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834.8","aria-hidden":"true"},g6={class:"MathJax",jax:"SVG",style:{position:"relative"}},h6={style:{"vertical-align":"-0.357ex"},xmlns:"http://www.w3.org/2000/svg",width:"13.008ex",height:"1.889ex",role:"img",focusable:"false",viewBox:"0 -677 5749.5 834.8","aria-hidden":"true"};function p6(H6,T){return l(),a("div",null,[T[512]||(T[512]=m('

故障分类

事务故障：
- 逻辑错误：由于某些内部错误条件（例如 A 的余额不足，无法扣款），事务无法完成。
- 系统错误：数据库系统必须由于错误条件（例如死锁）终止活动事务。
系统崩溃：
- 由于电源故障或其他硬件或软件故障，系统崩溃。
停止故障假设：
- 假设非易失性存储内容在系统崩溃时不会被破坏。
- 数据库系统具有许多完整性检查来防止磁盘数据的损坏。
磁盘故障：
- 磁头碰撞或类似的磁盘故障破坏了全部或部分磁盘存储。
- 假设破坏是可检测的：磁盘驱动器使用校验和来检测故障。

恢复算法

',3)),Q("ul",null,[Q("li",null,[T[2]||(T[2]=t("考虑一个将 50 美元从账户 A 转账到账户 B 的事务")),Q("mjx-container",g,[(l(),a("svg",h,T[0]||(T[0]=[m('',1)]))),T[1]||(T[1]=Q("mjx-assistive-mml",{unselectable:"on",display:"inline"},[Q("math",{xmlns:"http://www.w3.org/1998/Math/MathML"},[Q("msub",null,[Q("mi",null,"T"),Q("mi",null,"i")])])],-1))]),T[3]||(T[3]=t("： ")),T[4]||(T[4]=Q("ol",null,[Q("li",null,"读取(A)"),Q("li",null,"A := A - 50"),Q("li",null,"写入(A)"),Q("li",null,"读取(B)"),Q("li",null,"B := B + 50"),Q("li",null,"写入(B)")],-1))]),Q("li",null,[T[7]||(T[7]=t("事务")),Q("mjx-container",p,[(l(),a("svg",H,T[5]||(T[5]=[m('',1)]))),T[6]||(T[6]=Q("mjx-assistive-mml",{unselectable:"on",display:"inline"},[Q("math",{xmlns:"http://www.w3.org/1998/Math/MathML"},[Q("msub",null,[Q("mi",null,"T"),Q("mi",null,"i")])])],-1))]),T[8]||(T[8]=t("需要将对 A 和 B 的更新输出到数据库。"))]),T[9]||(T[9]=Q("li",null,"在这些修改中的某个修改完成之后，但两者都未完成之前，可能发生故障。",-1)),T[10]||(T[10]=Q("li",null,"在没有确保事务将提交的情况下修改数据库可能会导致数据库处于不一致的状态。",-1)),T[11]||(T[11]=Q("li",null,"如果在事务提交后发生故障，不修改数据库可能会导致更新丢失。",-1)),T[12]||(T[12]=Q("li",null,[t("恢复算法分为两个部分： "),Q("ol",null,[Q("li",null,"在正常事务处理期间采取的操作，以确保存在足够的信息以从故障中恢复。"),Q("li",null,"在发生故障后采取的操作，将数据库内容恢复到确保原子性、一致性和持久性的状态。")])],-1))]),T[513]||(T[513]=m('

存储结构

易失性存储：
- 不会在系统崩溃后保存数据。
- 示例：主存储器、缓存存储器。
非易失性存储：
- 在系统崩溃后能够保存数据。
- 示例：磁盘、磁带、闪存存储器、非易失性（电池备份）RAM。
- 但仍可能发生故障导致数据丢失。
稳定存储：
- 一种虚构的存储形式，能够在所有故障情况下都保留数据。
- 通过在不同的非易失性介质上维护多个副本来近似实现稳定存储。
- 有关如何实现稳定存储的更多详细信息，请参阅相关书籍。

实现稳定存储(Stable-Storage Implementation)

在单独的磁盘上维护每个块的多个副本：

副本可以位于远程站点，以防止火灾或洪水等灾害。
在数据传输过程中发生故障仍可能导致不一致的副本：
- 块传输可能导致：
  - 成功完成
  - 部分失败：目标块包含错误的信息
  - 完全失败：目标块从未更新

