DBMS - 2 Mark QA

CS 2255 DATABASE MANAGEMENT SYSTEMS

(Common to CSE & IT)

2 Mark Question Answer

UNIT I

PART A

1.What is Database Management System? Why do we need a DBMS ?

DBMS is a collection of interrelated data and a set of programs to access those data. It is to provide a way to store and retrieve information that is both convenient and efficient.

2.List any two advantages of database systems.

Security, Integrity, Atomicity, Concurrent access anomalies

3.Define Data Model and its types.

Data model is a collection of tools for describing Data, Data relationships, Data semantics, Data constraints. Types: Relational model, Entity-Relationship data model Object-based data models, Semi structured data model (XML), Network model, Hierarchical model.

4.Explain the role and functions of the database administrator(DB Manager).

Coordinates all the activities of the database system, Storage structure and access method definition, Schema and physical organization modification, Granting users authority to access the database, Backing up data, Monitoring performance and responding to changes.

5.Give the limitations of E-R model? How do you overcome this?

·          Limited constraint representation

·          Limited relationship representation

·          No data manipulation language

·          Loss of information content

6.What are the limitations of  file system.

Data redundancy and inconsistency, Difficulty in access data, data isolation, integrity, atomicity, concurrent access anomalies.

7.What is logical data Independency ?.

Application programs are said to exhibit physical data independence if they do not depend on the physical schema and thus need not be rewritten if the physical schema changes.

8.Define DML?.

DML is a language that enables users to access or manipulate data as organized by the appropriate data model. Data Manipulation Language Insert, Select, Delete

9.Define Data Dictionary?.

A data dictionary is a data structure which stores meta data about the structure of the database ie. the schema of the database.

10.Define Data independence.

Application programs are said to exhibit physical data independence if they do not depend on the physical schema and thus need not be rewritten if the physical schema changes.

11.With an relevant example explain Ternary Relationship.

A relationship is an association among several entities. Ternary relationship has three relationship. Eg. Parent related to child.

 

12.List any eight applications of DBMS.

a)  Banking

b)  Airlines

c)  Universities

d)  Credit card transactions

e)  Tele communication

f)  Finance

g)  Sales

h)  Manufacturing

i)  Human resources

13.What are the advantages of DBMS?.

Controlling redundancy

b)  Restricting unauthorized access

c)  Providing multiple user interfaces

d)  Enforcing integrity constraints.

e)  Providing back up and recovery

14.Give the levels of data abstraction?

a)  Physical level

b)  logical level

c)  view level

15.Define instance and schema?

Instance: Collection of data stored in the data base at a particular moment is called an Instance of the database.

Schema: The overall design of the data base is called the data base schema.

16. Define the terms 1) physical schema 2) logical schema.

Physical schema: The physical schema describes the database design at the physical level, which is the lowest level of abstraction describing how the data are actually stored.

Logical schema: The logical schema describes the database design at the logical level, which describes what data are stored in the database and what relationship exists among the data.

17.What is conceptual schema?

The schemas at the view level are called subschemas that describe different views of the database.

18. What is storage manager?

A storage manager is a program module that provides the interface between the

low level data stored in a database and the application programs and queries submitted to the system.

19.What are the components of storage manager?

The storage manager components include

a)  Authorization and integrity manager

b)  Transaction manager

c)  File manager

d)  Buffer manager

20.What is the purpose of storage manager?

The storage manager is responsible for the following

a)  Interaction with he file manager

b)  Translation of DML commands in to low level file system commands

c)  Storing, retrieving and updating data in the database

21.List the data structures implemented by the storage manager. The storage manager implements the following data structure

a)  Data files

b)  Data dictionary

c)  indices

UNIT II

PART A

1.Define query language. Give the classification of query language.

Query language is used to query a database. It can define the structure of the data, modify and specify security constraints. Procedural and non procedural query language

2.What are three characteristics of a Relational Database System?

Three characteristics of a Relational Database System are BCNF, Lossless join and Dependency preservation.

3.State the differences between Security and Integrity?

Integrity ensures that changes made to the database by authorized users do not result in loss of data consistency. Eg. Check constraints. Security refers to protection from malicious access. It is done in database systems, OS, Network physical, Human.

4.What are the pitfalls in relational database design?.

Repetition of Information.

Inability to represent certain information.

