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Answer to Question #68822 in Java | JSP | JSF for Maurice Ragland

Question #68822
1) What affect do you think a heavily fragmented disk has on the performance of Sequential and Random access methods?
2) Why do you want a File System to cache file data in memory? What are the downsides to caching File data? When is the cached contents written to disk?
3) How does Cluster Size affect the efficiency of the File System?
Expert's answer
1. Comparing random versus sequential operations is one way of assessing application efficiency in terms of disk use. Accessing data sequentially is much faster than accessing it randomly because of the way in which the disk hardware works. The seek operation, which occurs when the disk head positions itself at the right disk cylinder to access data requested, takes more time than any other part of the I/O process. Because reading randomly involves a higher number of seek operations than does sequential reading, random reads deliver a lower rate of throughput. The same is true for random writing.

2. File data in the system file cache is written to the disk at intervals determined by the operating system, and the memory previously used by that file data is freed—this is referred to as flushing the cache. The time at which a block of file data is flushed is partially based on the amount of time it has been stored in the cache and the amount of time since the data was last accessed in a read operation. This ensures that file data that is frequently read will stay accessible in the system file cache for the maximum amount of time.
Why does a cluster's size vary depending on the hard disk's size and file system? To see why, let's look at the FAT file system. As a 16-bit file system, it is limited to using a maximum of 65,536 clusters. Because the hard disk is limited to using a fixed number of clusters, the disk cluster size must change depending on the capacity of the partition. To calculate the disk cluster size, take the size of the partition and divide it among the number of available clusters. For example, the maximum size of a FAT-16 partition is 2 GB.

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