Locating the data blocks belonging to a file implies locating its inode in the inode table first. The inode of the desired file is generally not known at the time the open operation is issued. What we know is the path of the file. For example:

int $fd=$ open $(\frac{?}{\text{h}\text{o}\text{m}}\frac{e}{e}$altier$\frac{i}{h}$ello.$txt,O_R$DONLY$)$;

The desired file is hello.txt$,$ while its path is $/$home$/$ealtieri$/$hello$.$txt$.$

To find out the inode belonging to the file we first need to descend through its path, starting from the root directory, until we reach the file's parent directory. At this point we can locate the ext$2\_$dir$\_$entry$\_2$ entry corresponding to hello.txt and then its inode number.

Once the inode of the file is known, the data blocks belonging to the hello.txt are specified by the inode.blockl$]$ array.

The root directory is always located at inode $2.$

Directory entries in the inode table require special attention. To test if an inode refers to a directory file we can use the $S_{\text{I}}$SDIR $($mode$)$ macro:

if $($S$\_$ISDIR$($inode$.$i$\_$mode$))...$

In the case of directory entries, the data blocks pointed by i$\_$blockl$]$ contain a list of the files in the directory and their respective inode numbers.

The super block also tells us the number of blocks per group with s$\_$blocks$\_$per$\_$group. Using mkfs$.$ext$2,$ we can see that this value is $8192$ on a floppy disk. Because there are $1440$ blocks on a floppy, there can only be one group.

question Q$1.$ What is the difference in terms of internal fragmentation and average file size between using a small block size $(1024)$ and a big block size $($e$.$g$.8192)?$ question Q$2.$ What is the size in bytes of a block if s$\_$log$\_$block$\_$size $=3$ in the super$-$block?

The superblock is located at offset $1024$ of a floppy. The code to read the superblock from a floppy is shown below. This code also checks the magic number of the super block $($EXT$2\_$SUPER$\_$MAGIC$)$ to see if we are reading from an Ext$2$ filesystem. For simplicity, error checking has been omitted.

Limiting the size of a bitmap to one block also limits the size of a block$-$group, because a bitmap always refers to the blocks$/$inodes in the group it belongs to$.$ Consider the blocks bitmap: given a block size of $1024$ bytes, and knowing that each byte is made of $8$ bits, we can calculate the maximum number of blocks that the blocks bitmap can represent: $8**1024=8192$ blocks. Therefore, $8192$ blocks is the size of a block$-$group using a $1024-$byte block size, as we also see from the output of mkfs$.$ext$2$ in the first section.

question Q$3.$ Calculate the number of blocks in a block$-$group given a block size of $4096$ bytes. The following code fragment reads the block bitmap from disk:

In the case of our floppy disk, we can see from the output of mkfs$.$ext $2$ that we have $184$ inodes per group and a block size of $1024$ bytes. The size of an inode is $128$ bytes, therefore the inode table will take $\frac{184}{\frac{1024}{128}}=23$ blocks.

question Q$4.$ Assume a block size of $2048$ bytes. What is the offset of the inode table if bg$\_$inode$\_$table$=5$ in the group descritor?

The inode table contains everything the operating system needs to know about a file, including the type of file, permissions, owner, and, most important, where its data blocks are located on disk. It is no surprise therefore that this table needs to be accessed very frequently and its read access time should be minimized as much as possible. Reading an inode from disk every time it is needed is usually a very bad idea. However, in this context we will adopt this method to keep the example code as simple as possible. We provide a general function to read an inode from the inode table:

The offset of the inode to read is calculated by adding together the absolute offset of the inode table the distance of the desired inode from the beginning of the inode table.

question Q$5.$ Calculate the offset of inode $93$ on disk. Assume a block size of $1024$ bytes $b{g}_{-}$inode$\_$table $=5.$ To which block does the inode belong to$?($The size of an inode structure is bytes$)$

Inodes are numbered starting from $1.$ An inode is defined as a struct ext$2\_$inode, in ext$2\_$fs$.$h$.$ most important fields of this structure have been reported below: