Disk cache
A hard disk cache refers to the fast intermediate storage of a hard disk that buffers read and write accesses to a data carrier in order to reduce the influence of the latency and data transfer rate of data carrier access on the system.
Details
Hard drives and solid-state drives available in 2009 usually had between 8 and 128 MiB of RAM for intermediate storage. Current hard disks and solid-state drives are now available with up to 256 MiB cache.
The cache is divided into pure read cache and write cache. With read cache, only the data that has already been loaded from the data carrier is temporarily stored for reuse.
With a write cache, data can also be saved with a delay , which means that the hard drive processes write accesses more quickly from the system's point of view. The execution of the write request is reported as soon as the data is in the cache. The actual access to the data carrier, however, takes place a little later. A power failure in the meantime usually results in the loss of the data that has not yet been written. In this case, current RAID controllers use a BBU ( Battery Backup Unit ), which ensures the power supply until all data has been written from the write cache. Another advantage in addition to lower latency for sporadic write accesses is the possibility of re-sorting the write accesses so that movements within the hard disk can be minimized.
In addition to this specialized memory in the hard disk itself, many operating systems use part of the main memory as a cache in order to further shorten access times by eliminating communication via the relatively slow hard disk bus . In the times of MS-DOS , the Smartdrive program was often used on PCs for this purpose . In current operating systems, this functionality is normally set by the system itself, depending on the main memory available ; the user usually does not have to make any special configuration.