5.Distinguish between primary key and candidate key.

Primary key denotes a candidate key for identify entities with in entity set. Super keys for which no proper subset is a super key is called candidate key.

6.Give the reasons why Null values might be introduces into the database.

Null values are introduced into the database to indicate the absence of information about the value. Eg. Select loan number form loan where amount is null.

7.What is static SQL? How does it differ from dynamic SQL?

In static SQL, SQL queries must be present in compile time. Dynamic SQL allows programs to construct and submit SQL queries at run time.

8.What are the different types of integrity constraints used in designing a relational database?

Domain constraints, Referential Integrity, Check constraints, Assertion, Triggers.

9.With an example explain a weak entity in an ER diagram.

An entity without primary key is weak entity. Eg. Payment number in payment relation having payment number payment date and payment amount as fields.

10.What is  referential integrity.

The condition that value which is appearing in one relation for a given set of attributes also appears for a certain set of attributes in another relation is called as referential integrity.

11.What is domain integrity? Give example.

There are many domain types, such as integer, character and date/time. Create domain can be used to define new domains. Value of one type can be cast to other.

Eg. Create domain Dollars numeric (12,2) Create domain Pounds numeric (12,2) Cast r.A as pounds

12.Consider the following relations.

Empno(Eno,Name,Date_of_Birth, Sex,Date_Of_Joining,Basic_Pay,Dept)

Develop an SQL query that will find and display the Dept and Average Basic_Pay in eac Dept.

Select dept, Average(Basic_Pay) from empno Group by dept.

13.Define a distributed database management system.

A distributed database system consists of loosely coupled sites that share no physical component. Database systems that run on each site are independent of each other. Transactions may access data at one or more sites

14.List the SQL statements used for Transaction control. Commit, roll back.

15.What is a nested relation ?

Attributes in relations have multiple values. Example: library information system. Each book has title, a set of authors, Publisher, and a set of keywords.

16.State the advantages of distributed systems.

·         A distributed database system consists of loosely coupled sites that share no physical component

·         Database systems that run on each site are independent of each other

·         Transactions may access data at one or more sites

17.Name the different types of joins supported in SQL.

Inner join, Natural inner join, natural left join, natural right join and full outer join.

18.Define the terms fragmentation and replication, in terms of where data is stored.

System maintains several identical replica of relation and stores each replica and a different site. Fragmentation is that the system partitions the relation into several fragments and stores each fragment at a different site.

19.What are the types of transparencies that a distributed database must support? Why?

Fragmentation, Replication and location. The user of a distributed database system should not be required to know either where the data are physically located ore how the data can be accessed at the specific local site.

20.What is an entity relationship model?

The entity relationship model is a collection of basic objects called entities and relationship among those objects. An entity is a thing or object in the real world that is distinguishable from other objects.

21.Define the terms

i)  Entity set

ii)   Relationship set

Entity set: The set of all entities of the same type is termed as an entity set. Relationship set: The set of all relationships of the same type is termed as a relationship set.

22. Define single valued and multivalued attributes.

Single valued attributes: attributes with a single value for a particular entity are called single valued attributes.

Multivalued attributes: Attributes with a set of value for a particular entity are called multivalued attributes.

23.What are stored and derived attributes?

Stored attributes: The attributes stored in a data base are called stored attributes. Derived attributes: The attributes that are derived from the stored attributes are called derived attributes.

 

24.What are composite attributes?

Composite attributes can be divided in to sub parts.

25.Define null values.

In some cases a particular entity may not have an applicable value for an attribute

or if we do not know the value of an attribute for a particular entity. In these cases null value is used.

26.Define the terms

i)  Entity type

ii)   Entity set

Entity type: An entity type defines a collection of entities that have the same attributes.

Entity set: The set of all entities of the same type is termed as an entity set.

27.What is a candidate key?

Minimal super keys are called candidate keys.

28.What is a primary key?

Primary key is chosen by the database designer as the principal means of identifying an entity in the entity set.

29.What is a super key?

A super key is a set of one or more attributes that collectively allows us to identify uniquely an entity in the entity set.

30.Define- relational algebra.

The relational algebra is a procedural query language. It consists of a set of operations that take one or two relation as input and produce a new relation as output.

31.Write short notes on domain relational calculus

The domain relational calculus uses domain variables that take on values from an attribute domain rather than values for entire tuple.

