Database Management System

Unit 8: Crash Recovery

Lecture 1
Unit 8: Crash Recovery
8.1 Failure Classification
8.2 Recovery and Atomicity
8.3 Log-based Recovery
8.4 Shadow paging
8.5 Advanced Recovery Techniques

Remote Backup System

2
Outline
• Failure Classification
• Recovery and Atomicity
• Log-based Recovery

3
Failure Classification
• Transaction failure :
• Logical errors: transaction cannot complete due to some internal error condition
• System errors: the database system must terminate an active transaction due to an error
condition (e.g., deadlock)
• System crash: a power failure or other hardware or software failure causes the system to crash.
• Fail-stop assumption: non-volatile storage contents are assumed to not be corrupted by
system crash
• Database systems have numerous integrity checks to prevent corruption of disk data
• Disk failure: a head crash or similar disk failure destroys all or part of disk storage
• Destruction is assumed to be detectable: disk drives use checksums to detect failures

4
Recovery Algorithms
• Suppose transaction Ti transfers $50 from account A to account B
• Two updates: subtract 50 from A and add 50 to B
• Transaction Ti requires updates to A and B to be output to the database.
• A failure may occur after one of these modifications have been made but before
both of them are made.
• Modifying the database without ensuring that the transaction will commit may
leave the database in an inconsistent state
• Not modifying the database may result in lost updates if failure occurs just after
transaction commits
• Recovery algorithms have two parts
1. Actions taken during normal transaction processing to ensure enough information
exists to recover from failures
2. Actions taken after a failure to recover the database contents to a state that ensures
atomicity, consistency and durability

5
Storage Structure
• Volatile storage:
• Does not survive system crashes
• Examples: main memory, cache memory
• Nonvolatile storage:
• Survives system crashes
• Examples: disk, tape, flash memory, non-volatile RAM
• But may still fail, losing data
• Stable storage:
• A mythical form of storage that survives all failures
• Approximated by maintaining multiple copies on distinct nonvolatile media
• See book for more details on how to implement stable storage

6
Stable-Storage Implementation
• Maintain multiple copies of each block on separate disks
• copies can be at remote sites to protect against disasters such as fire or
flooding.
• Failure during data transfer can still result in inconsistent copies: Block transfer can result in
• Successful completion
• Partial failure: destination block has incorrect information
• Total failure: destination block was never updated
• Protecting storage media from failure during data transfer (one solution):
• Execute output operation as follows (assuming two copies of each block):
1. Write the information onto the first physical block.
2. When the first write successfully completes, write the same information onto the second
physical block.
3. The output is completed only after the second write successfully completes.

7
Protecting storage media from failure (Cont.)

• Copies of a block may differ due to failure during output operation.

• To recover from failure:
1. First find inconsistent blocks:
1. Expensive solution: Compare the two copies of every disk block.
2. Better solution:
• Record in-progress disk writes on non-volatile storage (Flash, Non-volatile RAM or special area
of disk).
• Use this information during recovery to find blocks that may be inconsistent, and only
compare copies of these.
• Used in hardware RAID systems

2. If either copy of an inconsistent block is detected to have an error (bad

checksum), overwrite it by the other copy. If both have no error, but are
different, overwrite the second block by the first block.
8
Data Access
• Physical blocks are those blocks residing on the disk.
• Buffer blocks are the blocks residing temporarily in main memory.
• Block movements between disk and main memory are initiated through the following two
operations:
• input (B) transfers the physical block B to main memory.
• output (B) transfers the buffer block B to the disk, and replaces the appropriate physical
block there.
• We assume, for simplicity, that each data item fits in, and is stored inside, a single block.

