TAILIEUCHUNG - HandBooks Professional Java-C-Scrip-SQL part 154

Tham khảo tài liệu 'handbooks professional java-c-scrip-sql part 154', công nghệ thông tin, kỹ thuật lập trình phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | of less than 32 bytes is 0. Since the original check for sufficient space in a quantum tested whether the space code for the quantum was greater than or equal to the space code for the new item even blocks with no free space . those having a free space code of 0 were considered possible storage places for these very small items. This resulted in a large number of unnecessary disk accesses to read those quanta so that their actual free space could be calculated. Changing from to in the test fixed this of course I could also have made the size calculation round up rather than down. Another problem I encountered with the free space handling in the new algorithm is that blocks and quanta are no longer synonymous as they were in the old program. That is the virtual memory system deals not only with quanta . blocks containing user data but also with free space blocks and the block s occupied by the main object list. At one point some routines dealing with the free space list were using physical block numbers and others were using logical quantum numbers however since only blocks containing user data are controlled by the free space list the correct solution was to use only logical quantum numbers in all such routines. The FreeSpaceArray FindEmptyBlock Function The last function that we will cover in this class is FindEmptyBlock which is shown in Figure . The FreeSpaceArray FindEmptyBlock function from quantum Figure codelist As you can see this code uses the member variable called m_CurrentLowestFreeBlock to reduce the amount of time needed to find a free block as the file fills up. Each time that a free block with a block number less than the current value is created or located the m_CurrentLowestFreeBlock variable is updated to point to that block and FindEmptyBlock starts at that location when looking for an empty block. FindEmptyBlock has an understandable resemblance to FindSpaceForItem since both of them search

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