VNSISPL_DBMS_Concepts_ch17

Chapter 17: Recovery System

Database System Concepts, 1st Ed.
©VNS InfoSolutions Private Limited, Varanasi(UP), India 221002
See www.vnsispl.com for conditions on re-use

Chapter 17: Recovery System
s Failure Classification
s Storage Structure
s Recovery and Atomicity
s Log-Based Recovery
s Shadow Paging
s Recovery With Concurrent Transactions
s Buffer Management
s Failure with Loss of Nonvolatile Storage
s Advanced Recovery Techniques
s ARIES Recovery Algorithm
s Remote Backup Systems

Database System Concepts, 1st Ed. 17.2 ©VNS InfoSolutions Private Limited, Varanasi(UP), India 221002

Failure Classification
s Transaction failure :
q Logical errors: transaction cannot complete due to some
internal error condition
q System errors: the database system must terminate an active
transaction due to an error condition (e.g., deadlock)
s System crash: a power failure or other hardware or software failure
causes the system to crash.
q 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
s Disk failure: a head crash or similar disk failure destroys all or part
of disk storage
q Destruction is assumed to be detectable: disk drives use
checksums to detect failures


Recovery Algorithms
s Recovery algorithms are techniques to ensure database consistency
and transaction atomicity and durability despite failures
q Focus of this chapter
s 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


Storage Structure
s Volatile storage:
q does not survive system crashes
q examples: main memory, cache memory
s Nonvolatile storage:
q survives system crashes
q examples: disk, tape, flash memory,
non-volatile (battery backed up) RAM
s Stable storage:
q a mythical form of storage that survives all failures
q approximated by maintaining multiple copies on distinct nonvolatile
media


Stable-Storage Implementation
s Maintain multiple copies of each block on separate disks
q copies can be at remote sites to protect against disasters such as
fire or flooding.
s Failure during data transfer can still result in inconsistent copies: Block
transfer can result in
q Successful completion
q Partial failure: destination block has incorrect information
q Total failure: destination block was never updated
s Protecting storage media from failure during data transfer (one
solution):
q 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.


Stable-Storage Implementation
(Cont.)

s Protecting storage media from failure during data transfer (cont.):
s Copies of a block may differ due to failure during output operation. To
recover from failure:
1. First find inconsistent blocks:
Expensive solution: Compare the two copies of every disk block.
Better solution:
q Record in-progress disk writes on non-volatile storage (Non-
volatile RAM or special area of disk).
q Use this information during recovery to find blocks that may be
inconsistent, and only compare copies of these.
q 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.


Data Access
s Physical blocks are those blocks residing on the disk.
s Buffer blocks are the blocks residing temporarily in main memory.
s Block movements between disk and main memory are initiated
through the following two operations:
q input(B) transfers the physical block B to main memory.
q output(B) transfers the buffer block B to the disk, and replaces
the appropriate physical block there.
s Each transaction Ti has its private work-area in which local copies of
all data items accessed and updated by it are kept.
q Ti's local copy of a data item X is called xi.
s We assume, for simplicity, that each data item fits in, and is stored
inside, a single block.


Data Access (Cont.)
s Transaction transfers data items between system buffer blocks and its
private work-area using the following operations :
q read(X) assigns the value of data item X to the local variable xi.

q write(X) assigns the value of local variable xi to data item {X} in
the buffer block.
q both these commands may necessitate the issue of an input(BX)
instruction before the assignment, if the block BX in which X
resides is not already in memory.
s Transactions
q Perform read(X) while accessing X for the first time;
q All subsequent accesses are to the local copy.
q After last access, transaction executes write(X).
s output(BX) need not immediately follow write(X). System can
perform the output operation when it deems fit.


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


Recovery and Atomicity
s Modifying the database without ensuring that the transaction will commit
may leave the database in an inconsistent state.
s Consider transaction Ti that transfers $50 from account A to account B;
goal is either to perform all database modifications made by Ti or none
at all.
s Several output operations may be required for Ti (to output A and B). A
failure may occur after one of these modifications have been made but
before all of them are made.


Recovery and Atomicity (Cont.)
s To ensure atomicity despite failures, we first output information
describing the modifications to stable storage without modifying the
database itself.
s We study two approaches:
q log-based recovery, and
q shadow-paging
s We assume (initially) that transactions run serially, that is, one after
the other.


Log-Based Recovery
s A log is kept on stable storage.
q
The log is a sequence of log records, and maintains a record of
update activities on the database.
s When transaction Ti starts, it registers itself by writing a
<Ti start>log record
s Before Ti executes write(X), a log record <Ti, X, V1, V2> is written,
where V1 is the value of X before the write, and V2 is the value to be
written to X.
q Log record notes that Ti has performed a write on data item Xj Xj
had value V1 before the write, and will have value V2 after the write.
s When Ti finishes it last statement, the log record <Ti commit> is
written.
s We assume for now that log records are written directly to stable
storage (that is, they are not buffered)
s Two approaches using logs
q Deferred database modification
q Immediate database modification

Deferred Database Modification
s The deferred database modification scheme records all
modifications to the log, but defers all the writes to after partial
commit.
s Assume that transactions execute serially
s Transaction starts by writing <Ti start> record to log.

s A write(X) operation results in a log record <Ti, X, V> being written,
where V is the new value for X
q Note: old value is not needed for this scheme
s The write is not performed on X at this time, but is deferred.
s When Ti partially commits, <Ti commit> is written to the log
s Finally, the log records are read and used to actually execute the
previously deferred writes.


(Cont.)
s During recovery after a crash, a transaction needs to be redone if and
only if both <Ti start> and<Ti commit> are there in the log.
s Redoing a transaction Ti ( redoTi) sets the value of all data items
updated by the transaction to the new values.
s Crashes can occur while
q the transaction is executing the original updates, or
q while recovery action is being taken
s example transactions T0 and T1 (T0 executes before T1):

T0: read (A) T1 : read (C)
A: - A - 50 C:- C- 100
Write (A) write (C)
read (B)
B:- B + 50
write (B)


(Cont.)
s Below we show the log as it appears at three instances of time.

s If log on stable storage at time of crash is as in case:
(a) No redo actions need to be taken
(b) redo(T0) must be performed since <T0 commit> is present
(c) redo(T0) must be performed followed by redo(T1) since
<T0 commit> and <Ti commit> are present


Immediate Database Modification
s The immediate database modification scheme allows database
updates of an uncommitted transaction to be made as the writes are
issued
q since undoing may be needed, update logs must have both old
value and new value
s Update log record must be written before database item is written
q We assume that the log record is output directly to stable storage
q Can be extended to postpone log record output, so long as prior to
execution of an output(B) operation for a data block B, all log
records corresponding to items B must be flushed to stable
storage
s Output of updated blocks can take place at any time before or after
transaction commit
s Order in which blocks are output can be different from the order in
which they are written.


Example
Log Write Output

<T0 start>
<T0, A, 1000, 950>
To, B, 2000, 2050
A = 950
B = 2050
<T0 commit>
<T1 start>
<T1, C, 700,x600>
1

C = 600
BB, BC
<T1 commit>
BA
s Note: BX denotes block containing X.


(Cont.)
s Recovery procedure has two operations instead of one:
q 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
q 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
s Both operations must be idempotent
q That is, even if the operation is executed multiple times the effect is
the same as if it is executed once
 Needed since operations may get re-executed during recovery
s When recovering after failure:
q Transaction Ti needs to be undone if the log contains the record
<Ti start>, but does not contain the record <Ti commit>.
q Transaction Ti needs to be redone if the log contains both the record
<Ti start> and the record <Ti commit>.
s Undo operations are performed first, then redo operations.


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.
(b) undo (T1) and redo (T0): C is restored to 700, and then A 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 C is set to 600


Checkpoints
s Problems in recovery procedure as discussed earlier :
1. searching the entire log is time-consuming
2. we might unnecessarily redo transactions which have already
3. output their updates to the database.
s Streamline recovery procedure by periodically performing
checkpointing
q Output all log records currently residing in main memory onto
stable storage.
q Output all modified buffer blocks to the disk.
q Write a log record < checkpoint> onto stable storage.


