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Silberschatz Korth Sudarshan: Database System Concepts, Fourth Edition
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however, are subject to failure (for example, head crash), which may result in loss of information At the current state of technology, nonvolatile storage is slower than volatile storage by several orders of magnitude This is because disk and tape devices are electromechanical, rather than based entirely on chips, as is volatile storage In database systems, disks are used for most nonvolatile storage Other nonvolatile media are normally used only for backup data Flash storage (see Section 111), though nonvolatile, has insuf cient capacity for most database systems Stable storage Information residing in stable storage is never lost (never should be taken with a grain of salt, since theoretically never cannot be guaranteed for example, it is possible, although extremely unlikely, that a black hole may envelop the earth and permanently destroy all data!) Although stable storage is theoretically impossible to obtain, it can be closely approximated by techniques that make data loss extremely unlikely Section 1722 discusses stable-storage implementation The distinctions among the various storage types are often less clear in practice than in our presentation Certain systems provide battery backup, so that some main memory can survive system crashes and power failures Alternative forms of nonvolatile storage, such as optical media, provide an even higher degree of reliability than do disks
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To implement stable storage, we need to replicate the needed information in several nonvolatile storage media (usually disk) with independent failure modes, and to update the information in a controlled manner to ensure that failure during data transfer does not damage the needed information Recall (from 11) that RAID systems guarantee that the failure of a single disk (even during data transfer) will not result in loss of data The simplest and fastest form of RAID is the mirrored disk, which keeps two copies of each block, on separate disks Other forms of RAID offer lower costs, but at the expense of lower performance RAID systems, however, cannot guard against data loss due to disasters such as res or ooding Many systems store archival backups of tapes off-site to guard against such disasters However, since tapes cannot be carried off-site continually, updates since the most recent time that tapes were carried off-site could be lost in such a disaster More secure systems keep a copy of each block of stable storage at a remote site, writing it out over a computer network, in addition to storing the block on a local disk system Since the blocks are output to a remote system as and when they are output to local storage, once an output operation is complete, the output is not lost, even in the event of a disaster such as a re or ood We study such remote backup systems in Section 1710 In the remainder of this section, we discuss how storage media can be protected from failure during data transfer Block transfer between memory and disk storage can result in
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17
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Successful completion The transferred information arrived safely at its destination Partial failure A failure occurred in the midst of transfer, and the destination block has incorrect information Total failure The failure occurred suf ciently early during the transfer that the destination block remains intact We require that, if a data-transfer failure occurs, the system detects it and invokes a recovery procedure to restore the block to a consistent state To do so, the system must maintain two physical blocks for each logical database block; in the case of mirrored disks, both blocks are at the same location; in the case of remote backup, one of the blocks is local, whereas the other is at a remote site An output operation is executed as follows: 1 Write the information onto the rst physical block 2 When the rst write completes successfully, write the same information onto the second physical block 3 The output is completed only after the second write completes successfully During recovery, the system examines each pair of physical blocks If both are the same and no detectable error exists, then no further actions are necessary (Recall that errors in a disk block, such as a partial write to the block, are detected by storing a checksum with each block) If the system detects an error in one block, then it replaces its content with the content of the other block If both blocks contain no detectable error, but they differ in content, then the system replaces the content of the rst block with the value of the second This recovery procedure ensures that a write to stable storage either succeeds completely (that is, updates all copies) or results in no change The requirement of comparing every corresponding pair of blocks during recovery is expensive to meet We can reduce the cost greatly by keeping track of block writes that are in progress, using a small amount of nonvolatile RAM On recovery, only blocks for which writes were in progress need to be compared The protocols for writing out a block to a remote site are similar to the protocols for writing blocks to a mirrored disk system, which we examined in 11, and particularly in Exercise 114 We can extend this procedure easily to allow the use of an arbitrarily large number of copies of each block of stable storage Although a large number of copies reduces the probability of a failure to even lower than two copies do, it is usually reasonable to simulate stable storage with only two copies
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