Side-by-side migration in the context of SQL Server using Backup and restore

 Side-by-side migration in the context of SQL Server using Backup and restore 

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Capture all SQL and Stored Proc calls using Extended Events in SQL Server

-- Capture all SQL and Stored Proc calls

-- Part of the SQL Server DBA Toolbox at https://github.com/DavidSchanzer/Sql-Server-DBA-Toolbox

-- This script creates an Extended Events session called "CaptureAllSQLAndStoredProcCalls" that includes the following events:

-- error_reported (for severity 20 and above, as well as certain error numbers)

-- existing_connection

-- login

-- logout

-- rpc_completed

-- sql_batch_completed

CREATE EVENT SESSION [CaptureAllSQLAndStoredProcCalls]

ON SERVER

    ADD EVENT sqlserver.error_reported

    (ACTION

     (

         package0.callstack,

         sqlserver.client_app_name,

         sqlserver.client_hostname,

         sqlserver.database_name,

         sqlserver.nt_username,

         sqlserver.session_id,

         sqlserver.session_nt_username,

         sqlserver.sql_text,

         sqlserver.tsql_stack,

         sqlserver.username

     )

     WHERE (

               [severity] >= (20)

               OR

               (

                   [error_number] = (17803)

                   OR [error_number] = (701)

                   OR [error_number] = (802)

                   OR [error_number] = (8645)

                   OR [error_number] = (8651)

                   OR [error_number] = (8657)

                   OR [error_number] = (8902)

                   OR [error_number] = (41354)

                   OR [error_number] = (41355)

                   OR [error_number] = (41367)

                   OR [error_number] = (41384)

                   OR [error_number] = (41336)

                   OR [error_number] = (41309)

                   OR [error_number] = (41312)

                   OR [error_number] = (41313)

               )

           )

    ),

    ADD EVENT sqlserver.existing_connection

    (ACTION

     (

         package0.event_sequence,

         sqlserver.client_hostname,

         sqlserver.session_id

     )

    ),

    ADD EVENT sqlserver.login

    (SET collect_options_text = (1)

     ACTION

     (

         package0.event_sequence,

         sqlserver.client_hostname,

         sqlserver.session_id

     )

    ),

    ADD EVENT sqlserver.logout

    (ACTION

     (

         package0.event_sequence,

         sqlserver.session_id

     )

    ),

    ADD EVENT sqlserver.rpc_completed

    (SET collect_statement = (1)

     ACTION

     (

         package0.event_sequence,

         sqlserver.client_app_name,

         sqlserver.client_hostname,

         sqlserver.database_name,

         sqlserver.nt_username,

         sqlserver.session_id,

         sqlserver.session_nt_username,

         sqlserver.sql_text,

         sqlserver.tsql_stack,

         sqlserver.username

     )

     WHERE ([package0].[equal_boolean]([sqlserver].[is_system], (0)))

    ),

    ADD EVENT sqlserver.sql_batch_completed

    (ACTION

     (

         package0.event_sequence,

         sqlserver.client_app_name,

         sqlserver.client_hostname,

         sqlserver.database_name,

         sqlserver.nt_username,

         sqlserver.session_id,

         sqlserver.session_nt_username,

         sqlserver.sql_text,

         sqlserver.tsql_stack,

         sqlserver.username

     )

     WHERE ([package0].[equal_boolean]([sqlserver].[is_system], (0)))

    )

    ADD TARGET package0.event_file

    (SET filename = N'c:\temp\CaptureAllSQLAndStoredProcCalls.xel')

WITH

(

    MAX_MEMORY = 16384KB,

    EVENT_RETENTION_MODE = ALLOW_SINGLE_EVENT_LOSS,

    MAX_DISPATCH_LATENCY = 5 SECONDS,

    MAX_EVENT_SIZE = 0KB,

    MEMORY_PARTITION_MODE = PER_CPU,

    TRACK_CAUSALITY = ON,

    STARTUP_STATE = OFF

);

GO 

Here are key points about the buffer cache in SQL Server:

    The buffer cache in SQL Server is a part of the SQL Server memory architecture that is responsible for caching database pages in memory. When SQL Server reads data from a disk, it stores a copy of that data in the buffer cache. Subsequent queries that need the same data can then be served from the in-memory buffer rather than reading from disk, which is significantly faster.

