Today we continue discussing the best practice from storage engineering:

443) Sections of the file can be locked for multi-process access and even to map sections of a file on virtual memory systems. The latter is called memory mapping and it enables multiple processes to share the data. Each sharing process' virtual memory map points to the same page of physical memory - the page that holds a copy of the disk block.

444) File Structure is dependent on the file types.  Internal file structure is operating system dependent. Disk access is done in units of block. Since logical records vary in size, several of them are packed in single physical block as for example at byte size. The logical record size, the physical block size and the packing technique determine how many logical records are in each physical block. There are three major methods of allocation methods: contiguous, linked and indexed. Internal fragmentation is a common occurrence from the wasted bytes in block size.

445) Access methods are either sequential or direct. The block number is relative to the beginning of the file. The use of relative block number helps the program to determine where the file should be placed and helps to prevent the users from accessing portions of the file system that may not be part of his file.

446) File system is broken into partitions. Each disk on the system contains at least one partition. Partitions are like separate devices or virtual disks. Each partition contains information about files within it and is referred to as the directory structure. The directory can be viewed as a symbol table that translates file names into their directory entries. Directories are represented as a tree structure. Each user has a current directory. A tree structure prohibits the sharing of a files or directories. An acyclic graph allows directories to have shared sub-directories and files. Sharing means there's one actual file and changes made by one user are visible to the other. Shared files can be implemented via a symbolic link which is resolved via the path name. Garbage collection may be necessary to avoid cycles.

447) Protection involves access lists and groups. Consistency is maintained via open and close operation wrapping.

448) File system is layered in the following manner:
1) application programs, 2) logical file system, 3) file-organization module, 4) basic file system, 4) i/o control  and 5) devices. The last layer is the hardware. The i/o control is the consists of device drivers and interrupt handlers, the basic file system issues generic commands to the appropriate device driver. The file organization module knows about files and their logical blocks. The logical file system uses the directory structure to inform the file organization module. The application program is responsible for creating and deleting files. 

#codingexercise: 

a player can draw a coin from the ends of a sequence. Determine the winning strategy:

int GetCoinsDP(List<int> coins, int i, int j)
{
If (i > j) return 0;
If (i==j) return coins[i];
If (j == i+1) return max (coins [i], coins [j]);
return max (
coins[I] + GetCoinsDP(sequence, 1, j), 
coins[j] + GetCoinsDP (sequence, 0,j-1));
}
The selections in each initiation level of GetCoinsDP can be added to a list and alternate additions can be skipped as belonging to the other player since the method remains the same for both.



# potential trend with object storage: 

https://1drv.ms/w/s!Ashlm-Nw-wnWuQB7rNqURAxQf9hF