Checkpoints (Cont.)
s During recovery we need to consider only the most recent transaction
Ti that started before the checkpoint, and transactions that started
after Ti.
q Scan backwards from end of log to find the most recent
<checkpoint> record
q Continue scanning backwards till a record <Ti start> is found.
q Need only consider the part of log following above start record.
Earlier part of log can be ignored during recovery, and can be
erased whenever desired.
q For all transactions (starting from Ti or later) with no <Ti
commit>, execute undo(Ti). (Done only in case of immediate
modification.)
q Scanning forward in the log, for all transactions starting
from Ti or later with a <Ti commit>, execute redo(Ti).


Example of Checkpoints

Tc Tf
T1
T2
T3
T4

checkpoint system failure
s T1 can be ignored (updates already output to disk due to checkpoint)
s T2 and T3 redone.

s T4 undone


Recovery With Concurrent
Transactions
s We modify the log-based recovery schemes to allow multiple
transactions to execute concurrently.
q All transactions share a single disk buffer and a single log
q A buffer block can have data items updated by one or more
transactions
s We assume concurrency control using strict two-phase locking;
q 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?
s Logging is done as described earlier.
q Log records of different transactions may be interspersed in the log.
s The checkpointing technique and actions taken on recovery have to be
changed
q since several transactions may be active when a checkpoint is
performed.


Recovery With Concurrent Transactions
(Cont.)
s Checkpoints are performed as before, except that the checkpoint log record
is now of the form
< checkpoint L>
where L is the list of transactions active at the time of the checkpoint
q We assume no updates are in progress while the checkpoint is carried
out (will relax this later)
s When the system recovers from a crash, it first does the following:
q Initialize undo-list and redo-list to empty
q Scan the log backwards from the end, stopping when the first
<checkpoint L> record is found.
For each record found during the backward scan:
5 if the record is <Ti commit>, add Ti to redo-list

if the record is <Ti start>, then if Ti is not in redo-list, add Ti to
5
undo-list
q For every Ti in L, if Ti is not in redo-list, add Ti to undo-list


Recovery With Concurrent Transactions
(Cont.)

s At this point undo-list consists of incomplete transactions which must
be undone, and redo-list consists of finished transactions that must be
redone.
s Recovery now continues as follows:
q Scan log backwards from most recent record, stopping when
<Ti start> records have been encountered for every Ti in undo-
list.
s During the scan, perform undo for each log record that
belongs to a transaction in undo-list.
q Locate the most recent <checkpoint L> record.
q Scan log forwards from the <checkpoint L> record till the end of
the log.
s During the scan, perform redo for each log record that
belongs to a transaction on redo-list


Example of Recovery
s Go over the steps of the recovery algorithm on the following log:
<T0 start>
<T0, A, 0, 10>
<T0 commit>
<T1 start> /* Scan at step 1 comes up to here */
<T1, B, 0, 10>
<T2 start>
<T2, C, 0, 10>
<T2, C, 10, 20>
<checkpoint {T1, T2}>
<T3 start>
<T3, A, 10, 20>
<T3, D, 0, 10>
<T3 commit>


Log Record Buffering
s Log record buffering: log records are buffered in main memory,
instead of of being output directly to stable storage.
q 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.
s Log force is performed to commit a transaction by forcing all its log
records (including the commit record) to stable storage.
s Several log records can thus be output using a single output operation,
reducing the I/O cost.