32.What is foreign key?

A relation schema r1 derived from an ER schema may include among its

attributes the primary key of another relation schema r2.this attribute is called a foreign key from r1 referencing r2.

33.What is horizontal fragmentation?

Horizontal fragmentation splits the relation by assuming each tuple of r to one or more fragments.

34.What is vertical fragmentation?

Vertical fragmentation splits the relation by decomposing the scheme R of relation r.

35. Give the forms of triggers?

_ The triggering event can be insert or delete. _ For updated the trigger can specify columns. _ The referencing old row as clause

_ The referencing new row as clause

_ The triggers can be initiated before the event or after the event.

UNIT III

PART A

1. With an example explain a Functional Dependency.

Let R be a relation schema, α subset of  R.β is also subset of R. The functional dependency α->βholds on R if and only if for any legal relations r(R), whenever any two tuples t1 and t2 of r agree on the attributes α, they also agree on the attributes β . That is, t1[α] = t2 [α] =>t1[ β ] = t2 [ β ] eg.

Here A->C.

2.Define Normalization?

Normalization is to generate a set of relation schemas that allows to store information with out redundancy and to retrieve information easily

3.Justify the need for normalization.

A bad relational database have the following pit falls: inability to represent some data and redundancy. To overcome this pit falls, normalization is to be done. Good database design should be Lossless join and Dependency preservation. Decomposition of databases are done to make the database consistent and efficient.

4.Why it is necessary to decompose a relation?

A relation is to be decomposed in order to avoid repetition of information and inability to represent certain information.

5.What is decomposition and how does it address redundancy?

Decomposition is reducing a large database to set of smaller database. When splitting database, some of the fields will be repeated in databases to maintain association with records.

6.Give the comparison between BCNF and 3NF.

3NF can be achieved without sacrificing losslessness or dependency preservation. Disadvantage: Use null values to represent some of the meaningful relation and repetition of information. If it is difficult to get a dependency-preserving BCNF algorithm, it is preferable to opt for BCNF and use techniques such as materialized views.

7.Explain trivial dependency?

Functional dependency of the form α->β is trivial if β is subset of α. Trivial functional dependencies are satisfied by all the relations.

8.Define canonical cover?

A canonical cover Fc for F is a set of dependencies such that F logically implies all dependencies in FC and Fc logically implies all dependencies in F. Fc must have the following properties.

9.List the properties of canonical cover.

Fc must have the following properties.

_ No functional dependency in Fc contains an extraneous attribute. _ Each left side of a functional dependency in Fc is unique.

10. Explain the desirable properties of decomposition.

_ Lossless-join decomposition _ Dependency preservation

_ Repetition of information

11.What is 2NF?

A relation schema R is in 2NF if it is in 1NF and every non-prime attribute A in R is fully functionally dependent on primary key.

 

12.Define Boyce codd normal form

A relation schema R is in BCNF with respect to a set F of functional dependencies if, for all functional dependencies in F+ of the form α->β.

13.What is first normal form?

The domain of attribute must include only atomic (simple, indivisible) values.

14.What is meant by computing the closure of a set of functional dependency?

The closure of F denoted by F+ is the set of functional dependencies logically implied by F.

15.What are axioms?

Axioms or rules of inference provide a simpler technique for reasoning about functional dependencies.

16.What are the uses of functional dependencies?

_ To test relations to see whether they are legal under a given set of functional dependencies.

_ To specify constraints on the set of legal relations.

17.What is meant by normalization of data?

It is a process of analyzing the given relation schemas based on their Functional Dependencies (FDs) and primary key to achieve the properties

_ Minimizing redundancy

_ Minimizing insertion, deletion and updating anomalies.

18.Why certain Functional dependencies are called trivial dependency?.

The functional dependency α->βis trivial if βC α

19.What is multi valued dependency?

If A->B then, two tuples with same A value have different B value. Eg. Customer name -> customer street, customer city.

20. What is fourth normal form?.

A relation schema R is in 4NF with respect to a set D of functional and multivalued dependencies if for all multivalued dependencies in D+ of the form

a ®® b, where a Í R and b Í R, at least one of the following hold: 1.a ®® b is trivial (i.e., b Í a or a È b = R)

2.a is a superkey for schema R If a relation is in 4NF it is in BCNF

21.What are the closure set of functional dependency?.

