In computer storage, disk cache is the embedded memory in a hard disk drive (HDD) acting as a buffer between the rest of the computer and the physical hard disk platter that is used for storage. We know that disk memory is smaller and faster than main memory and that is interposed between main memory and the processor. Hence, we introduce disk cache to reduce average memory access time by exploiting the principle of locality.The cache contains a copy of some of the sectors on the disk. When an I/O request is made for a particular sector, a check is made to determine if the sector is in the disk cache. If so, the request is satisfied via the cache. If not, the requested sector is read into the disk cache from the disk. Because of the phenomenon of locality of reference, when a block of data is fetched into the cache to satisfy a single I/O request, it is likely that there will be future references to that same block.Design issues:1. When an I/O request is satisfied from the disk cache, the data in the disk cache must be delivered to the requesting process.We could satisfy this requirement 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 using the readers/writers model.2. Replacement strategyLRU: Replace that block that has been in the cache longest with no reference to it. LRU is used most commonly. Logically, the cache consists of a stack of blocks, with the most recently referenced block 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 secondary memory, remove the block that is on the bottom of the stack and push the incoming block onto the top of the stack. Naturally, it is not necessary actually to move these blocks around in main memory; a stack of pointers can be associated with the cache.LFU: Replace that block in the set that has experienced the fewest references. LFU could be implemented by associating a counter with each block. When a block is brought in, it is assigned a count of 1; with each reference to the block, its count is incremented by 1. When replacement is required, the block with the smallest count is selected. Intuitively, it might seem that LFU is more appropriate than LRU because LFU makes use of more pertinent information about each block in the selection process.Regardless of the particular replacement strategy, the replacement can take place on demand or preplanned. In the former case, a sector is replaced only when the slot is needed. In the latter case, a number of slots are released at a time. The reason for this latter approach is related to the need to write back sectors. If a sector is brought into the cache and only read, then when it is replaced, it is not necessary to write it back out to the disk. However, if the sector has been updated, then it is necessary to write it back out before replacing it. In this latter case, it makes sense to cluster the writing and to order the writing to minimize seek time.Performance issues:The issue of cache performance reduces itself to a question of whether a given miss ratio can be achieved. This will depend on the locality behavior of the disk references, the replacement algorithm, and other design factors. Principally, however, the miss ratio is a function of the size of the disk cache.LRU: figure 1Frequency-based replacement algorithm: figure 2The figures appear to show that LRU outperforms the frequency-based replacement algorithm. However, when identical reference patterns using the same cache structure are compared, the frequency-based replacement algorithm is superior. Thus, the exact sequence of reference patterns, plus related design issues such as block size, will have a profound influence on the performance achieved.

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