Log Record Buffering (Cont.)
s The rules below must be followed if log records are buffered:
q Log records are output to stable storage in the order in which they
are created.
q Transaction Ti enters the commit state only when the log record
<Ti commit> has been output to stable storage.
q 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
– Strictly speaking WAL only requires undo information to be
output


Database Buffering
s Database maintains an in-memory buffer of data blocks
q When a new block is needed, if buffer is full an existing block needs to
be removed from buffer
q If the block chosen for removal has been updated, it must be output to
disk
s If a block with uncommitted updates is output to disk, log records with undo
information for the updates are output to the log on stable storage first
q (Write ahead logging)
s No updates should be in progress on a block when it is output to disk. Can
be ensured as follows.
q Before writing a data item, transaction acquires exclusive lock on block
containing the data item
q Lock can be released once the write is completed.
 Such locks held for short duration are called latches.
q Before a block is output to disk, the system acquires an exclusive latch
on the block
 Ensures no update can be in progress on the block


Buffer Management (Cont.)
s Database buffer can be implemented either
q in an area of real main-memory reserved for the database, or
q in virtual memory
s Implementing buffer in reserved main-memory has drawbacks:
q Memory is partitioned before-hand between database buffer and
applications, limiting flexibility.
q Needs may change, and although operating system knows best
how memory should be divided up at any time, it cannot change
the partitioning of memory.


Buffer Management (Cont.)
s Database buffers are generally implemented in virtual memory in spite
of some drawbacks:
q When operating system needs to evict a page that has been
modified, the page is written to swap space on disk.
q When database decides to write buffer page to disk, buffer page
may be in swap space, and may have to be read from swap space
on disk and output to the database on disk, resulting in extra I/O!
 Known as dual paging problem.
q Ideally when OS needs to evict a page from the buffer, it should
pass control to database, which in turn should
1. Output the page to database instead of to swap space (making
sure to output log records first), if it is modified
2. Release the page from the buffer, for the OS to use
Dual paging can thus be avoided, but common operating systems
do not support such functionality.


Failure with Loss of Nonvolatile
Storage
s So far we assumed no loss of non-volatile storage
s Technique similar to checkpointing used to deal with loss of non-
volatile storage
q Periodically dump the entire content of the database to stable
storage
q No transaction may be active during the dump procedure; a
procedure similar to checkpointing must take place
 Output all log records currently residing in main memory onto
stable storage.
 Output all buffer blocks onto the disk.
 Copy the contents of the database to stable storage.
 Output a record <dump> to log on stable storage.


Recovering from Failure of Non-Volatile
Storage
s To recover from disk failure
q restore database from most recent dump.
q Consult the log and redo all transactions that committed after
the dump
s Can be extended to allow transactions to be active during dump;
known as fuzzy dump or online dump
q Will study fuzzy checkpointing later


Advanced Recovery Algorithm


Advanced Recovery: Key Features
s Support for high-concurrency locking techniques, such as those used
for B+-tree concurrency control, which release locks early
q Supports “logical undo”
s Recovery based on “repeating history”, whereby recovery executes
exactly the same actions as normal processing
q including redo of log records of incomplete transactions, followed
by subsequent undo
q Key benefits
 supports logical undo
 easier to understand/show correctness


Advanced Recovery: Logical Undo
Logging

s Operations like B+-tree insertions and deletions release locks early.
q They cannot be undone by restoring old values (physical undo),
since once a lock is released, other transactions may have updated
the B+-tree.
q Instead, insertions (resp. deletions) are undone by executing a
deletion (resp. insertion) operation (known as logical undo).
s For such operations, undo log records should contain the undo operation
to be executed
q Such logging is called logical undo logging, in contrast to
physical undo logging
 Operations are called logical operations
q Other examples:
 delete of tuple, to undo insert of tuple
– allows early lock release on space allocation information
 subtract amount deposited, to undo deposit
– allows early lock release on bank balance


Advanced Recovery: Physical Redo
s Redo information is logged physically (that is, new value for each
write) even for operations with logical undo
q Logical redo is very complicated since database state on disk may
not be “operation consistent” when recovery starts
q Physical redo logging does not conflict with early lock release


Advanced Recovery: Operation
Logging
s Operation logging is done as follows:
q When operation starts, log <Ti, Oj, operation-begin>. Here Oj is a
unique identifier of the operation instance.
q While operation is executing, normal log records with physical redo
and physical undo information are logged.
q When operation completes, <Ti, Oj, operation-end, U> is logged,
where U contains information needed to perform a logical undo
information.
Example: insert of (key, record-id) pair (K5, RID7) into index I9
<T1, O1, operation-begin>
….
<T1, X, 10, K5> Physical redo of steps in insert
<T1, Y, 45, RID7>
<T1, O1, operation-end, (delete I9, K5, RID7)>


Advanced Recovery: Operation Logging
(Cont.)

s If crash/rollback occurs before operation completes:
q the operation-end log record is not found, and
q the physical undo information is used to undo operation.
s If crash/rollback occurs after the operation completes:
q the operation-end log record is found, and in this case
q logical undo is performed using U; the physical undo information
for the operation is ignored.
s Redo of operation (after crash) still uses physical redo information.


