Datbase Snapshot

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What is database snapshot?
A database snapshot provides a read-only, static view of a source database as it existed at snapshot creation, minus any uncommitted transactions. Uncommitted transactions are rolled back in a newly created database snapshot because the Database Engine runs recovery after the snapshot has been created. 

 

Is database snapshot transitionally consistent?
Yes.
Each database snapshot is transitionally consistent with the source database at the moment of the snapshot’s creation. When we create a database snapshot, the source database will typically have number open transactions. Before the snapshot becomes available, the open transactions are rolled back to make the database snapshot transitionally consistent. Just like it follows the recovery interval step and it will not affect the source database. 

 

What are the uses of database snapshot?
Reporting Purpose
we can recover damaged database using database snapshot
also useful if we are planning to do major change in the source database
Mirroring database is always in recovering mode, to read that database we can use database snapshot.
Which edition of SQL Server 2005 supports database snapshot? Enterprise Edition 

 

Is Developer edition of SQL Server 2005 supports database snapshot?
No. 

 

What is copy-on-write operation in database snapshot?
Database snapshots operate at the data-page level. Before a page of the source database is modified for the first time, the original page is copied from the source database to the snapshot. This process is called a copy-on-write operation. The snapshot stores the original page, preserving the data records as they existed when the snapshot was created.

 

What is Sparse file in Database Snapshot?

To store the copied original pages, the snapshot uses one or more sparse files. Initially, a sparse file is an essentially empty file that contains no user data and has not yet been allocated disk space for user data. As more and more pages are updated in the source database, the size of the file grows. When a snapshot is taken, the sparse file takes up little disk space. As the database is updated over time, however, a sparse file can grow into a very large file.

 

What is NTFS File System?

The NTFS acronym stands for New Technology File System. The name derives from the implementation of very innovative data storage techniques that were refined in NTFS. While none of the techniques are unique to NTFS, it is the first time that so many innovations were released at once on a production file system. The FAT file system had long been criticized for not including some of the more obvious improvements such as journaling, disk quotas, and file compression. However, these improvements made NTFS incompatible with previous versions of Windows, and also with hard disk tools designed for FAT file systems. For example, data recovery tools such as GetDataBack and partitioning tools such as PartitionMagic would run on Windows NT, yet could not function on the newer file system. This led to much frustration with users who had purchased licenses for these products before upgrading to Windows NT.

 

Explain Sparse file Size in database snapshot

At the creation time sparse file will take very little space, but as the data changes occurred into the parent database, data page before the changes copied into the sparse file. Thus sparse file grows. Sparse files are the feature of NTFS file system. As the sparse file grows NTFS will allocate the space to sparse file gradually.

 

Why does the size of a sparse file slightly exceed than the space actually filled by pages in it?

Sparse files grow in 64-kilobyte (KB) increments; thus, the size of a sparse file on disk is always a multiple of 64 KB. The latest 64-KB increment holds from one to eight 8-KB pages, depending on how many pages have been copied from the source database. This means that, on the average, the size of a sparse file slightly exceeds the space actually filled by pages.

 

How to create database snapshot?

  1. Based on the current size of the source database, ensure that you have sufficient disk space to hold the database snapshot. The maximum size of a database snapshot is the size of the source database at snapshot creation.
  2. Issue a CREATE DATABASE statement on the files using the AS SNAPSHOT OF clause. Creating a snapshot requires specifying the logical name of every database file of the source database.

 

Syntax

CREATE DATABASE AdventureWorks_SS ON

(

NAME = AdventureWorks_Data,

FILENAME = ‘D:\Program Files\Microsoft SQL Server\MSSQL.1\MSSQL\Data\AdventureWorks_Data.ss’

)

AS SNAPSHOT OF AdventureWorks

 

 

 

 

What does database performance means?

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What does database performance means?

Database performance enhancement refers to below terms.

Response time: Refers to the interval between the time when a request is submitted and when the first character of the response is received.

Throughput: Refers to the number of transactions that can be processed in a fixed unit of time.

Scalability: Refers to how the throughput and/or the response time changes as we add more hardware resources. In simple terms, scalability means that if you are hitting a hardware bottleneck, you can alleviate it simply by adding more resources.

 

SQL Server and Networking

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How do you test proper TCP/IP configuration Windows machine?

Windows NT: IPCONFIG/ALL, Windows 95: WINIPCFG, Ping or ping ip.add.re.ss

What is RAID and what are different types of RAID configurations?

RAID stands for Redundant Array of Inexpensive Disks, used to provide fault tolerance to database servers. There are six RAID levels 0 through 5 offering different levels of performance, fault tolerance.

How do you define testing of network layers?

Reviewing with your developers to identify the layers of the Network layered architecture, your Web client and Web server application interact with. Determine the hardware and software configuration dependencies for the application under test.

What are the steps you will take to improve performance of a poor performing query?