-Given a set F of functional dependencies, there are certain other functional dependencies that are logically implied by F.

- For example: If A ® B and B ® C, then we can infer that A ® C -The set of all functional dependencies logically implied by F is the closure of F. -We denote the closure of F by F+.

-F+ is a superset of F.

22.What are the goals of normalization?.

Let R be a relation scheme with a set F of functional dependencies. Decide whether a relation scheme R is in “good” form.

In the case that a relation scheme R is not in “good” form, decompose it into a set of relation scheme {R1, R2, ..., Rn} such that

-       each relation scheme is in good form

-       the decomposition is a lossless-join decomposition

-       Preferably, the decomposition should be dependency preserving.

 

23.How would you find the extraneous attribute?.

Consider a set F of functional dependencies and the functional dependency a ® b in

F.

-     Attribute A is extraneous in a if A Î a

and F logically implies (F – {a ® b}) È {(a – A) ® b}. - Attribute A is extraneous in b if A Î b

and the set of functional dependencies

(F  – {a ® b}) È {a ®(b – A)} logically implies F.

24. What do you mean by lossless join decomposition?.

A decomposition of R into R1 and R2 is lossless join if and only if at least one of the following dependencies is in F+:

o R1 Ç R2 ® R1 o R1 Ç R2 ® R2

25.Give the uses of Multivalued dependency?.

We use multivalued dependencies in two ways:

-       To test relations to determine whether they are legal under a given set of functional and multivalued dependencies

-       To specify constraints on the set of legal relations. We shall thus concern ourselves only with relations that satisfy a given set of functional and multivalued dependencies.

UNIT IV

PART A

1.State the atomicity property of a transaction.

Atomicity: Either all operations of the transaction are properly reflected in the database or none.

Correctness: Execution of a transaction in isolation preserves the consistency Isolation.: Each transaction must be unaware of other concurrently executing transactions. Durability: After a transaction completes successfully, the changes it has made to the database persist, even if there are system failures.

2.Define deadlock.

A transaction waits for another transaction to release lock on data item. Mean while, transaction also waits for some other data item to be released by first transaction.

3.When are two schedules conflict equivalent?

If a schedule S can be transformed into a schedule S´ by a series of swaps of nonconflicting instructions, we say that S and S´ are conflict equivalent.

4.State the benefits of strict two-phase locking.

Cascading rollbacks can be avoided by a modification of two-phase locking. Any data written by an uncommitted transaction are locked in exclusive mode until the transaction commits, preventing any other transactions from reading the data.

5.What benefit is provided by strict-two-phase locking? What are the disadvantages results?

The transactions can be serialized in the order of their lock points. In strict two-phase locking, transaction must hold all its exclusive locks till it commits/aborts.

6.What is concurrency control ?

Concurrency control is maintaining consistency

7.What is transaction?

Collections of operations that form a single logical unit of work are called transactions.

8.What are the two statements regarding transaction?

The two statements regarding transaction of the form: _ Begin transaction

_ End transaction

9.When is a transaction rolled back?

Any changes that the aborted transaction made to the database must be undone. Once the changes caused by an aborted transaction have been undone, then the transaction has been rolled back.

10.What are the states of transaction?

The states of transaction are

-       Active

-        Partially committed

-        Failed

-        Aborted

-       Commited

-       Terminated

11.What is a shadow copy scheme?

It is simple, but efficient, scheme called the shadow copy schemes. It is based on making copies of the database called shadow copies that one transaction is active at a time. The scheme also assumes that the database is simply a file on disk.

12.Give the reasons for allowing concurrency?

The reasons for allowing concurrency is if the transactions run serially, a short transaction may have to wait for a preceding long transaction to complete, which can lead to unpredictable delays in running a transaction.

So concurrent execution reduces the unpredictable delays in running transactions.

13.What is average response time?

The average response time is that the average time for a transaction to be completed after it has been submitted.

14.What are the two types of serializability?

The two types of serializability is _ Conflict serializability

_ View serializability

15.Define lock?

Lock is the most common used to implement the requirement is to allow a transaction to access a data item only if it is currently holding a lock on that item.

16.What are the different modes of lock?

The modes of lock are: _ Shared

_ Exclusive

17.Differentiate strict two phase locking protocol and rigorous two phase locking protocol.

