Secondary Storage

Disk Caching

A disk cache is a mechanism for improving the time it takes to read from or write to a hard disk and it is usually included as part of the hard disk. A disk cache can also be a specified portion of random access memory (RAM). Disk cache holds data that has recently been read and, in some cases, adjacent data areas that are likely to be accessed next. Write caching is also provided with some disk caches. Once the data is stored in the cache, it can be used in the future by accessing the cached copy rather than re-fetching or recomputing the original data. When a program needs to access new data, the operating system will first checks to see if the data is in the cache before reading it from the disk accessing data from RAM much faster than from a disk, disk caching can significantly increase performance.

Redundant Arrays of Inexpensive disks(RAID)

It's a technology that allowed computer users to achieve high levels of storage reliability from low-cost and less reliable PC-class disk-drive components,through the technique of arranging the devices into arrays for redundancy.Redundancy is achieved by either writing the same data to multiple drives or collecting data across the array, calculated such that the failure of one disks in the array will not result in loss of data. A failed disk may be replaced by a new one, and the lost data reconstructed from the remaining data and the parity data. There are three key concepts in RAID: mirroring, the copying of data to more than one disk; striping , the splitting of data across more than one disk; and error correction, where redundant data is stored to allow problems to be detected and possibly fixed. Different RAID levels use one or more of these techniques, depending on the system requirements. RAID's main aim is either to improve reliability and availability of data, ensuring that important data is available more often than not or merely to improve the access speed to files.

File compression and decompression