This is a very open ended question and there could be a lot of reasons behind the poor performance of a query. But some general issues that you could talk about would be: No indexes, table scans, missing or out of date statistics, blocking, excess recompilations of stored procedures, procedures and triggers without SET NOCOUNT ON, poorly written query with unnecessarily complicated joins, too much normalization, excess usage of cursors and temporary tables.
Some of the tools/ways that help you troubleshooting performance problems are: SET SHOWPLAN_ALL ON, SET SHOWPLAN_TEXT ON, SET STATISTICS IO ON, SQL Server Profiler, Windows NT /2000 Performance monitor, Graphical execution plan in Query Analyzer.

How many layers of TCP/IP protocol combined of?

Five. (Application, Transport, Internet, Data link, Physical).

How many bits IP Address consist of?

An IP Address is a 32-bit number.

What is a deadlock and what is a live lock? How will you go about resolving deadlocks?

Deadlock is a situation when two processes, each having a lock on one piece of data, attempt to acquire a lock on the other’s piece. Each process would wait indefinitely for the other to release the lock, unless one of the user processes is terminated. SQL Server detects deadlocks and terminates one user’s process.
A livelock is one, where a request for an exclusive lock is repeatedly denied because a series of overlapping shared locks keeps interfering. SQL Server detects the situation after four denials and refuses further shared locks. A livelock also occurs when read transactions monopolize a table or page, forcing a write transaction to wait indefinitely.

What is blocking and how would you troubleshoot it?

Blocking happens when one connection from an application holds a lock and a second connection requires a conflicting lock type. This forces the second connection to wait, blocked on the first.

How to restart SQL Server in single user mode? How to start SQL Server in minimal configuration mode?

SQL Server can be started from command line, using the SQLSERVR.EXE. This EXE has some very important parameters with which a DBA should be familiar with. -m is used for starting SQL Server in single user mode and -f is used to start the SQL Server in minimal confuguration mode.

As a part of your job, what are the DBCC commands that you commonly use for database maintenance?

DBCC CHECKDB, DBCC CHECKTABLE, DBCC CHECKCATALOG, DBCC CHECKALLOC, DBCC SHOWCONTIG, DBCC SHRINKDATABASE, DBCC SHRINKFILE etc. But there are a whole load of DBCC commands which are very useful for DBAs.

What is the difference between them (Ethernet networks and token ring networks)?

With Ethernet, any devices on the network can send data in a packet to any location on the network at any time. With Token Ring, data is transmitted in ‘tokens’ from computer to computer in a ring or star configuration. Token ring speed is 4/16 Mbit/sec , Ethernet – 10/100 Mbit/sec.

What are triggers? How many triggers you can have on a table? How to invoke a trigger on demand?

Triggers are special kind of stored procedures that get executed automatically when an INSERT, UPDATE or DELETE operation takes place on a table.

In SQL Server 6.5 you could define only 3 triggers per table, one for INSERT, one for UPDATE and one for DELETE. From SQL Server 7.0 onwards, this restriction is gone, and you could create multiple triggers per each action. But in 7.0 there’s no way to control the order in which the triggers fire. In SQL Server 2000 you could specify which trigger fires first or fires last using sp_settriggerorder

Triggers can’t be invoked on demand. They get triggered only when an associated action (INSERT, UPDATE, DELETE) happens on the table on which they are defined.

Triggers are generally used to implement business rules, auditing. Triggers can also be used to extend the referential integrity checks, but wherever possible, use constraints for this purpose, instead of triggers, as constraints are much faster.

Till SQL Server 7.0, triggers fire only after the data modification operation happens. So in a way, they are called post triggers. But in SQL Server 2000 you could create pre triggers also.

What is the system function to get the current user’s user id?

USER_ID(). Also check out other system functions like USER_NAME(), SYSTEM_USER, SESSION_USER, CURRENT_USER, USER, SUSER_SID(), HOST_NAME().

What is a traditional Network Library for SQL Servers?

Named Pipes.

Table Locking enhancement – SQL Server 2008

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In SQL Server 2008 ALTER TABLE statement, you can find a new option called SET LOCK_ESCALATION. This is one of the enhancements in Locking in SQL Server 2008. This option can have three value, Auto, Table, Disable

FROM BOL
SET ( LOCK_ESCALATION = { AUTO | TABLE | DISABLE } )
Specifies the allowed methods of lock escalation for a table.

AUTO
This option allows SQL Server Database Engine to select the lock escalation granularity that is appropriate for the table schema.
• If the table is partitioned, lock escalation will be allowed to the heap or B-tree (HoBT) granularity. After the lock is escalated to the HoBT level, the lock will not be escalated later to TABLE granularity.
• If the table is not partitioned, the lock escalation will be done to the TABLE granularity.

TABLE
Lock escalation will be done at table-level granularity regardless whether the table is partitioned or not partitioned. This behavior is the same as in SQL Server 2005. TABLE is the default value.

DISABLE
Prevents lock escalation in most cases. Table-level locks are not completely disallowed. For example, when you are scanning a table that has no clustered index under the serializable isolation level, Database Engine must take a table lock to protect data integrity.

Note : If you use partitions then After you create a partitioned table, consider setting the LOCK_ESCALATION option for the table to AUTO. This can improve concurrency by enabling locks to escalate to partition (HoBT) level instead of the table.