Advanced Recovery: Txn Rollback
Rollback of transaction Ti is done as follows:
s Scan the log backwards
q If a log record <Ti, X, V1, V2> is found, perform the undo and log a
special redo-only log record <Ti, X, V1>.
q If a <Ti, Oj, operation-end, U> record is found
 Rollback the operation logically using the undo information U.
– Updates performed during roll back are logged just like
during normal operation execution.
– At the end of the operation rollback, instead of logging an
operation-end record, generate a record
<Ti, Oj, operation-abort>.
 Skip all preceding log records for Ti until the record
<Ti, Oj operation-begin> is found


Advanced Recovery: Txn Rollback (Cont.)

s Scan the log backwards (cont.):
q If a redo-only record is found ignore it
q If a <Ti, Oj, operation-abort> record is found:
5 skip all preceding log records for Ti until the record
<Ti, Oj, operation-begin> is found.
q Stop the scan when the record <Ti, start> is found
q Add a <Ti, abort> record to the log
Some points to note:
s Cases 3 and 4 above can occur only if the database crashes while a
transaction is being rolled back.
s Skipping of log records as in case 4 is important to prevent multiple
rollback of the same operation.


Advanced Recovery: Txn Rollback
Example

s Example with a complete and an incomplete operation

<T1, start>
….
<T1, X, 10, K5>
<T1, Y, 45, RID7>
<T1, O1, operation-end, (delete I9, K5, RID7)>
<T1, Z, 45, 70>
 T1 Rollback begins here
<T1, Z, 45>  redo-only log record during physical undo (of incomplete O2)
<T1, Y, .., ..>  Normal redo records for logical undo of O1
…
<T1, O1, operation-abort>  What if crash occurred immediately after this?
<T1, abort>

Advanced Recovery: Crash
Recovery
The following actions are taken when recovering from system crash
s (Redo phase): Scan log forward from last < checkpoint L> record
till end of log
q Repeat history by physically redoing all updates of all
transactions,
q Create an undo-list during the scan as follows
 undo-list is set to L initially
 Whenever <Ti start> is found Ti is added to undo-list
 Whenever <Ti commit> or <Ti abort> is found, Ti is deleted
from undo-list
This brings database to state as of crash, with committed as well as
uncommitted transactions having been redone.
Now undo-list contains transactions that are incomplete, that is,
have neither committed nor been fully rolled back.


Advanced Recovery: Crash Recovery
(Cont.)

Recovery from system crash (cont.)
s (Undo phase): Scan log backwards, performing undo on log records
of transactions found in undo-list.
q Log records of transactions being rolled back are processed as
described earlier, as they are found
 Single shared scan for all transactions being undone
q When <Ti start> is found for a transaction Ti in undo-list, write a
<Ti abort> log record.
q Stop scan when <Ti start> records have been found for all Ti in
undo-list
s This undoes the effects of incomplete transactions (those with neither
commit nor abort log records). Recovery is now complete.


Advanced Recovery: Checkpointing
s Checkpointing is done as follows:
q Output all log records in memory to stable storage
q Output to disk all modified buffer blocks
q Output to log on stable storage a < checkpoint L> record.
Transactions are not allowed to perform any actions while
checkpointing is in progress.
s Fuzzy checkpointing allows transactions to progress while the most
time consuming parts of checkpointing are in progress
q Performed as described on next slide


Advanced Recovery: Fuzzy Checkpointing

s Fuzzy checkpointing is done as follows:
q Temporarily stop all updates by transactions
q Write a <checkpoint L> log record and force log to stable storage
q Note list M of modified buffer blocks
q Now permit transactions to proceed with their actions
q Output to disk all modified buffer blocks in list M
5 blocks should not be updated while being output
5 Follow WAL: all log records pertaining to a block must be output
before the block is output
q Store a pointer to the checkpoint record in a fixed position
last_checkpoint on disk
……
<checkpoint L>
…..
<checkpoint L>
last_checkpoint
…..

