Crash Recovery

Review: The ACID properties A A tomicity: All actions in the Xaction happen, or none happen. C C onsistency: If each Xaction is consistent, and the DB starts consistent, it ends up consistent. I I solation: Execution of one Xaction is isolated from that of other Xacts. D D urability: If a Xaction commits, its effects persist. CC guarantees Isolation and Atomicity. The Recovery Manager guarantees Atomicity & Durability.

Why is recovery system necessary? Transaction failure :  Logical errors: application errors (e.g. div by 0, segmentation fault)  System errors: deadlocks System crash: hardware/software failure causes the system to crash. Disk failure: head crash or similar disk failure destroys all or part of disk storage The data we will lose can be in main memory or in disk

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

Recovery and Durability To achieve Durability: Put data on stable storage To approximate stable storage make two copies of data Problem: data transfer failure

Stable-Storage Implementation Solution:  Write to the first disk  Write to the second disk when the first disk completes  The process is complete only after the second write completes successfully Recovery (from disk failures, etc):  Detect bad blocks with the checksum (e.g. parity)  Two good copies, equal blocks: done  One good, one bad : copy good to bad  Two bad copies: ignore write  Two good, unequal blocks? Ans: Copy the second to the first

Recovery and Atomicity Durability is achieved by making 2 copies of data What about atomicity…  Crash may cause inconsistencies…

Recovery and Atomicity Example: transfer $50 from account A to account B  goal is either to perform all database modifications made by T i or none at all. Requires several inputs (reads) and outputs (writes) Failure after output to account A and before output to B….  DB is corrupted!

Recovery Algorithms Recovery algorithms are techniques to ensure database consistency and transaction atomicity and durability despite failures 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

Background: Data Access Physical blocks: blocks on disk. Buffer blocks: blocks in main memory. Data transfer:  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. Each transaction T i has its private work-area in which local copies of all data items accessed and updated by it are kept.  T i 's local copy of a data item X is called x i. Assumption: each data item fits in and is stored inside, a single block.

Data Access (Cont.) Transaction transfers data items between system buffer blocks and its private work-area using the following operations :  read(X) assigns the value of data item X to the local variable x i.  write(X) assigns the value of local variable x i to data item {X} in the buffer block.  both these commands may necessitate the issue of an input(B X ) instruction before the assignment, if the block B X in which X resides is not already in memory. Transactions  Perform read(X) while accessing X for the first time;  All subsequent accesses are to the local copy.  After last access, transaction executes write(X). output(B X ) need not immediately follow write(X). System can perform the output operation when it deems fit.

X Y A B x1x1 y1y1 buffer Buffer Block A Buffer Block B input(A) output(B) read(X) write(Y) disk work area of T 1 work area of T 2 memory x2x2

Recovery and Atomicity (Cont.) To ensure atomicity, first output information about modifications to stable storage without modifying the database itself. We study :  log-based recovery  Database Files storing the actual data  Log –Another file storing the actions of transactions

Log-Based Recovery Simplifying assumptions:  Transactions run serially  logs are written directly on the stable stogare Log: a sequence of log records; maintains a record of update activities on the database. (Write Ahead Log, W.A.L.) Log records for transaction Tj:  Two approaches using logs  Deferred database modification  Immediate database modification

Log example Transaction T1 Read(A) A =A-50 Write(A) Read(B) B = B+50 Write(B) Log

Deferred Database Modification Write the modifications to a log And Defer execution of write operations till database commits Example T i starts: write a record to log. T i write(X)  write to log: V is the new value for X  The write is deferred  Note: old value is not needed for this scheme T i partially commits:  Write to the log DB updates by reading and executing the log:  ……

Deferred Database Modification How to use the log for recovery after a crash?  Redo: if both and are there in the log.  Ignore otherwise. Crashes can occur while  the transaction is executing the original updates, or  while recovery action is being taken  REDO should be idempotent (i.e., executing several times should be the same as once) example transactions T 0 and T 1 (T 0 executes before T 1 ): T 0 : read (A) T 1 : read (C) A: - A - 50 C:-C- 100 Write (A) write (C) read (B) B:- B + 50 write (B)

Deferred Database Modification (Cont.) Below we show the log as it appears at three instances of time. (a) (b) (c) - Only new values of an item are recorded in the log (old values can be omitted). - At partial commit time, after the log records are on stable storage, the items are written to the database.

