I wonder if everywhere I find the FF FF FF FF it indicates an MFT block there. Because here they say:
you can quite clearly see the 0xFFFFFFFF end of file marker that marks
the end of the new MFT entry (byte offset 504).
Not exactly. All MFT entries are aligned to a cluster or file record size which are multiples of the sector size of the disk. Usually file records in the MFT are 1024 bytes long, the size of a file record and the offset to the MFT are stored in the first sector of the volume in a structure called NTFS bios parameter block. You can find the structure in the ReactOS source code here.
All MFT file entries have at offset 0 a magic number (e.g. FILE or BAAD). The exact size of the MFT can be obtained by parsing the attributes of the first file record of the MFT which is the MFT itself. An unnamed non-resident data attribute contains the size of the entire MFT and a data run of it's fragments on the disk.
The 0xFFFFFFFF that article talks about is found in the type field of last attribute that denotes the end of the file record's attributes. It would be a waste of time and not entirely accurate to parse for this value since other files might contain it.
Related
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?
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).
If I use inode file system, and I have only 55 free block, than how big is the biggest file, that I can store?
As Inode keep references to all the data blocks where corresponding file data is stored,you can store file which can be accommodated by these 55 free blocks. Also file size depends size of blocks.
Instead of storing references to next nodes in a table, why couldn't it be just stored like a conventional linked list, that is, with a next pointer?
This is due to alignment. FAT (and just about any other file system) stores file data in one or more whole sectors of the underlying storage. Because the underlying storage can only read and write whole sectors such allocation allows efficient access to the contents of a file.
Issues with interleaving
When a program wants to store something in a file it provides a buffer, say 1MB of data to store. Now if the file's data sectors have to also keep next pointers to their next sector, this pointer information will need to be interleaved with the actual user data. So the file system would need to build another buffer (of slightly more than the provided 1MB), for each output sector copy some of the user data and the corresponding next pointer and give this new buffer to the storage. This would be somewhat inefficient. Unless the file system always stores file data to new sectors (and most usually don't), rewriting these next pointers will also be redundant.
The bigger problem would be when read operation is attempted on the file. Files will now work like tape devices: with only the location of the first sector known in the file's primary metadata, in order to reach sector 1000, the file system will need to read all sectors before it in order: read sector 0, find the address of sector 1 from the loaded next pointer, read sector 1, etc. With typical seek times of around 10 ms per random I/O (assuming a hard disk drive), reaching sector 1000 will take about 10 seconds. Even if sectors are sequentially ordered, while the file system driver processes sector N's data, the disk head will be flying over the next sector and when the read for sector N+1 is issued it may be too late, requiring the disk to rotate entire revolution (8.3ms for 7200 RPM drive) before being able to read the next sector again. On-disk cache can and will help with that though.
Writing single sector is usually atomic operation (depends on hardware): reading back the sector after power failure returns either its old content or the new one without intermediate states. Database applications usually need to know which writes would be atomic. If the file system interleaves file data and metadata in the same sectors, it will need to report smaller than the actual sector size to the application. For example instead of say 512 bytes it may need to report 504. But it can't do it because sector size is usually assumed by applications to be power of 2. Furthermore file stored on such filesystem would very likely be unusable if copied to another file system with different reported sector size.
Better approaches
The FAT format is better because all next pointers are stored in adjacent sectors. For FAT12, FAT16 and not very large FAT32 volumes the entire table is small enough to fit in memory. FAT still records the blocks of a file in a linked list, so to have efficient random access, an implementation needs to cache the chain per file. On large enough volumes (that can sport large enough file) such cache may no longer fit in memory.
ext3 uses direct and indirect blocks. This simple format avoids the need for preprocessing that FAT requires and goes by with only minimal amount of additional reads per I/O when indirect blocks are needed. These additional reads are cached by the operating system so that their overhead is often negligible.
Other variants are also possible and used by various file systems.
Random notes
For the sake of completeness, some hard disk drives can be formatted with slightly larger sector sizes (say 520 bytes) so that the file system can pack 512 bytes of file data with several bytes of metadata in the same sector. Yet because of the above, I don't believe anyone has used such formats for storing the address of the file's next sector. These additional bytes can be put to better use: additional checksums and timestamping come to mind. The timestamping I believe is used to improve the performance of some RAID systems. Still such usage is rare, and most software can't work with them at all.
Some file systems can save the content of small enough files in the file metadata directly without occupying distinct sectors. ReiserFS has the controversial tail packing. This is not important here: large files still benefit from having proper mapping to storage sectors.
Any modern OS requires much more than a pointer to the next data block for its file system: attributes (encryption, compression, hidden, ...), security descriptors (ACL list items), support for different hardware, buffering. This is just a tiny fraction of functionality that any good file system does.
Have a look at file system at Wikipedia to learn what else any modern file system does.
If we ignore the detail of FAT12 sharing a byte between two items to compact 12 bite as 1.5 bytes, then we can concentrate on the deeper meaning of the question.
It turns out that the FAT system is equivalent to a linked list with the following points:
The "next" pointer is located in an array (the FAT) instead of being appended or prepended to the actual data
The value written in "next" is an integer instead of the more familiar memory address of the next node.
The nodes are not reserved dynamically but represented by another array. That array is the entire data part of the hard drive.
One fascinating exercise we were assigned as part of the software engineer education was to convert an application using memory pointer to an equivalent application which use integer value. The rationale was that some processors (PDP-11? or another PDP-xx) would perform integer arithmetic much faster than memory pointer operation or maybe even did forbid any arithmetic on pointers.
I need a function which will create a file with fixed size in linux. Function like truncate or fopen,fseek,fclose, is not a solution because, they will fill opened file with zeros, but it is not necessary and I have no time for this. So is there some function, which will only open a file with fixed length and not fill buffer?
Thanks in advance.
The system call truncate(2) doesn't fill the file with zeros. It simply advances the file's reported size and leaves holes in it.
When you read from it, you do get zeros, but that's just a convenience of the OS.
The truncate() and ftruncate() functions cause the regular file named
by path or referenced by fd to be truncated to a size of precisely
length bytes.
If the file previously was shorter, it is extended, and the extended
part reads as null bytes ('\0').
About holes (from TLPI):
The existence of holes means that a file’s nominal size may be larger
than the amount of disk storage it utilizes (in some cases,
considerably larger).
Filesystems and holes:
Rather than allocate blocks of null bytes for the holes in a file, the
file system can just mark (with the value 0) appropriate pointers in
the i-node and in the indirect pointer blocks to indicate that they
don't refer to actual disk blocks.
As Per Johansson notes, this is dependent of the filesystem.
Most native UNIX file systems support the concept of file holes, but
many nonnative file systems (e.g., Microsoft’s VFAT) do not. On a
file system that doesn’t support holes, explicit null bytes are
written to the file.