Log

Advanced Rec: Fuzzy Checkpointing
(Cont.)

s When recovering using a fuzzy checkpoint, start scan from the
checkpoint record pointed to by last_checkpoint
q Log records before last_checkpoint have their updates
reflected in database on disk, and need not be redone.
q Incomplete checkpoints, where system had crashed while
performing checkpoint, are handled safely


ARIES Recovery Algorithm


ARIES
s ARIES is a state of the art recovery method
q Incorporates numerous optimizations to reduce overheads during
normal processing and to speed up recovery
q The “advanced recovery algorithm” we studied earlier is modeled
after ARIES, but greatly simplified by removing optimizations
s Unlike the advanced recovery algorithm, ARIES
q Uses log sequence number (LSN) to identify log records
 Stores LSNs in pages to identify what updates have already
been applied to a database page
2. Physiological redo
3. Dirty page table to avoid unnecessary redos during recovery
4. Fuzzy checkpointing that only records information about dirty
pages, and does not require dirty pages to be written out at
checkpoint time
 More coming up on each of the above …


ARIES Optimizations
s Physiological redo
q Affected page is physically identified, action within page can be
logical
 Used to reduce logging overheads
– e.g. when a record is deleted and all other records have to be
moved to fill hole
» Physiological redo can log just the record deletion
» Physical redo would require logging of old and new values
for much of the page
 Requires page to be output to disk atomically
– Easy to achieve with hardware RAID, also supported by some
disk systems
– Incomplete page output can be detected by checksum
techniques,
» But extra actions are required for recovery
» Treated as a media failure


ARIES Data Structures
s ARIES uses several data structures
q Log sequence number (LSN) identifies each log record
 Must be sequentially increasing
 Typically an offset from beginning of log file to allow fast access
– Easily extended to handle multiple log files
q Page LSN
q Log records of several different types
q Dirty page table


ARIES Data Structures: Page LSN
s Each page contains a PageLSN which is the LSN of the last log
record whose effects are reflected on the page
q To update a page:
 X-latch the page, and write the log record
 Update the page
 Record the LSN of the log record in PageLSN
 Unlock page
q To flush page to disk, must first S-latch page
 Thus page state on disk is operation consistent
– Required to support physiological redo
q PageLSN is used during recovery to prevent repeated redo
 Thus ensuring idempotence


ARIES Data Structures: Log Record
s Each log record contains LSN of previous log record of the same transaction

LSN TransID PrevLSN RedoInfo UndoInfo
q LSN in log record may be implicit
s Special redo-only log record called compensation log record (CLR)
used to log actions taken during recovery that never need to be undone
q Serves the role of operation-abort log records used in advanced recovery
algorithm
q Has a field UndoNextLSN to note next (earlier) record to be undone
 Records in between would have already been undone
 Required to avoid repeated undo of already undone actions

LSN TransID UndoNextLSN RedoInfo

1 2 3 4 4' 3'
2' 1'


ARIES Data Structures: DirtyPage
Table
s DirtyPageTable
q List of pages in the buffer that have been updated
q Contains, for each such page
 PageLSN of the page
 RecLSN is an LSN such that log records before this LSN have
already been applied to the page version on disk
– Set to current end of log when a page is inserted into dirty
page table (just before being updated)
Page LSNs
on disk – Recorded in checkpoints, helps to minimize redo work

P1 P6 P23
P1 16 Page PLSN RLSN
25 16 19
… P1 25 17
P6 12 P6 16 15
..
P23 19 18
P15 9 P15
..
9
P23 11 Buffer Pool DirtyPage Table


