Disk Design Considerations
Several design issues are of interest. Disk should be designed according to the memory and performance requirements.
First, when an I/O request is satisfied from the disk cache, the data in the disk cache must be delivered to the requesting process. Delivery can be made either by transferring the block of data within main memory from the disk cache to memory assigned to the user process, or simply by using a shared memory capability and passing a pointer to the appropriate slot in the disk cache. The latter approach saves the time of a memory to memory transfer and also allows shared access by other processes.
A second design issue has to do with the replacement strategy. When a new sector is brought into the disk cache, one of the existing blocks must be replaced. A number of algorithms have been tried. The most commonly used algorithm is the recently used (LRU) algorithm, in which the commonly used algorithm, in which the block that has been in the cache longest with no reference to it is replaced. Logically, the cache consists of a stack of blocks, with the most recently referenced block being on the top of the stack. When a block in the cache is referenced, it is moved from its existing position on the stack to the top of the stack. When a block is brought in from the secondary memory, the block that is on the bottom of the stack is removed, pushing the incoming block onto the top of the stack. It is not necessary to move these blocks around in main memory.
Another possibility is the least frequently associated used (LFU) algorithm, in which that block in the cache that has experienced the fewest references is replaced. LFU could be implemented by associating a counter with each block. When a block is brought in, it is assigned a count of 1 and subsequently it is incremented by 1. When replacement is required, the block with the smallest count is selected. Intuitively, it might seem the LFU is more appropriate than LRU because more information about each block is used in the selection process.
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A simple LFU algorithm has the following problem. It may be that certain blocks are referenced relatively infrequently overall, but when they are referenced there are short intervals of repeated re-references due to locality, thus building up high reference counts in an interval.
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