Here are key points about the buffer cache in SQL Server:

Buffer Pool:

The buffer cache is often referred to as the "buffer pool" or "data cache." It is a region in the SQL Server memory space dedicated to storing data pages.

Pages and Extents:

SQL Server divides its storage into fixed-size pages (usually 8 KB). These pages are grouped into larger structures called extents. The buffer cache holds these pages in memory.

Data Access:

When a query needs data, SQL Server first checks if the required pages are already in the buffer cache. If the data is present, it's called a "cache hit," and the data can be retrieved quickly from memory. If not, it's a "cache miss," and the data must be read from disk.

Read-Ahead Mechanism:

SQL Server uses a read-ahead mechanism to anticipate and pre-fetch pages into the buffer cache before they are needed. This helps to minimize the impact of physical I/O on query performance.

LRU (Least Recently Used) Algorithm:

The buffer cache uses an LRU algorithm to manage the contents of the cache. When the cache becomes full, pages that haven't been used recently are candidates for removal to make room for new pages.

Dirty Pages and Checkpoints:

When modifications are made to data in the buffer cache, the modified pages become "dirty." SQL Server periodically writes these dirty pages back to the data files during a process called a checkpoint. This ensures that changes are persisted to disk.

Monitoring Buffer Cache:

Performance monitoring tools and DMVs (Dynamic Management Views) can be used to monitor the state of the buffer cache. For example, the sys.dm_os_buffer_descriptors view provides information about the pages currently in the buffer cache.

SELECT * FROM sys.dm_os_buffer_descriptors;

Configuring Buffer Cache:

SQL Server provides configuration options for the size and behavior of the buffer cache. The "max server memory" configuration option limits the amount of memory that SQL Server can use for the buffer cache.

sp_configure 'max server memory', <value>;

The buffer cache plays a crucial role in optimizing SQL Server performance by reducing the need to perform expensive disk I/O operations. Properly configuring and monitoring the buffer cache is important for maintaining optimal database performance.

Here are common SQL Server performance issues and potential solutions:

 SQL Server performance issues can arise for various reasons, and resolving them often involves identifying bottlenecks, optimizing queries, and configuring the server appropriately. Here are common SQL Server performance issues and potential solutions:

High CPU Usage:

Issue: Excessive CPU utilization.

Solutions:

Identify and optimize poorly performing queries.

Consider adding indexes to improve query performance.

Scale up resources (CPU, memory).

Review and adjust the SQL Server configuration for parallelism.

Memory Pressure:

Issue: Insufficient available memory for SQL Server.

Solutions:

Configure SQL Server memory settings appropriately.

Identify and optimize memory-consuming queries.

Monitor and adjust memory-related configuration settings.

I/O Bottlenecks:

Issue: Slow disk I/O affecting query performance.

Solutions:

Optimize queries to reduce I/O load.

Consider adding more/faster disks.

Use storage with higher IOPS capabilities.

Review and optimize file placement, such as database and log files.

Blocking and Deadlocks:

Issue: Transactions waiting on locks, leading to performance degradation.

Solutions:

Optimize queries and transactions.

Use appropriate isolation levels.

Monitor and identify blocking using tools like SQL Server Profiler.

Implement proper indexing.

Inefficient Query Plans:

Issue: SQL Server generates suboptimal query execution plans.

Solutions:

Update statistics to ensure accurate query plans.

Use index hints to force specific indexes.

Rewrite queries to improve performance.

Indexing Issues:

Issue: Missing or poorly designed indexes.

Solutions:

Regularly analyze and create missing indexes.

Remove unnecessary indexes to improve write performance.