ARIES Data Structures: Checkpoint
Log
s Checkpoint log record
q Contains:
 DirtyPageTable and list of active transactions
 For each active transaction, LastLSN, the LSN of the last log
record written by the transaction
q Fixed position on disk notes LSN of last completed
checkpoint log record
s Dirty pages are not written out at checkpoint time
 Instead, they are flushed out continuously, in the background
s Checkpoint is thus very low overhead
q can be done frequently


ARIES Recovery Algorithm
ARIES recovery involves three passes
s Analysis pass: Determines
q Which transactions to undo
q Which pages were dirty (disk version not up to date) at time of crash
q RedoLSN: LSN from which redo should start
s Redo pass:
q Repeats history, redoing all actions from RedoLSN
 RecLSN and PageLSNs are used to avoid redoing actions already
reflected on page
s Undo pass:
q Rolls back all incomplete transactions
 Transactions whose abort was complete earlier are not undone
– Key idea: no need to undo these transactions: earlier undo
actions were logged, and are redone as required


Aries Recovery: 3 Passes
s Analysis, redo and undo passes
s Analysis determines where redo should start
s Undo has to go back till start of earliest incomplete transaction

Last checkpoint End of Log
Time

Log Analysis pass
Redo pass

Undo pass


ARIES Recovery: Analysis

Analysis pass
s Starts from last complete checkpoint log record
q Reads DirtyPageTable from log record
q Sets RedoLSN = min of RecLSNs of all pages in DirtyPageTable
 In case no pages are dirty, RedoLSN = checkpoint record’s
LSN
q Sets undo-list = list of transactions in checkpoint log record
q Reads LSN of last log record for each transaction in undo-list from
checkpoint log record
s Scans forward from checkpoint
s .. Cont. on next page …


ARIES Recovery: Analysis (Cont.)
Analysis pass (cont.)
s Scans forward from checkpoint
q If any log record found for transaction not in undo-list, adds
transaction to undo-list
q Whenever an update log record is found
 If page is not in DirtyPageTable, it is added with RecLSN set to
LSN of the update log record
q If transaction end log record found, delete transaction from undo-list
q Keeps track of last log record for each transaction in undo-list
 May be needed for later undo
s At end of analysis pass:
q RedoLSN determines where to start redo pass
q RecLSN for each page in DirtyPageTable used to minimize redo work
q All transactions in undo-list need to be rolled back


ARIES Redo Pass
Redo Pass: Repeats history by replaying every action not already
reflected in the page on disk, as follows:
s Scans forward from RedoLSN. Whenever an update log record is
found:
1. If the page is not in DirtyPageTable or the LSN of the log record is
less than the RecLSN of the page in DirtyPageTable, then skip
the log record
2. Otherwise fetch the page from disk. If the PageLSN of the page
fetched from disk is less than the LSN of the log record, redo the
log record
NOTE: if either test is negative the effects of the log record have
already appeared on the page. First test avoids even fetching the
page from disk!


ARIES Undo Actions
s When an undo is performed for an update log record
q Generate a CLR containing the undo action performed (actions
performed during undo are logged physicaly or physiologically).
 CLR for record n noted as n’ in figure below
q Set UndoNextLSN of the CLR to the PrevLSN value of the update log
record
 Arrows indicate UndoNextLSN value
s ARIES supports partial rollback
q Used e.g. to handle deadlocks by rolling back just enough to release
reqd. locks
q Figure indicates forward actions after partial rollbacks
 records 3 and 4 initially, later 5 and 6, then full rollback
1 2 3 4 4' 3' 5 6 6' 5' 2' 1'


ARIES: Undo Pass
Undo pass:
s Performs backward scan on log undoing all transaction in undo-list
q Backward scan optimized by skipping unneeded log records as follows:
 Next LSN to be undone for each transaction set to LSN of last log
record for transaction found by analysis pass.
 At each step pick largest of these LSNs to undo, skip back to it and
undo it
 After undoing a log record
– For ordinary log records, set next LSN to be undone for
transaction to PrevLSN noted in the log record
– For compensation log records (CLRs) set next LSN to be undo to
UndoNextLSN noted in the log record
» All intervening records are skipped since they would have
been undone already
s Undos performed as described earlier