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Data Access (Cont.)
• Each transaction Ti has its private work-area in which local copies of all data items accessed and
updated by it are kept.
• Ti 's local copy of a data item X is called xi.
• Transferring data items between system buffer blocks and its private work-area done by:
• read(X) assigns the value of data item X to the local variable xi.
• write(X) assigns the value of local variable xi to data item {X} in the buffer
block.
• Note: output(BX) need not immediately follow write(X)
• Transactions
• Must perform read(X) before accessing X for the first time (subsequent reads
can be from local copy)
• write(X) can be executed at any time before the transaction commits

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Example of Data Access
buffer
Buffer Block A input(A)
X A
Buffer Block B Y B
output(B)
read(X)
write(Y)

x2
x1
y1

work area work area

of T1 of T2

memory disk

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Recovery and Atomicity
• To ensure atomicity despite failures, we first output information describing the modifications to
stable storage without modifying the database itself.
• log-based recovery mechanisms
• Less used alternative: shadow-copy and shadow-paging

shadow-copy

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Log-Based Recovery
• A log is a sequence of log records. The records keep information about update activities on the
database.
• The log is kept on stable storage
• When transaction Ti starts, it registers itself by writing a
<Ti start> log record
• Before Ti executes write(X), a log record
<Ti, X, V1, V2>
is written, where V1 is the value of X before the write (the old value), and V2 is the value to be
written to X (the new value).
• When Ti finishes it last statement, the log record <Ti commit> is written.
• Two approaches using logs
• Immediate database modification
• Deferred database modification.

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Immediate Database Modification
• The immediate-modification scheme allows updates of an uncommitted transaction to be made
to the buffer, or the disk itself, before the transaction commits
• Update log record must be written before database item is written
• We assume that the log record is output directly to stable storage
• Output of updated blocks to disk can take place at any time before or after transaction commit
• Order in which blocks are output can be different from the order in which they are written.
• The deferred-modification scheme performs updates to buffer/disk only at the time of
transaction commit
• Simplifies some aspects of recovery
• But has overhead of storing local copy

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Transaction Commit
• A transaction is said to have committed when its commit log record is output to stable storage
• All previous log records of the transaction must have been output already
• Writes performed by a transaction may still be in the buffer when the transaction commits, and
may be output later

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Immediate Database Modification Example

Log Write Output

<T0 start>
<T0, A, 1000, 950>
<T0, B, 2000, 2050>
A = 950
B = 2050
<T0 commit>
<T1 start>
<T1, C, 700, 600> BC output before T1
C = 600 commits
BB , BC
<T1 commit>
BA
• Note: BX denotes block containing X. BA output after T0
commits
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Concurrency Control and Recovery
• With concurrent transactions, all transactions share a single disk buffer and a single log
• A buffer block can have data items updated by one or more transactions
• We assume that if a transaction Ti has modified an item, no other transaction can modify the
same item until Ti has committed or aborted
• i.e., the updates of uncommitted transactions should not be visible to other
transactions
• Otherwise, how to perform undo if T1 updates A, then T2 updates A and commits, and
finally T1 has to abort?
• Can be ensured by obtaining exclusive locks on updated items and holding the
locks till end of transaction (strict two-phase locking)
• Log records of different transactions may be interspersed in the log.

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Undo and Redo Operations
• Undo and Redo of Transactions
• undo(Ti) -- restores the value of all data items updated by Ti to their old values, going
backwards from the last log record for Ti
• Each time a data item X is restored to its old value V a special log record <Ti , X, V> is
written out
• When undo of a transaction is complete, a log record
<Ti abort> is written out.
• redo(Ti) -- sets the value of all data items updated by Ti to the new values, going forward
from the first log record for Ti
• No logging is done in this case

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Recovering from Failure
• When recovering after failure:
• Transaction Ti needs to be undone if the log
• Contains the record <Ti start>,
• But does not contain either the record <Ti commit> or <Ti abort>.
• Transaction Ti needs to be redone if the log
• Contains the records <Ti start>
• And contains the record <Ti commit> or <Ti abort>

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Recovering from Failure (Cont.)
• Suppose that transaction Ti was undone earlier and the <Ti abort> record was written to the log,
and then a failure occurs,
• On recovery from failure transaction Ti is redone
• Such a redo redoes all the original actions of transaction Ti including the steps that restored
old values
• Known as repeating history
• Seems wasteful, but simplifies recovery greatly

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 Immediate DB Modification Recovery Example
Below we show the log as it appears at three instances of time.

