I am trying to understand, how does the inode pointer work. Suppose I have the following file structure with 512 byte block size. How will the pointer read the file, if the file partially occupies the 14th block - will it read the whole 14th block (all 128x128 blocks), or will it read only non-empty blocks?
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Examaple problem:
Given:
2KB block size
Inode consists of:
5 direct pointers
3 indirect pointers
2 doubly indirect pointers
1 triply indirect pointer
Assume a pointer is 8B.
What is the maximum size of a file that an inode can point to in this system? How many distinct blocks must be read to read the entirety of a 128MB file?
I've been seeing inconsistent answers online and am not sure who to trust on this.
I like to know how data is stored in storage. So what I know about a simple file system organizational struct that contains meta data about a file called inode is that it has two member fields
struct inode {
blkcnt_t i_blocks;
...
loff_t i_size;
}
I am assuming that i_blocks is storing block numbers. but how block numbers are numbered? its of type u64
so the question is if this field contains all the block [numbers] then how they are stored u64 means 64 bit and if I represent each 4 bit relate to block numbers then there are 16 blocks per inode. so for example if i_blocks field is 0b1111 1110.... so 1111 is block number 15 and 1110 is block number 14 and so on. so I like to know if number of bits to represent a block number is 4 bit then there can be only 15 blocks in inode so this way I have block numbers and number of blocks but I still could not field the third field which is >>> what is the base address of data block so for example if inode number is 1111 that correspond to some.txt text file with data hello world then where is the offset of hello world data in storage device. This data offset field array of corresponding inode numbers I could not find. Can any one please direct me to the answer in where I can find the data offset byte in storage medium and it has to be in inode struct?
Short sketch for finding inode number ii:
find the inode block where ii lives: ii/InodesPerBlock; use this as an index into the inodeblocks.
find the index into this block : ii%InodesPerBlock
treat (cast) this location as an Inode, and use the first entry in the blocks[] array as the number of the first data block.
For finding a file, this operation must be precededed by a similar operation for finding the directory entry and finding the file's inodeNumber
NOTE: there are a lot of manifest constants, these can all be found in the superblock
Block size
filesystem Type
size of a blockNumber
number of Inode Blocks (or: number of Inodes)
size of an inode (or: InodesPerBlock)
Number of Data Blocks
Location of the inode containing the root Directory
Location of the freelist containing the unused block numbers
State/backup/...
et cetera ...
NOTE2: this is a simplified scheme. Modern file systems may contain additional structures for efficiency or redundancy. Also: a filesystem may combine more than one physical block into one logical block (e.g. 8*512 -->> 4096)
NOTE3: the superblock is located at blocknumber=0 (at least in UFS) That means that 0 can be used as sentinel value for blocknumbers referring to actual (non-root) blocks. So, the blocknumber arrays inside the inodes can be initialized to all-zeros.
NOTE4: these structures all reside on disk. The kernel may maintain (it will!) additional, similar structures in memory. These structures will refer to both themselves (using pointers, or offsets, or indices) or they will refer to disk blocks (numbers).
I have a binary file which contains data in 128-byte blocks that are spread across the file. Each block starts with a char array of length 8.
How do I reorganize the data in this binary file such that all 128-byte blocks are ordered sequentially and that there is no unused space between these blocks?
Unused/unallocated space is just represented by 0 in this file and strings are null terminated.
I'm quite lost.
Your question is not a complete specification, e.g.:
- what is unused space?
- do the char arrays contain string terminator or not?
- is the file small enough to read into memory, or is it large?
So I cannot write it to you. However you can program it easily if you follow these instructions:
1. Build a list of pointers to the used 128 bytes blocks.
2. Sort that list.
3. You can now overwrite the file looping through the ordered list of pointers.
When a file is written, the write function corresponding with .write in fuse_operations is called for each file segment.
This means that for a larger file (e.g. 12720 bytes), the write function could be called 4 times with
1. size=4096, offset=0
2. size=4096, offset=4096
3. size=4096, offset=8192
4. size=432, offset=12288
because it has 4 segments with max segment size of 4096 bytes.
Inside the write function, I'd like to determine when the last segment size is being writen. I'm intending to put all the segments into a buffer, and use this last written segment to signal that the buffer now contains the entire object, so that it can be put somewhere else (such as an object store). By knowing the size of the object being written before it's written, I can just do a simple equality test file_size == size + offset to determine when the last segment is being written.
Apparently, I can't. I can only put the entire object somewhere else (such as an object store) after the file handler is closed.
If the length of your chunk is less than 4096, you know that you're at the end of the file. Go ahead and write out the contents of your buffer!
Lets say a system stores files in blocks of 16000 bytes. Then calculating the wasted file space from the incomplete filling of the last block for a file of size 4,064,0000 bytes and 640,000 bytes..
Divide them by 16000 and I get NO wasted space...What am I doing wrong?