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    Recovery

    A recovery scheme can restore the database to the consistent

    state that existed before the failure. It must minimize the time

    for which the database is not usable after a crash.

    We study two approaches:

    log-based recovery, and

    shadow-pagingWe assume (initially) that transactions run serially, that

    is, one after the other.

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    A transaction creates a log record prior to modifying the database.

    The log records allow the system to undo changes made by atransaction in the event that the transaction must be aborted.

    They also allow the system to redo changes made by a transaction

    if the transaction has committed.

    If a transaction does not modify the database until it has

    committed, it is said to use the deferredmodification

    technique.

    Ifdatabase modifications occur while the transaction is still

    active, the transaction is said to use theimmediatemodification

    technique.

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    Deferred Database Modification

    This scheme ensures atomicity despite failures byrecording all modifications to log, but deferring all thewrites to after partial commit.

    Assume that transactions execute serially

    Transaction starts by writing record to log.

    A write(X) operation results in a log record being written, where Vis the new value forX.

    The write is not performed onXat this time, but isdeferred.

    When Tipartially commits, is written to thelog

    Finally, log records are used to actually execute thepreviously deferred writes.

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    Immediate Database Modification Example

    Log Write Output

    To, B, 2000, 2050

    A = 950B = 2050

    C= 600

    BB, BCBA

    Note: BXdenotes block containingX.

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    Recovery procedure has two operationsinstead of one:

    undo(Ti) restores the value of all data items updated by Tito their old values,going backwards from the last log record for Ti

    redo(Ti) sets the value of all data items updated by Tito the new values, goingforward from the first log record for Ti

    Both operations must be idempotent

    Idempotent: property of an operation they can be appliedmultiple times without changing the result.

    When recovering after failure: Transaction Tineeds to be undone if the log contains the record , but

    does not contain the record .

    Transaction Tineeds to be redone if the log contains both the record and the record .

    Undo operations are performed first, thenredo operations.

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    Immediate DB Modification Recovery Example

    Below we show the log as it appears at three instances of time.

    (a)(b)

    Recovery actions in each case above are:(a) undo (T

    0

    ): B is restored to 2000 and A to 1000.(b) undo (T1) and redo (T0): C is restored to 700, and thenA and B are

    set to 950 and 2050 respectively.(c) redo (T0) and redo (T1): A and B are set to 950 and 2050

    respectively. Then Cis set to 600

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    Checkpoints

    Problems in recovery procedure as discussed earlier :

    1. searching the entire log is time-consuming2. we might unnecessarily redo transactions which have

    already

    3. output their updates to the database.

    Streamline recovery procedure by periodicallyperforming check pointing

    1. Output all log records currently residing in main

    memory onto stable storage.2. Output all modified buffer blocks to the disk.

    3 Write a log record < checkpoint> onto stable storage.

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    During recovery we need to consider only the mostrecent transaction Ti that started before thecheckpoint, and transactions that started after Ti.

    Scan backwards from end of log to find the mostrecent record

    Continue scanning backwards till a record is found.

    Need only consider the part of log following abovestart record. Earlier part of log can be ignored duringrecovery, and can be erased whenever desired.

    For all transactions (starting from Tior later) with no, execute undo(Ti). (Done only in case of

    immediate modification.) Scanning forward in the log, for all transactions

    starting from Tior later with a ,execute redo(Ti).

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    Example of Checkpoints

    T1 can be ignored (updates already output to disk due

    to checkpoint) T2 and T3 redone.

    T4 undone

    Tc Tf

    T1

    T2

    T3

    T4

    checkpoint system failure

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    Shadow Paging

    Alternative to log-based recovery; this scheme is useful iftransactions execute serially

    Idea: maintain two page tables during the lifetime of a transactionthe currentpage table, and the shadowpage table

    Store the shadow page table in nonvolatile storage, such that stateof the database prior to transaction execution may be recovered.Shadow page table is never modified during execution

    To start with, both the page tables are identical. Only current pagetable is used for data item accesses during execution of thetransaction.

    Whenever any page is about to be written for the first time, a copyof this page is made onto an unused page. The current page tableis then made to point to the copy, and the update is performed onthe copy

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    Example of Shadow Paging

    Shadow and current page tables after write to page 4

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    To commit a transaction :

    1. Flush all modified pages in main memory to disk

    2. Output current page table to disk

    3. Make the current page the new shadow page table

    keep a pointer to the shadow page table at a fixed (known) location ondisk.

    to make the current page table the new shadow page table, simply updatethe pointer to point to current page table on disk

    Once pointer to shadow page table has been written,transaction is committed.

    No recovery is needed after a crash new transactions canstart right away, using the shadow page table.

    Pages not pointed to from current/shadow page table should befreed (garbage collected).

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    Commit overhead is high (many pages need to be flushed)

    Data gets fragmented (related pages get separated)

    After every transaction completion, the database pages containing

    old versions of modified data need to be garbage collected and put

    into

    the list of unused pages

    Hard to extend algorithm to allow transactions to run concurrently

    recovery is trivial(perfect)

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    Advantages of shadow-paging over log-

    based schemes no overhead of writing

    log records; recovery is trivial(perfect)

    Disadvantages :

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    Lock-Based Protocols

    A lock is a mechanism to control concurrentaccess to a data item

    Data items can be locked in two modes

    1. Exclusive (X) mode: read/write

    2. Shared (S) mode: read

    Lock requests are made to concurrency-

    control manager. Transaction can proceed

    only after request is granted.

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    Lock-compatibility matrix

    A transaction may be granted a lock on an item ifthe requested lock is compatible with locks alreadyheld on the item by other transactions

    Any number of transactions can hold shared lockson an item

    No other transaction may hold any lock on the itemif any transaction holds an exclusive lock on theitem

    If a lock cannot be granted, the requestingtransaction must wait till all incompatible locks heldby other transactions have been released.

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    Lock-Based Protocols

    Examplelock-S(A);

    read (A);

    unlock(A);

    lock-S(B);

    read (B);

    unlock(B);

    display(A+B)

    Locking as above is not sufficient to guarantee serializability

    IfA and B get updated between the read ofA and B, the displayed sum would be

    wrong. A locking protocol is a set of rules followed by all transactions while requesting

    and releasing locks. Locking protocols restrict the set of possible schedules.

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    Pitfalls of Lock-Based Protocols

    Deadlock

    Neither T3 nor T4 can make progress executing lock-S(B) causes T4 to waitfor T3 to release its lock on B, while executing lock-X(A) causes T3 to wait forT4 to release its lock onA.

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    Starvation A transaction may be waiting for an X-lock on an item, while a

    sequence of other transactions request and are granted an S-lock

    on the same item.

    Concurrency control manager can be designed to prevent starvation.

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