Use the Database Engine Tuning Advisor (DTA) to recommend index changes.

TempDB Contention:

Issue: High contention in the TempDB database.

Solutions:

Split TempDB data files equally across multiple disks.

Adjust the number of TempDB files based on CPU cores.

Monitor and optimize queries that heavily use TempDB.

Out-of-date Statistics:

Issue: Query optimizer relies on outdated statistics.

Solutions:

Regularly update statistics on tables and indexes.

Consider enabling the AUTO_UPDATE_STATISTICS database option.

Network Latency:

Issue: Slow communication between the application and the database.

Solutions:

Optimize network infrastructure.

Use the appropriate network protocols.

Consider deploying closer Azure regions for Azure SQL Database.

Fragmented Indexes:

Issue: Fragmentation affecting index scan/seek performance.

Solutions:

Regularly rebuild or reorganize fragmented indexes.

Monitor index fragmentation using DMVs.

Long-running Queries:

Issue: Queries taking too long to execute.

Solutions:

Optimize queries using proper indexing.

Use execution plans to identify and address performance bottlenecks.

Insufficient Server Resources:

Issue: Not enough CPU, memory, or disk resources.

Solutions:

Consider upgrading hardware or moving to a larger VM size.

Optimize queries to use resources more efficiently.

Regular monitoring, proper configuration, and ongoing performance tuning are essential for maintaining optimal SQL Server performance. It's often helpful to use tools like SQL Server Profiler, SQL Server Management Studio (SSMS), and dynamic management views (DMVs) to diagnose and address performance issues. Additionally, regularly reviewing and implementing best practices for SQL Server performance can help prevent and mitigate potential problems.

Version control in SQL Server

 Version control in SQL Server refers to the practice of managing and tracking changes to database objects, such as tables, views, stored procedures, and functions, over time. Using version control helps in maintaining a history of changes, collaborating with multiple developers, and rolling back to previous states if needed. Here are common approaches and tools for version control in SQL Server:

1. Scripting and Source Control Systems:

• Manual Scripting: Developers manually create and maintain SQL scripts for database objects. These scripts are then stored in a version control system such as Git.
• Source Control Integration: Many version control systems offer integrations with SQL Server Management Studio (SSMS) or other database development tools. Developers can directly commit changes to version control from within the tool.

2. Database Projects in Visual Studio:

• SQL Server Data Tools (SSDT): Visual Studio includes a project type known as SQL Server Data Tools, which allows developers to create and manage database projects. These projects can be version-controlled using Git, TFS (Team Foundation Server), or other source control systems.

3. Migrations and Change Tracking:

• Database Migrations: Tools like FluentMigrator, DbUp, or Entity Framework Migrations can be used to create scripts that represent changes to the database schema. These scripts can be version-controlled and applied in a structured manner.
• Change Tracking: SQL Server has built-in features like Change Data Capture (CDC) and Change Tracking that can help track changes to data. While not a complete version control solution, these features complement version control practices.

4. Third-Party Tools:

• Redgate SQL Source Control: This tool integrates with SSMS and supports popular version control systems. It allows developers to link databases to version control repositories and track changes.
• Liquibase and Flyway: These are database migration tools that support version control for databases. They use scripts or configurations to manage changes and can be integrated with source control systems.

5. Git Hooks and Database CI/CD:

• Git Hooks: Pre-commit and post-commit hooks in Git can be used to automate checks and tasks related to version control, such as running tests, enforcing coding standards, or triggering continuous integration (CI) builds.
• Database CI/CD: Implementing a continuous integration and continuous delivery (CI/CD) pipeline for databases helps automate the process of deploying database changes from version control to different environments.

When implementing version control for SQL Server, it's essential to establish best practices, including documentation, naming conventions, and a clear process for branching and merging. Regularly syncing the database schema with version control and ensuring that changes are traceable are critical aspects of effective version control practices.