In strict two phase locking protocol all exclusive mode locks taken by a transaction is held until that transaction commits.

Rigorous two phase locking protocol requires that all locks be held until the transaction commits.

18.How the time stamps are implemented

-Use the value of the system clock as the time stamp. That is a transaction’s time stamp is equal to the value of the clock when the transaction enters the system.

-Use a logical counter that is incremented after a new timestamp has been assigned; that is the time stamp is equal to the value of the counter.

 

19. What are the time stamps associated with each data item?

-W-timestamp (Q) denotes the largest time stamp if any transaction that executed WRITE (Q) successfully.

- R-timestamp (Q) denotes the largest time stamp if any transaction that executed READ (Q) successfully.

20.What is recovery management component?

Ensuring durability is the responsibility of a software component of the base system called the recovery management component.

21.Define the phases of two phase locking protocol

_ Growing phase: a transaction may obtain locks but not release any lock.

_ Shrinking phase: a transaction may release locks but may not obtain any new locks.

22.What is meant by log-based recovery?

The most widely used structures for recording database modifications is the log.

The log is a sequence of log records, recording all the update activities in the database. There are several types of log records.

23.What are the two methods for dealing deadlock problem?

The two methods for dealing deadlock problem is deadlock detection and deadlock recovery.

24.What are the two types of errors?

The two types of errors are: _ Logical error

_ System error

25. What are the storage types?

The storage types are: _ Volatile storage

_ Nonvolatile storage

UNIT V

PART A

1)What are ordered Indices?

In an ordered index, index entries are stored sorted on the search key value. E.g., author catalog in library.

Two types: Sparse Index and Dense index.

2)Distinguish between sparse index and dense index.

An index record appears for every search key value in the file is dense index. An index record appears for only some of the search key values is sparse index. Each index record contains a search key value and a pointer to the first data record with that search value.

3.What is a heap file? How are pages organized in a heap file?

Heap file:- Any record can be placed anywhere in the file where there is space for the record. There is no ordering of records.

4.How does a B-tree differ from B+ - tree? Why is a B+ - tree usually preferred as an access structure to a data file?

B-tree allows search-key values to appear only once; eliminates redundant

storage of search keys. Search keys in nonleaf nodes appear nowhere else in the B-tree; All paths from root to leaf are of the same length. Each node that is not a root or a

leaf has between n/2 and n children. A leaf node has between (n-1)/2 and n-1 values.

 

5.Define Static hashing. Static Hashing:

Hash function h is a function from the set of all search-key values K to the set of all bucket addresses B. Hash function is used to locate records for access, insertion as well as deletion. In static hashing, function h maps search-key values to a fixed set of B of bucket addresses.

6.Define Dynamic hashing.

Dynamic Hashing: Good for database that grows and shrinks in size, allows the hash function to be modified dynamically. The number of buckets also changes dynamically due to coalescing and splitting of buckets.

7.How does a DBMS represent a relational query plan?

Parsing and translation: Translate the query into its internal form. This is then translated into relational algebra. Parser checks syntax, verifies relations Evaluation: The query-execution engine takes a query-evaluation plan, executes that plan, and returns the answers to the query.

8.Define RAID:

Redundant Arrays of Independent Disks.

9.Compare sequential access devices versus random access devices with an example.

Records are stored in sequential order according to the value of a search key. If n th record has to be accessed then all n-1 records has to be passed eg. Tape. In random access device, n th record can be pointed directly eg. Disk.

10.What can be done to reduce the occurrences of bucket overflows in a hash file organization?

The bucket overflows can be reduced by using overflow buckets. Overflow handling can be done using linked list. This is called overflow chaining.

11.Give the measures of quality of a disk.

_ Capacity

_ Access time _ Seek time

_ Data transfer rate _ Reliability

_ Rotational latency time.

12.  Compare sequential access devices versus random access devices with an example sequential access devices random access devices

Must be accessed from the beginning It is possible to read data from any location Eg:- tape storage Eg:-disk storage

Access to data is much slower Access to data is faster Cheaper than disk Expensive when compared with disk

13.  What are the types of storage devices?

_ Primary storage

_ Secondary storage _ Tertiary storage

14.Define access time.

Access time is the time from when a read or write request is issued to when data transfer begins.

15.Define seek time.

