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C H A P T E R

14

Transactions

Practice Exercises

14.1 Answer: Even in this case the recovery manager is needed to perform

roll-back of aborted transactions.

14.2

Answer: There are several steps in the creation of a ﬁle. A storage area

is assigned to the ﬁle in the ﬁle system, a unique i-number is given to the

ﬁle and an i-node entry is inserted into the i-list. Deletion of ﬁle involves

exactly opposite steps.

For the ﬁle system user in UNIX, durability is important for obvious rea-

sons, but atomicity is not relevant generally as the ﬁle system doesn’t

support transactions. To the ﬁle system implementor though, many of the

internal ﬁle system actions need to have transaction semantics. All the

steps involved in creation/deletion of the ﬁle must be atomic, otherwise

there will be unreferenceable ﬁles or unusable areas in the ﬁle system.

14.3 Answer: Database systems usually perform crucial tasks whose effects

need to be atomic and durable, and whose outcome affects the real world

in a permanent manner. Examples of such tasks are monetary transac-

tions, seat bookings etc. Hence the ACID properties have to be ensured. In

contrast, most users of ﬁle systems would not be willing to pay the price

(monetary, disk space, time) of supporting ACID properties.

14.4

Answer: If a transaction is very long or when it fetches data from a

slow disk, it takes a long time to complete. In absence of concurrency,

other transactions will have to wait for longer period of time. Average

responce time will increase. Also when the transaction is reading data from

disk, CPU is idle. So resources are not properly utilized. Hence concurrent

execution becomes important in this case. However, when the transactions

are short or the data is available in memory, these problems do not occur.

14.5

Answer: Most of the concurrency control protocols (protocols for en-

suring that only serializable schedules are generated) used in practice are

based on conﬂict serializability—they actually permit only a subset of

1

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