保护存储介质免受数据传输期间的故障（一种解决方案）：

执行输出操作的步骤（假设每个块有两个副本）：
1. 将信息写入第一个物理块。
2. 当第一次写入成功完成后，将相同的信息写入第二个物理块。
3. 只有在第二次写入成功完成后，输出才算完成。
由于输出操作期间的故障，块的副本可能不同。为了从故障中恢复：
- 首先找到不一致的块：
  - 昂贵的解决方案：比较每个磁盘块的两个副本。
  - 更好的解决方案：
    - 在非易失性存储（非易失性 RAM 或磁盘的特殊区域）中记录正在进行的磁盘写操作。
    - 在恢复过程中使用此信息找到可能不一致的块，并仅比较这些块的副本。
    - 在硬件 RAID 系统中使用。
- 如果检测到不一致块的任一副本存在错误（校验和错误），则使用另一个副本覆盖它。如果两个副本都没有错误，但是不同，那么使用第一个副本覆盖第二个块。

数据访问(Data Access)

物理块(Physical blocks)是指存储在磁盘上的块。
缓冲块(Buffer blocks)是临时存储在主存储器中的块。
磁盘和主存储器之间的块移动是通过以下两个操作发起的：
- input(B)：将物理块 B 传输到主存储器。
- output(B)：将缓冲块 B 传输到磁盘，并替换相应的物理块。
为简单起见，我们假设每个数据项适合并存储在单个块内。

恢复与原子性

为了确保在发生故障时实现原子性，我们首先将描述修改的信息输出到稳定存储中，而不修改数据库本身。

我们详细研究基于日志的恢复机制。
首先介绍关键概念。
然后介绍实际的恢复算法。
较少使用的备选方案是影子分页（在书中有简要介绍）。

基于日志的恢复

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无需执行重做操作",-1)),Q("li",null,[T[91]||(T[91]=t("(b) 由于存在 ")),Q("mjx-container",T1,[(l(),a("svg",t1,T[87]||(T[87]=[m('',1)]))),T[88]||(T[88]=Q("mjx-assistive-mml",{unselectable:"on",display:"inline"},[Q("math",{xmlns:"http://www.w3.org/1998/Math/MathML"},[Q("mo",null,"<"),Q("msub",null,[Q("mi",null,"T"),Q("mn",null,"0")]),Q("mstyle",{scriptlevel:"0"},[Q("mspace",{width:"0.278em"})]),Q("mstyle",{scriptlevel:"0"},[Q("mspace",{width:"0.278em"})]),Q("mi",null,"c"),Q("mi",null,"o"),Q("mi",null,"m"),Q("mi",null,"m"),Q("mi",null,"i"),Q("mi",null,"t"),Q("mo",null,">")])],-1))]),T[92]||(T[92]=t("，因此必须执行 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")),Q("mjx-container",r1,[(l(),a("svg",i1,T[99]||(T[99]=[m('',1)]))),T[100]||(T[100]=Q("mjx-assistive-mml",{unselectable:"on",display:"inline"},[Q("math",{xmlns:"http://www.w3.org/1998/Math/MathML"},[Q("mi",null,"r"),Q("mi",null,"e"),Q("mi",null,"d"),Q("mi",null,"o"),Q("mo",{stretchy:"false"},"("),Q("msub",null,[Q("mi",null,"T"),Q("mn",null,"1")]),Q("mo",{stretchy:"false"},")")])],-1))])])]),T[516]||(T[516]=m('

方法 2: 立即数据库修改(Immediate Database Modification)

在故障发生前的常规做法：

先写日志，后执行写操作：
- 必须在写入数据库项之前写入更新的日志记录。
- 我们假设日志记录直接输出到稳定存储器中。
- （稍后将了解如何在一定程度上延迟日志记录的输出）
写操作立即执行：
- 立即修改方案允许未提交的事务的更新操作被写入缓冲区或磁盘本身，在事务提交之前就可以进行修改。
输出可能发生在任意时刻：
- 更新后的块输出到稳定存储器可以在事务提交之前或之后的任何时刻发生。
输出的顺序是任意的：
- 输出块的顺序可以与写入顺序不同。

