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IV Data Storage and Querying
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11 Storage and File Structure
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of the block older than the version in the buffer The subsystem responsible for the allocation of buffer space is called the buffer manager
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Programs in a database system make requests (that is, calls) on the buffer manager when they need a block from disk If the block is already in the buffer, the buffer manager passes the address of the block in main memory to the requester If the block is not in the buffer, the buffer manager rst allocates space in the buffer for the block, throwing out some other block, if necessary, to make space for the new block The thrown-out block is written back to disk only if it has been modi ed since the most recent time that it was written to the disk Then, the buffer manager reads in the requested block from the disk to the buffer, and passes the address of the block in main memory to the requester The internal actions of the buffer manager are transparent to the programs that issue disk-block requests If you are familiar with operating-system concepts, you will note that the buffer manager appears to be nothing more than a virtual-memory manager, like those found in most operating systems One difference is that the size of the database may be much more than the hardware address space of a machine, so memory addresses are not suf cient to address all disk blocks Further, to serve the database system well, the buffer manager must use techniques more sophisticated than typical virtualmemory management schemes: Buffer replacement strategy When there is no room left in the buffer, a block must be removed from the buffer before a new one can be read in Most operating systems use a least recently used (LRU) scheme, in which the block that was referenced least recently is written back to disk and is removed from the buffer This simple approach can be improved on for database applications Pinned blocks For the database system to be able to recover from crashes ( 17), it is necessary to restrict those times when a block may be written back to disk For instance, most recovery systems require that a block should not be written to disk while an update on the block is in progress A block that is not allowed to be written back to disk is said to be pinned Although many operating systems do not support pinned blocks, such a feature is essential for a database system that is resilient to crashes Forced output of blocks There are situations in which it is necessary to write back the block to disk, even though the buffer space that it occupies is not needed This write is called the forced output of a block We shall see the reason for forced output in 17; brie y, main-memory contents and thus buffer contents are lost in a crash, whereas data on disk usually survive a crash
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The goal of a replacement strategy for blocks in the buffer is to minimize accesses to the disk For general-purpose programs, it is not possible to predict accurately
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Silberschatz Korth Sudarshan: Database System Concepts, Fourth Edition
IV Data Storage and Querying
11 Storage and File Structure
The McGraw Hill Companies, 2001
11
Storage and File Structure
for each tuple b of borrower do for each tuple c of customer do if b[customer-name] = c[customer-name] then begin let x be a tuple de ned as follows: x[customer-name] := b[customer-name] x[loan-number] := b[loan-number] x[customer-street] := c[customer-street] x[customer-city] := c[customer-city] include tuple x as part of result of borrower end end end Figure 115 Procedure for computing join
customer
which blocks will be referenced Therefore, operating systems use the past pattern of block references as a predictor of future references The assumption generally made is that blocks that have been referenced recently are likely to be referenced again Therefore, if a block must be replaced, the least recently referenced block is replaced This approach is called the least recently used (LRU) block-replacement scheme LRU is an acceptable replacement scheme in operating systems However, a database system is able to predict the pattern of future references more accurately than an operating system A user request to the database system involves several steps The database system is often able to determine in advance which blocks will be needed by looking at each of the steps required to perform the user-requested operation Thus, unlike operating systems, which must rely on the past to predict the future, database systems may have information regarding at least the short-term future To illustrate how information about future block access allows us to improve the LRU strategy, consider the processing of the relational-algebra expression borrower
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