Other ARIES Features
s Recovery Independence
q Pages can be recovered independently of others
 E.g. if some disk pages fail they can be recovered from a backup
while other pages are being used
s Savepoints:
q Transactions can record savepoints and roll back to a savepoint
 Useful for complex transactions
 Also used to rollback just enough to release locks on deadlock


Other ARIES Features (Cont.)
s Fine-grained locking:
q Index concurrency algorithms that permit tuple level locking on
indices can be used
 These require logical undo, rather than physical undo, as in
advanced recovery algorithm
s Recovery optimizations: For example:
q Dirty page table can be used to prefetch pages during redo
q Out of order redo is possible:
 redo can be postponed on a page being fetched from disk,
and
performed when page is fetched.
 Meanwhile other log records can continue to be processed


Remote Backup Systems


Remote Backup Systems
s Remote backup systems provide high availability by allowing transaction
processing to continue even if the primary site is destroyed.


Remote Backup Systems (Cont.)
s Detection of failure: Backup site must detect when primary site has
failed
q to distinguish primary site failure from link failure maintain several
communication links between the primary and the remote backup.
q Heart-beat messages
s Transfer of control:
q 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.
q When the backup site takes over processing it becomes the new
primary
q 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.



s 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.
s Hot-Spare configuration permits very fast takeover:
q Backup continually processes redo log record as they arrive,
applying the updates locally.
q When failure of the primary is detected the backup rolls back
incomplete transactions, and is ready to process new transactions.
s Alternative to remote backup: distributed database with replicated data
q Remote backup is faster and cheaper, but less tolerant to failure
 more on this in Chapter 19



s Ensure durability of updates by delaying transaction commit until update is
logged at backup; avoid this delay by permitting lower degrees of durability.
s One-safe: commit as soon as transaction’s commit log record is written at
primary
q Problem: updates may not arrive at backup before it takes over.
s Two-very-safe: commit when transaction’s commit log record is written
at primary and backup
q Reduces availability since transactions cannot commit if either site fails.
s 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.
q Better availability than two-very-safe; avoids problem of lost
transactions in one-safe.


End of Chapter


Shadow Paging
s Shadow paging is an alternative to log-based recovery; this scheme is
useful if transactions execute serially
s Idea: maintain two page tables during the lifetime of a transaction –the
current page table, and the shadow page table
s Store the shadow page table in nonvolatile storage, such that state of the
database prior to transaction execution may be recovered.
q Shadow page table is never modified during execution
s To start with, both the page tables are identical. Only current page table is
used for data item accesses during execution of the transaction.
s Whenever any page is about to be written for the first time
q A copy of this page is made onto an unused page.
q The current page table is then made to point to the copy
q The update is performed on the copy


Sample Page Table


Example of Shadow Paging
Shadow and current page tables after write to page 4


Shadow Paging (Cont.)
s 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:
q keep a pointer to the shadow page table at a fixed (known) location
on disk.
q to make the current page table the new shadow page table, simply
update the pointer to point to current page table on disk
s Once pointer to shadow page table has been written, transaction is
committed.
s No recovery is needed after a crash — new transactions can start right
away, using the shadow page table.
s Pages not pointed to from current/shadow page table should be freed
(garbage collected).


Show Paging (Cont.)
s Advantages of shadow-paging over log-based schemes
q no overhead of writing log records
q recovery is trivial
s Disadvantages :
q 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
q Commit overhead is high even with above extension
 Need to flush every updated page, and page table
q Data gets fragmented (related pages get separated on disk)
q After every transaction completion, the database pages containing old
versions of modified data need to be garbage collected
q Hard to extend algorithm to allow transactions to run concurrently
 Easier to extend log based schemes


Block Storage Operations


Portion of the Database Log
Corresponding to T 0 and T 1


State of the Log and Database


Portion of the System Log Corresponding
to T 0 and T 1


State of System Log and Database


VNSISPL_DBMS_Concepts_ch17

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