The time for repositioning the arm is called the seek time and it increases with the distance that the arm is called the seek time.

16.Define average seek time.

The average seek time is the average of the seek times, measured over a sequence of random requests.

 

17.Define rotational latency time.

The time spent waiting for the sector to be accessed to appear under the head is called the rotational latency time.

18.Define average latency time.

The average latency time of the disk is one-half the time for a full rotation of the disk.

19.What is meant by data-transfer rate?

The data-transfer rate is the rate at which data can be retrieved from or stored to the disk.

20.What is meant by mean time to failure?

The mean time to failure is the amount of time that the system could run continuously without failure.

21.What are a block and a block number?

A block is a contiguous sequence of sectors from a single track of one platter.

Each request specifies the address on the disk to be referenced. That address is in the form of a block number.

22.What is the use of RAID?

A variety of disk-organization techniques, collectively called redundant arrays of independent disks are used to improve the performance and reliability.

23.Explain how reliability can be improved through redundancy?

The simplest approach to introducing redundancy is to duplicate every disk. This technique is called mirroring or shadowing. A logical disk then consists of two physical disks, and write is carried out on both the disk. If one of the disks fails the data can be read from the other. Data will be lost if the second disk fails before the first fail ed disk is repaired.

24.What is called bit-level striping?

Data striping consists of splitting the bits of each byte across multiple disks. This is called bit-level striping.

25.What is called block-level striping?

Block level striping stripes blocks across multiple disks. It treats the array of disks as a large disk, and gives blocks logical numbers.

26.Distinguish between fixed length records and variable length records? Fixed length records

Every record has the same fields and field lengths are fixed.

Variable length records

File records are of same type but one or more of the fields are of varying size.

27.What are the ways in which the variable-length records arise in database systems?

_ Storage of multiple record types in a file.

_ Record types that allow variable lengths for one or more fields. _ Record types that allow repeating fields.

28.Explain the use of variable length records.

_ They are used for Storing of multiple record types in a file.

_ Used for storing records that has varying lengths for one or more fields. _ Used for storing records that allow repeating fields

29.What is known as heap file organization?

In the heap file organization, any record can be placed anywhere in the file where there is space for the record. There is no ordering of records. There is a single file for each relation.

 

30.What is known as sequential file organization?

In the sequential file organization, the records are stored in sequential order, according to the value of a “search key” of each record.

31.What is hashing file organization?

In the hashing file organization, a hash function is computed on some attribute of

each record. The result of the hash function specifies in which block of the file the record should be placed.

32.What is known as clustering file organization?

In the clustering file organization, records of several different relations are stored in the same file.

33.What is an index?

An index is a structure that helps to locate desired records of a relation quickly, without examining all records.

34.  What are the two types of ordered indices?

_ Primary index

_ Secondary index

35.  What are the types of indices?

_ Ordered indices _ Hash indices

36.What are the techniques to be evaluated for both ordered indexing and hashing?

_ Access types _ Access time _ Insertion time _ Deletion time

_ Space overhead

37.What is linear probing?

Linear probing is a type of open hashing. If a bucket is full the system inserts records in to the next bucket that has space. This is known as linear probing.

38.What is called query processing?

Query processing refers to the range of activities involved in extracting data from a database.

39.What are the steps involved in query processing?

The basic steps are:

_ parsing and translation _ optimization

_ evaluation

40.What is called an evaluation primitive?

A relational algebra operation annotated with instructions on how to evaluate is called an evaluation primitive.

41.What is called a query evaluation plan?

A sequence of primitive operations that can be used to evaluate ba query is a query evaluation plan or a query execution plan.

42.What is called a query –execution engine?

The query execution engine takes a query evaluation plan, executes that plan, and returns the answers to the query.

 

43.How do you measure the cost of query evaluation?

The cost of a query evaluation is measured in terms of a number of different resources including disk accesses, CPU time to execute a query, and in a distributed database system the cost of communication

44.List out the operations involved in query processing

Selection operation Join operations. Sorting.

Projection Set operations Aggregation

45.Define query optimization.

Query optimization refers to the process of finding the lowest –cost method of evaluating a given query.

46.Which level of RAID is best? Why?

RAID level 1 is the RAID level of choice for many applications with moderate

storage requirements and high I/O requirements. RAID 1 follows mirroring and provides best write performance.

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