This document discusses database buffering policies and write-ahead logging. It describes the no-force and no-steal policies, noting advantages like faster commits but also disadvantages like buffers filling with updated pages. The no-steal policy does not write modified blocks until transaction commit, while the steal policy may write blocks even if the transaction has not committed. It also explains that write-ahead logging is needed with steal policies to ensure consistency if a crash occurs, by requiring all log records be written to stable storage before committing a transaction or writing data blocks.

1. BUFFER MANAGEMENT-
DATABASE BUFFERING
Submitted by
Balasara Kshiar
MCA 4th
Sem

2. Database Buffering

3. Database Buffering contd..
• Advantages of no-force policy:
Faster commit of transaction.
Reduce the number of output operations
to disk.

4. Database Buffering contd..
• No-Steal policy:
– Blocks modified by a transaction that is still active
should not be written to disk.
• Steal-policy:
Modified blocks written to disk even if
transaction has not committed.

5. Database Buffering contd..
• Disadvantage of no-steal policy:
Buffer may be full of updated pages which
cannot be written to disk.

6. Database Buffering contd..
Steal policy needs to implement write-ahead logging(WAL)
rule.
• Ti enters commit state after <Ti commit> log record has
been o/p to stable storage
• Before <Ti commit> log record can be o/p to stable storage,
all log records pertaining to transaction Ti must have been
o/p to stable storage
• Before a block of data in main memory can be o/p to
database, all log records pertaining to data in that block
must have been o/p to stable storage

7. Database Buffering contd..
Example showing the need for WAL rule:
Let To be a transaction that transfers Rs.50
from account A to B
To :
read(A);
A:=A-50;
Write(A);
read(B);
B:=B+50;
write(B);

8. Database Buffering contd..
• Log record:
<T0 start>
<T0, A, 1000,950>
<T0, B, 2000,2050>
<T0 commit>

9. Database Buffering contd..
• Suppose state of the log is:
<T0 start>
<T0, A, 1000,950>
To issues a read(B).
B is not in memory and memory is full.
A is output to disk and system crash.
A-950,B-2000 -->inconsistent.
But using WAL rule it will not be inconsistent.

10. Database Buffering contd..
• No write to the block when the block is being output to
disk.
Lock the block when a write operation is in process. Then
released when done.
• When a block is output,no write operation is
performed.
These are ensured by special means of locking.
– Lock the block.
– log records to stable storage.
– Output block to disk.
– Release the lock.

11. Database Buffering contd..
• Latches:
Locks that are held for short duration.
• Locks are also needed when checkpoint is
in progress.

12. THANKYOU…