Short File System Guide 2

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File System Guide
Given a file system with:
● 4KB block size
● 6 direct block pointers
● 2 single indirect pointers
● 1 double indirect pointer
How do you calculate the maximum file size?
1. Calculate the amount of data accessible by each of the direct and indirect pointers.
a. From direct pointers = ​BLOCK SIZE x NUM DIRECT POINTERS
b. Calculate number of pointers that fit in a block = ​BLOCK SIZE / POINTER SIZE
c. From single indirect pointers = ​BLOCK SIZE x NUM POINTERS PER BLOCK x
NUM SINGLE INDIRECT POINTERS
d. From double indirect pointers = ​BLOCK SIZE x (NUM POINTERS PER
BLOCK)^2 x NUM DOUBLE INDIRECT POINTERS
e. From nth indirect pointers = ​BLOCK SIZE x (NUM POINTERS PER BLOCK)^n
x NUM NTH INDIRECT POINTERS
Example:
Note that 2^10 = 1024
From Direct Pointers = 4KB * 6 = 24KB
Number of Pointers per Block = 4KB / 4 bytes = 1024
From Single Indirect = 4KB * 1024 * 2 = 8192KB
From Double Indirect = 4KB * 1024^2 * 1 = 2^2 * 2^20 = 2^22KB
2. Sum the values for each type of pointer.
Example:
Maximum File Size = Direct + Single + Double
= 24 + 8192 + 2^22KB
= 8216 + 2^22KB

3. Remember that addressing bytes of a file starts at 0. Therefore, the bytes that can be
found through direct pointers are: [0, NUM DIRECT PTRS * BLOCK SIZE - 1].
Opening a File for Read or Write
1. Find the i-number of the file; open its i-node and check permissions, etc.
a. Start by opening the root i-node to retrieve a pointer to the roots data, which is
where the hard links can be found.
b. Open roots data block, retrieve the i-number of the next element of the path.

c. Repeat this process until the i-number for the file is found.
Example: Open /usr/bin/lua51 for reading.
Read root i-node.
Read root data; retrieve usr i-number.
Read usr i-node.
Read usr data; retrieve bin i-number.
Read bin i-node.
Read bin data; retrieve lua51 i-number.
Read lua51 i-node.

Creating Files
1. To create a file; find the directory where the file will “live” and add a hardlink to the new
file; then create the i-node for the new file
a. Find the i-number of the directory that will contain the file; open the i-node and
read the data block.
b. Read the i-bitmap; find an unused bit N ( bit[N] == 0); set bit to 1; N is the new
i-number
c. Read the i-node corresponding to N; write metadata to it
d. Write the i-node of the containing directory to indicate it was modified (a hardlink
was added!)
2. To write NEW data to a file
a. If a new direct block is needed; read the data-bitmap and search for free block;
then write to that block
b. If a new single indirect block is needed; and there is no block of pointers yet
i.
Read the data-bitmap and find TWO blocks; one for the block of pointers
AND one for the data
ii.
Write to BOTH blocks
Example: Create file /usr/bin/blah.txt
Read root i-node.
Read root data; retrieve usr i-number.
Read usr i-node.
Read usr data; retrieve bin i-number.
Read bin i-node.
Read bin data
Read i-node bitmap; find unused inode
Write i-node bitmap
Write bin data; add hardlink for blah.txt
Read i-node for blah.txt

Write i-node for blah.txt; adding metadata
Write bin i-node; update last edited time

Questions
1. How many i-nodes are Read and Written if we execute “ls /usr/bin/local”, where local is a
directory?
2. Does “ls -la x.txt” read any of x.txt’s data?
3. What must be Read and Written to delete the file /usr/bin/foo.txt?
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