Recovery actions in each case above are:

(a) undo (T0): B is restored to 2000 and A to 1000, and log records
<T0, B, 2000>, <T0, A, 1000>, <T0, abort> are written out
(b) redo (T0) and undo (T1): A and B are set to 950 and 2050 and C is restored to 700.
Log records <T1, C, 700>, <T1, abort> are written out.
(c) redo (T0) and redo (T1): A and B are set to 950 and 2050
respectively. Then C is set to 600
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Next
• Checkpoints
• Shadow paging
• Remote Backup System

22
Database Management System

Unit 8: Crash Recovery

Lecture 2
Previous
• Failure Classification
• Recovery and Atomicity
• Log-based Recovery
Outline
• Checkpoints
• Recovery Algorithm
• Shadow paging
• Remote Backup System
Checkpoints
• Redoing/undoing all transactions recorded in the log can be very slow
• Processing the entire log is time-consuming if the system has run for a long
time
• We might unnecessarily redo transactions which have already output their
updates to the database.
• Streamline recovery procedure by periodically performing checkpointing
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 L> onto stable storage where L is a list of all
transactions active at the time of checkpoint.
4. All updates are stopped while doing checkpointing
Checkpoints (Cont.)
• During recovery we need to consider only the most recent transaction Ti that started before the
checkpoint, and transactions that started after Ti.
• Scan backwards from end of log to find the most recent <checkpoint L>
record
• Only transactions that are in L or started after the checkpoint need to be
redone or undone
• Transactions that committed or aborted before the checkpoint already have
all their updates output to stable storage.
• Some earlier part of the log may be needed for undo operations
• Continue scanning backwards till a record <Ti start> is found for every
transaction Ti in L.
• Parts of log prior to earliest <Ti start> record above are not needed for
recovery, and can be erased whenever desired.
Example of Checkpoints
Tc Tf
T1
T2
T3
T4