',3)),Q("ul",null,[Q("li",null,[T[126]||(T[126]=t("恢复过程需要两个操作而不是一个： 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")),Q("mjx-container",E1,[(l(),a("svg",F1,T[150]||(T[150]=[m('',1)]))),T[151]||(T[151]=Q("mjx-assistive-mml",{unselectable:"on",display:"inline"},[Q("math",{xmlns:"http://www.w3.org/1998/Math/MathML"},[Q("mi",null,"u"),Q("mi",null,"n"),Q("mi",null,"d"),Q("mi",null,"o"),Q("mo",{stretchy:"false"},"("),Q("msub",null,[Q("mi",null,"T"),Q("mn",null,"0")]),Q("mo",{stretchy:"false"},")")])],-1))]),T[153]||(T[153]=t("：B 恢复为 2000，A 恢复为 1000。"))]),Q("li",null,[T[158]||(T[158]=t("(b) ")),Q("mjx-container",A1,[(l(),a("svg",X1,T[154]||(T[154]=[m('',1)]))),T[155]||(T[155]=Q("mjx-assistive-mml",{unselectable:"on",display:"inline"},[Q("math",{xmlns:"http://www.w3.org/1998/Math/MathML"},[Q("mi",null,"u"),Q("mi",null,"n"),Q("mi",null,"d"),Q("mi",null,"o"),Q("mo",{stretchy:"false"},"("),Q("msub",null,[Q("mi",null,"T"),Q("mn",null,"1")]),Q("mo",{stretchy:"false"},")")])],-1))]),T[159]||(T[159]=t("和")),Q("mjx-container",R1,[(l(),a("svg",z1,T[156]||(T[156]=[m('',1)]))),T[157]||(T[157]=Q("mjx-assistive-mml",{unselectable:"on",display:"inline"},[Q("math",{xmlns:"http://www.w3.org/1998/Math/MathML"},[Q("mi",null,"r"),Q("mi",null,"e"),Q("mi",null,"d"),Q("mi",null,"o"),Q("mo",{stretchy:"false"},"("),Q("msub",null,[Q("mi",null,"T"),Q("mn",null,"0")]),Q("mo",{stretchy:"false"},")")])],-1))]),T[160]||(T[160]=t("：C 恢复为 700，然后 A 和 B 恢复分别设置为 950 和 2050。"))]),Q("li",null,[T[165]||(T[165]=t("(c) 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600"))])]),T[517]||(T[517]=Q("h3",{id:"对比",tabindex:"-1"},[Q("a",{class:"header-anchor",href:"#对比"},[Q("span",null,"对比")])],-1)),T[518]||(T[518]=Q("ul",null,[Q("li",null,"延迟修改方案仅在事务提交时或之后才对缓冲区/磁盘执行更新，简化了恢复的某些方面，但有存储本地副本的开销"),Q("li",null,"接下来介绍的算法都采用立即数据库修改(Immediate Database 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")),Q("ul",null,[Q("li",null,[T[182]||(T[182]=t("否则，如果")),Q("mjx-container",Y1,[(l(),a("svg",$1,T[176]||(T[176]=[m('',1)]))),T[177]||(T[177]=Q("mjx-assistive-mml",{unselectable:"on",display:"inline"},[Q("math",{xmlns:"http://www.w3.org/1998/Math/MathML"},[Q("msub",null,[Q("mi",null,"T"),Q("mn",null,"1")])])],-1))]),T[183]||(T[183]=t("更新了 A，然后")),Q("mjx-container",_1,[(l(),a("svg",Q2,T[178]||(T[178]=[m('',1)]))),T[179]||(T[179]=Q("mjx-assistive-mml",{unselectable:"on",display:"inline"},[Q("math",{xmlns:"http://www.w3.org/1998/Math/MathML"},[Q("msub",null,[Q("mi",null,"T"),Q("mn",null,"2")])])],-1))]),T[184]||(T[184]=t("更新了 A 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故障分类

\\n

事务故障：\\n
- 逻辑错误：由于某些内部错误条件（例如 A 的余额不足，无法扣款），事务无法完成。
- 系统错误：数据库系统必须由于错误条件（例如死锁）终止活动事务。
\\n
系统崩溃：\\n
- 由于电源故障或其他硬件或软件故障，系统崩溃。
\\n
停止故障假设：\\n
- 假设非易失性存储内容在系统崩溃时不会被破坏。
- 数据库系统具有许多完整性检查来防止磁盘数据的损坏。
\\n
磁盘故障：\\n
- 磁头碰撞或类似的磁盘故障破坏了全部或部分磁盘存储。
- 假设破坏是可检测的：磁盘驱动器使用校验和来检测故障。
\\n

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