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As an illustration, consider the set of transactions {T0 , T1 , , T100 } executed in the order of the subscripts Suppose that the most recent checkpoint took place during the execution of transaction T67 Thus, only transactions T67 , T68 , , T100 need to be considered during the recovery scheme Each of them needs to be redone if it has committed; otherwise, it needs to be undone In Section 1763, we consider an extension of the checkpoint technique for concurrent transaction processing
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An alternative to log-based crash-recovery techniques is shadow paging The shadow-paging technique is essentially an improvement on the shadow-copy technique that we saw in Section 153 Under certain circumstances, shadow paging may require fewer disk accesses than do the log-based methods discussed previously There are, however, disadvantages to the shadow-paging approach, as we shall see, that limit its use For example, it is hard to extend shadow paging to allow multiple transactions to execute concurrently As before, the database is partitioned into some number of xed-length blocks, which are referred to as pages The term page is borrowed from operating systems, since we are using a paging scheme for memory management Assume that there are n pages, numbered 1 through n (In practice, n may be in the hundreds of thousands) These pages do not need to be stored in any particular order on disk (there are many reasons why they do not, as we saw in 11) However, there must be a way to nd the ith page of the database for any given i We use a page table, as in Figure 178, for this purpose The page table has n entries one for each database page Each entry contains a pointer to a page on disk The rst entry contains a pointer to the rst page of the database, the second entry points to the second page, and so on The example in Figure 178 shows that the logical order of database pages does not need to correspond to the physical order in which the pages are placed on disk The key idea behind the shadow-paging technique is to maintain two page tables during the life of a transaction: the current page table and the shadow page table When the transaction starts, both page tables are identical The shadow page table is never changed over the duration of the transaction The current page table may be changed when a transaction performs a write operation All input and output operations use the current page table to locate database pages on disk Suppose that the transaction Tj performs a write(X) operation, and that X resides on the ith page The system executes the write operation as follows: 1 If the ith page (that is, the page on which X resides) is not already in main memory, then the system issues input(X) 2 If this is the write rst performed on the ith page by this transaction, then the system modi es the current page table as follows: a It nds an unused page on disk Usually, the database system has access to a list of unused (free) pages, as we saw in 11
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17 Recovery System
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17
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1 2 3 4 5 6 7
n page table
pages on disk
Figure 178 Sample page table
b It deletes the page found in step 2a from the list of free page frames; it copies the contents of the ith page to the page found in step 2a c It modi es the current page table so that the ith entry points to the page found in step 2a 3 It assigns the value of xj to X in the buffer page Compare this action for a write operation with that described in Section 1723 The only difference is that we have added a new step Steps 1 and 3 here correspond to steps 1 and 2 in Section 1723 The added step, step 2, manipulates the current
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