checkpoint system failure

• T1 can be ignored (updates already output to disk due to

checkpoint)
• T2 and T3 redone.
• T4 undone
Recovery Algorithm
Recovery Algorithm
• Logging (during normal operation):
• <Ti start> at transaction start
• <Ti, Xj, V1, V2> for each update, and
• <Ti commit> at transaction end
• Transaction rollback
• Let Ti be the transaction to be rolled back
• Scan log backwards from the end, and for each log record of Ti of the form
<Ti, Xj, V1, V2>
• Perform the undo by writing V1 to Xj,
• Write a log record <Ti , Xj, V1>
• such log records are called compensation log records
• Once the record <Ti start> is found stop the scan and write the log record <Ti
abort>
Recovery Algorithm (Cont.)
• Recovery from failure: Two phases
• Redo phase: replay updates of all transactions, whether they committed,
aborted, or are incomplete
• Undo phase: undo all incomplete transactions
• Redo phase:
1. Find last <checkpoint L> record, and set undo-list to L.
2. Scan forward from above <checkpoint L> record
1. Whenever a record <Ti, Xj, V1, V2> or <Ti, Xj, V2> is found, redo it by writing V2 to Xj
2. Whenever a log record <Ti start> is found, add Ti to undo-list
3. Whenever a log record <Ti commit> or <Ti abort> is found, remove Ti from undo-list
Recovery Algorithm (Cont.)
• Undo phase:
1. Scan log backwards from end
1. Whenever a log record <Ti, Xj, V1, V2> is found where Ti is in undo-list perform same
actions as for transaction rollback:
1. perform undo by writing V1 to Xj.
2. write a log record <Ti , Xj, V1>
2. Whenever a log record <Ti start> is found where Ti is in undo-list,
1. Write a log record <Ti abort>
2. Remove Ti from undo-list
3. Stop when undo-list is empty
 i.e., <Ti start> has been found for every transaction in undo-list
• After undo phase completes, normal transaction processing can commence
Example of Recovery
Log Record Buffering
• Log record buffering: log records are buffered in main memory, instead of being output directly to
stable storage.
• Log records are output to stable storage when a block of log records in the buffer is full, or a
log force operation is executed.
• Log force is performed to commit a transaction by forcing all its log records (including the commit
record) to stable storage.
• Several log records can thus be output using a single output operation, reducing the I/O cost.
Log Record Buffering (Cont.)
• The rules below must be followed if log records are buffered:
• Log records are output to stable storage in the order in which they are created.
• Transaction Ti enters the commit state only when the log record
<Ti commit> has been output to stable storage.
• Before a block of data in main memory is output to the database, all log records pertaining to
data in that block must have been output to stable storage.
• This rule is called the write-ahead logging or WAL rule
Shadow Paging
Shadow Paging
• Shadow paging is an alternative to log-based recovery; this scheme is useful if transactions
execute serially
• Idea: maintain two page tables during the lifetime of a transaction –the current page table, and
the shadow page table
• Store the shadow page table in nonvolatile storage, such that state of 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 page table is used for data item
accesses during execution of the transaction.
Sample Page Table
Example of Shadow Paging
Shadow and current page tables after write to page 4
Shadow Paging (Cont.)
• 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 table the new shadow page table, as follows:
• keep a pointer to the shadow page table at a fixed (known) location on disk.
• to make the current page table the new shadow page table, simply update the
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 can start right away, using the shadow page
table.
• Pages not pointed to from current/shadow page table should be freed (garbage collected).
Show Paging (Cont.)
• Advantages of shadow-paging over log-based schemes
• no overhead of writing log records
• recovery is trivial
• Disadvantages:
• Copying the entire page table is very expensive
• Can be reduced by using a page table structured like a B+-tree
• No need to copy entire tree, only need to copy paths in the tree that lead to updated leaf nodes
• Commit overhead is high even with above extension
• Need to flush every updated page, and page table
• Data gets fragmented (related pages get separated on disk)
• After every transaction completion, the database pages containing old versions of
modified data need to be garbage collected
• Hard to extend algorithm to allow transactions to run concurrently
• Easier to extend log based schemes
Remote Backup System
 Remote Backup Systems
 Remote backup systems provide high availability by allowing transaction
processing to continue even if the primary site is destroyed.
Remote Backup Systems (Cont.)
• Detection of failure: Backup site must detect when primary site has failed
• to distinguish primary site failure from link failure maintain several
communication links between the primary and the remote backup.
• Transfer of control:
• To take over control backup site first perform recovery using its copy of the
database and all the long records it has received from the primary.
• Thus, completed transactions are redone and incomplete transactions are rolled back.
• When the backup site takes over processing it becomes the new primary
• To transfer control back to old primary when it recovers, old primary must
receive redo logs from the old backup and apply all updates locally.
Remote Backup Systems (Cont.)
• Time to recover: To reduce delay in takeover, backup site periodically proceses the redo log
records (in effect, performing recovery from previous database state), performs a checkpoint, and
can then delete earlier parts of the log.
• Hot-Spare configuration permits very fast takeover:
• Backup continually processes redo log record as they arrive, applying the updates locally.
• When failure of the primary is detected the backup rolls back incomplete transactions, and is
ready to process new transactions.
• Alternative to remote backup: distributed database with replicated data
• Remote backup is faster and cheaper, but less tolerant to failure
Remote Backup Systems (Cont.)
• Ensure durability of updates by delaying transaction commit until update is logged at backup;
avoid this delay by permitting lower degrees of durability.
• One-safe: commit as soon as transaction’s commit log record is written at primary
• Problem: updates may not arrive at backup before it takes over.
• Two-very-safe: commit when transaction’s commit log record is written at primary and backup
• Reduces availability since transactions cannot commit if either site fails.
• Two-safe: proceed as in two-very-safe if both primary and backup are active. If only the primary is
active, the transaction commits as soon as is commit log record is written at the primary.
• Better availability than two-very-safe; avoids problem of lost transactions in one-safe.
End of Unit 8