lec05-aries-rev.pdf
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ARIES: Logging and
Recovery
Slides derived from Joe Hellerstein;
Updated by A. Fekete
If you are going to be in the
logging business, one of the
things that you have to do is to
learn about heavy equipment.
-
Robert VanNatta,
Logging History of
Columbia County
Review: The ACID properties
• A tomicity: All actions in the Xact happen, or none happen.
• C onsistency: If each Xact is consistent, and the DB starts
consistent, it ends up consistent.
• I solation: Execution of one Xact is isolated from that of other
Xacts.
• D urability: If a Xact commits, its effects persist.
• The Recovery Manager guarantees Atomicity & Durability.
Motivation
• Atomicity:
– Transactions may abort (“Rollback”).
• Durability:
– What if DBMS stops running? (Causes?)
crash!
Desired Behavior after
system restarts:
– T1, T2 & T3 should be
durable.
– T4 & T5 should be
aborted (effects not seen).
T1
T2
T3
T4
T5
Intended Functionality
• At any time, each data item contains the value
produced by the most recent update done by a
transaction that committed
Assumptions
• Essential concurrency control is in effect.
– For read/write items: Write locks taken and held
till commit
• Eg Strict 2PL, but read locks not important for recovery.
– For more general types: operations of concurrent
transactions commute
• Updates are happening “in place”.
– i.e. data is overwritten on (deleted from) its
location.
• Unlike multiversion approaches
• Buffer in volatile memory; data persists on
disk
Challenge: REDO
• Need to restore value 1 to item
– Last value written by a committed transaction
Action Buffer Disk
Initially 0
T1 writes 1 1 0
T1 commits 1 0
CRASH 0
Challenge: UNDO
• Need to restore value 0 to item
– Last value from a committed transaction
Action Buffer Disk
Initially 0
T1 writes 1 1 0
Page flushed 1
CRASH 1
Handling the Buffer Pool
• Can you think of a simple scheme
to guarantee Atomicity &
Durability?
• Force write to disk at commit?
– Poor response time.
– But provides durability.
• No Steal of buffer-pool frames
from uncommited Xacts (“pin”)?
– Poor throughput.
– But easily ensure atomicity
Force
No Force
No Steal
Steal
Trivial
Desired
of 11
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