OS202
Checksum
A checksum is a value used to verify the integrity of a file or a data transfer. In other words, it is a sum that checks the validity of data.
Checksums are typically used to compare two sets of data to make sure they are the same. Some common applications include verifying a disk image or checking the integrity of a downloaded file.
If the checksums don’t match those of the original files, the data may have been altered or corrupted.
SHA-265
The SHA (Secure Hash Algorithm) is one of a number of cryptographic hash functions. A cryptographic hash is like a signature for a data set.
If you would like to compare two sets of raw data (source of the file, text or similar) it is always better to hash it and compare SHA256 values.
It is like the fingerprints of the data. Even if only one symbol is changed the algorithm will produce different hash value. SHA256 algorithm generates an almost-unique, fixed size 256-bit (32-byte) hash.
Single-Level vs Two-Levels Directory
A single-level directory in a multiuser system causes naming problems,
since each file must have a unique name. A two-level directory
solves this problem by creating a separate directory for each user’s files.
The directory lists the files by name and includes the file’s location on the
disk, length, type, owner, time of creation, time of last use, and so on.
Acylic-Graph directory structures vs General Graph structures
Acyclic-graph directory structures enable users to share
subdirectories and files but complicate searching and deletion. A general graph structure allows complete flexibility in the sharing of files and directories,
but sometimes requires garbage collection to recover unused disk space.
Further explanation for Garbage Collection in Genral Graph Structure
When we want to delete a file we use reference count. In a cycle, due to self-reference, the reference countmaybenon-zeroevenwhenitisnolonger count may be non-zero even when it is no longer possible to refer to a file or directory.
Thus garbage collection may needed. A garbage collector traverses the directory and marks files and directories that can be accessed
Directory Implementation : Linear-List
This method is simple to program but time-consuming to execute. To create a new file, we must first search the
directory to be sure that no existing file has the same name. Then, we add a new entry at the end of the directory.
To delete a file, we search the directory for the named file and then release the space allocated to it.
Directory Implementation : Hash-Table
A linear list stores the directory entries, but a hashdata structure is also used.
The hashtable takes a value computed from the file name and returns a pointer to the file name in the linear list.
Linear-List disadvantage : finding a file requires a linear search, directory information is used frequently, and users will notice if access to it is slow. advantage : sorted directory listing can be produced without a separate sort step. Hash-Table disadvantage : The major dificulties with a hash table are its generally fixed size and the dependence of the hash function on that size. advantage : it can greatly decrease the directory search time. Insertion and deletion are also fairly straightforward, although some provision must be made for collisions—situations in which two ile names hash to the same location.
Contiguous vs Linked vs Indexed Allocation
Contiguous allocation can suffer from external fragmentation. Direct access is very ineficient with linked allocation.
Indexed allocation may require substantial overhead for its index block. These algorithms can be optimized in many ways.
Contiguous space can be enlarged through extents to increase flexibility and to decrease external fragmentation.
Indexed allocation can be done in clusters of multiple blocks to increase throughput and to reduce the number of index entries needed.