Immediate Database Modification Allow database modifications to be OUTPUT to the database before transaction commits. Tighter logging rules are needed to ensure transaction are undoable  Write records must be of the form:  Both old and new values  Log record must be written before database item is written/output  Output of DB items can occur:  Before or after commit  In any order  But Log record should be written prior to Output of an item to database

Immediate Database Modification Example Log Database A = 950 B = 2050 C = 600

Immediate Database Modification (Cont.) Recovery procedure :  Undo : is in the log but is not. Undo:  restore the value of all data items updated by T i to their old values, going backwards from the last log record for T i  Redo: and are both in the log. Redo:  sets the value of all data items updated by T i to the new values, going forward from the first log record for T i Both operations must be idempotent: even if the operation is executed multiple times the effect is the same as if it is executed once Undo operations are performed first, then redo operations. Why?

I M Recovery Example (a) (b) (c) Recovery actions in each case above are: (a) undo (T 0 ): B is restored to 2000 and A to (b) undo (T 1 ) and redo (T 0 ): C is restored to 700, and then A and B are set to 950 and 2050 respectively. (c) redo (T 0 ) and redo (T 1 ): A and B are set to 950 and 2050 respectively. Then C is set to 600

Checkpoints Problems in recovery procedure as discussed earlier : 1. searching the entire log is time-consuming 2. we might unnecessarily redo transactions which have already output their updates to the database. How to avoid redundant redoes?  Put marks in the log indicating that at that point DB and log are consistent. Checkpoint!

Checkpoints At a checkpoint: Output all log records currently residing in main memory onto stable storage. Output all modified buffer blocks to the disk. Write a log record onto stable storage.

Checkpoints (Cont.) Recovering from log with checkpoints: 1. Scan backwards from end of log to find the most recent record 2. Continue scanning backwards till a record is found. 3. Need only consider the part of log following above start record. Why? 4. After that, recover from log with the rules that we had before.

Example of Checkpoints TcTc TfTf T1T1 T2T2 T3T3 T4T4 checkpoint system failure checkpoint T 1 can be ignored (updates already output to disk due to checkpoint) T 2 and T 3 redone. T 4 undone

Recovery With Concurrent Transactions To permit concurrency:  All transactions share a single disk buffer and a single log  Concurrency control: Strict 2PL :i.e. Release eXclusive locks only after commit. Why?  Logging is done as described earlier. The checkpointing technique and actions taken on recovery have to be changed (based on ARIES)  since several transactions may be active when a checkpoint is performed.

Recovery With Concurrent Transactions (Cont.) Checkpoints for concurrent transactions: L: the list of transactions active at the time of the checkpoint  We assume no updates are in progress while the checkpoint is carried out Recovery for concurrent transactions, 3 phases  Analysis: Construction of Undo, Redo-Lists  Perform Undo  Perform Redo

Recovery With Concurrent Transactions (Cont.) 1. ANALYSIS: Construction of Undo, Redo-lists a. Initialize undo-list and redo-list to empty b. Scan the log backwards from the end, stopping when the first record is found. For each record found during the backward scan:  if the record is, add T i to redo-list  if the record is, then if T i is not in redo-list, add T i to undo-list c. For every T i in L, if T i is not in redo-list, add T i to undo-list  This will add txns to undo-list that started prior to L but did not commit ANALYSIS

Recovery With Concurrent Transactions Perform UNDO: Scan log backwards  Perform undo(T) for every transaction in undo-list  Stop when reach for every T in undo-list. Perform REDO: Locate the most recent record. 1. Scan log forwards from the record till the end of the log. 2. perform redo for each log record that belongs to a transaction on redo-list UNDO REDO

Example of Recovery Go over the steps of the recovery algorithm on the following log: DB A B C D Initial At crash After rec Redo-list{T3} Undo-list{T4, T1, T2} Undo: Set C to 10 Set C to 0 Set B to 0 Redo: Set A to 20 Set D to 10

Recovery Summary Durability  Duplicate copies Ensuring Atomicity  Deferred Logging --- Only Redo  Immediate Logging-- Undo and Redo  Checkpoints : to limit log size  Recovery of concurrent txns with checkpoints  3 phases

What we covered Relational model - SQL  Formal & commercial query languages Functional Dependencies Normalization Interfacing with Databases: PL/SQL, JDBC… Physical Design File storage Indexing Query Processing and Optimization Txns Concurrency Control Recovery