocate new block (search and update bit map)
– update file descriptor with new block number
• write the buffer to disk block
• continue copying
– update file length in descriptor
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Seek in a file
• if the new position is not within the current block
– write the buffer to disk
– read the new block
• set the current position to the new position
• return status
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List the directory
• read directory file
• for each non-empty entry print file name
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The Bit Map (pg 217)
• BM size: # of bits needed = # of ldisk blocks
• represent bit map as an array of int (32 bits each):
BM[n]
• How to set, reset, and search for bits in BM?
• prepare a mask array: MASK[32]
– diagonal contains “1”, all other fields are “0”
– use bit operations (bitwise or/and) to manipulate bits
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The Bit Map
• MASK (assume 16 bits only)
0
10…
1
010…
2
0010…
3
00010…
…
…
15
0 … 01
• to set bit i of BM[j] to “1”:
BM[j] = BM[j] | MASK[i]
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The Bit Map
• how to create MASK?
MASK[0] = 0x8000 (1000 0000 0000 0000)
MASK[1] = 0x4000 (0100 0000 0000 0000)
MASK[2] = 0x2000 (0010 0000 0000 0000)
MASK[3] = 0x1000 (0001 0000 0000 0000)
MASK[4] = 0x0800 (0000 1000 0000 0000)
…
MASK[15] = 0x0001 (0000 0000 0000 0001)
• another approach:
MASK[15] = 1;
MASK[i] = MASK[i+1] <<
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The Bit Map
• to set a bit to “0”:
– create MASK2, where MASK2[i] = ~MASK[i]
e.g., 0010 0000 0000 0000  1101 1111 1111 1111
• set bit i of BM[j] to “0”:
BM[j] = BM[j] & MASK2[i]
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The Bit Map
• to search for a bit equal to “0” in BM:
for (i=0; …
/* search BM from the beginning
for (j=0; … /* check each bit in BM[i] for “0”
test = BM[i] & MASK[j])
if (test == 0) then
bit j of BM[i] is “0”;
stop search
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Disk and FS Specifications
•
•
•
•
•
•
ldisk: 64 blocks
block = 64 B =16 integer
block 0 holds bitmap: 64 bits (one per block) = 2 integers
Q: how many blocks to reserve for descriptors?
descriptor: 4 integers (file length plus 3 block #s)
number of descriptors depends on directory size
– each directory entry: 2 integers
• file name: maximum 4 chars, no extension (=1 int)
• descriptor index: 1 integer
– directory size = 3 blocks = 3*64 B = 48 integers
= 24 entries
• 24 descriptors = 24*4 = 96 integers = 6 blocks
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Disk and FS Specifications
• ldisk can be saved into a text file at any point with the sv
command
• ldisk can be restored from a previously saved text file
• a new empty ldisk is created if no saved file is given
– it consists of 64 blocks
– block 0 contains the initial bitmap
– next 6 blocks contain the descriptor slots
– slot 0 describes the empty directory
• directory is opened automatically with init (OFT index = 0)
• OFT has 4 entries: directory plus up to 3 other open files
• all files (including directory) must close with sv command
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Testing shell (driver)
• develop testing shell:
– repeatedly accept command (e.g. cr abc) from a file
– invoke corresponding FS function (e.g.
create(abc))
– write status/data to an output file
(e.g. abc created or error)
• project will be tested using an input file containing
multiple test sequences, each starting with the command
in (initialize or restore disk)
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Shell commands and Output
• cr <name>
– Output: <name> created
• de <name>
– Output: <name> destroyed
• op <name>
– Output: <name> opened <index>
• cl <index>
– Output: <index> closed
• rd <index> <count>
– Output: <xx...x>
• wr <index> <char> <count>
– Output: <count> bytes written
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Shell commands and Output
• sk <index> <pos>
– Output: position is <pos>
• dr
– Output: <file0> <file1> … <fileN>
• in <disk_cont.txt>
– if file does not exist, output: disk initialized
– if file does exist, output: disk restored
• sv <disk_cont.txt>
– Output: disk saved
• If any command fails, output: error
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Sample Interaction
• Input
• Output
in
cr
op
wr
wr
sk
rd
dr
sv
in
op
rd
cr
disk initialized
foo created
foo opened 1
60 bytes written
10 bytes written
position is 55
xxxxxyyyyy
foo
disk saved
disk restored
foo opened 1
xxx
error
foo
foo
1 x 60
1 y 10
1 55
1 10
dsk.txt
dsk.txt
foo
1 3
foo
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Summary of tasks
• design and implement I/O interface (ldisk array plus
read/write operations)
• design and implement FS using only read/write on ldisk
• develop test/presentation shell
• error checks on all commands
• submit documentation
• schedule testing
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