 

Isolation levels in SQL Server

 Isolation levels in SQL Server define the degree to which one transaction must be isolated from resource or data modifications made by other transactions. SQL Server supports several isolation levels, each providing a different level of consistency, concurrency, and isolation. The isolation levels in SQL Server are defined by the SQL standard and include the following:

1. READ UNCOMMITTED:

• Description: Allows a transaction to read data that is being modified by another transaction without waiting for the other transaction to complete.
• Issues: Non-repeatable reads, dirty reads, and phantom reads are possible.

2. READ COMMITTED:

• Description: Ensures that a transaction reads only committed data. It prevents dirty reads but still allows non-repeatable reads and phantom reads.
• Issues: Non-repeatable reads and phantom reads are possible.

3. REPEATABLE READ:

• Description: Ensures that if a transaction reads a value, it will get the same value if it reads it again within the same transaction. It prevents dirty reads and non-repeatable reads but allows phantom reads.
• Issues: Phantom reads are possible.

4. SNAPSHOT:

• Description: Allows a transaction to read a version of data as it existed at the start of the transaction. This provides a consistent snapshot of the data for the duration of the transaction.
• Issues: Avoids dirty reads, non-repeatable reads, and phantom reads.

5. SERIALIZABLE:

• Description: Provides the highest level of isolation. It ensures that transactions are completely isolated from one another. It prevents dirty reads, non-repeatable reads, and phantom reads.
• Issues: Increased contention and potential for slower performance due to locks.

The isolation level can be set for a session using the SET TRANSACTION ISOLATION LEVEL statement. For example:

SET TRANSACTION ISOLATION LEVEL READ COMMITTED;

it's important to note that selecting a higher isolation level typically comes with an increased risk of performance issues, such as blocking and decreased concurrency. Developers and database administrators need to carefully choose the appropriate isolation level based on the requirements of the application and the specific trade-offs they are willing to make in terms of consistency and performance.

Common SQL Server wait types

In SQL Server, wait types are events or conditions that cause a task (such as a query or a process) to wait for a specific resource or event to be available before it can continue processing. Monitoring and analyzing wait types can help identify performance bottlenecks and optimize the database system.

Here are some common SQL Server wait types:

1. PAGEIOLATCH_XX:

• Description: Indicates that a process is waiting for a data page to be read from disk into memory.
• Possible Causes: Slow I/O subsystem, high disk latency.

2. CXPACKET:

• Description: Related to parallel query execution. Indicates that a parallel query is waiting for another thread to complete its work.
• Possible Causes: Overloaded parallelism, uneven workload distribution.

3. LCK_XX:

• Description: Indicates a process is waiting to acquire a lock on a resource.
• Possible Causes: Contentious locks due to high concurrency.

4. ASYNC_NETWORK_IO:

• Description: Indicates a task is waiting for network packets to be sent or received.
• Possible Causes: Slow or congested network.

5. WRITELOG:

• Description: Indicates a process is waiting for a log flush to complete.
• Possible Causes: High transaction log activity, slow disk write performance.

6. SOS_SCHEDULER_YIELD:

• Description: Indicates that a task voluntarily yielded the scheduler to let other tasks run.
• Possible Causes: High CPU usage, resource contention.

7. PAGE_VERIFY:

• Description: Indicates a task is waiting for a page verification operation to complete.
• Possible Causes: Configuring database option CHECKSUM and experiencing high I/O.

8. OLEDB:

• Description: Indicates a task is waiting for an OLE DB operation to complete.
• Possible Causes: Issues with external data source or linked server.

9. WAITFOR:

• Description: Indicates a task is waiting for a specified amount of time to elapse.
• Possible Causes: Delays introduced in queries using the WAITFOR statement.

Monitoring and analyzing wait types can be done using dynamic management views (DMVs) such as sys.dm_os_wait_stats. By querying these views, you can identify which wait types are causing the most contention and focus on optimizing those areas for better performance. Additionally, tools like SQL Server Profiler and Extended Events can be used for more in-depth analysis of wait statistics.

 

What is Lazy Writer in SQL Server ?

The Lazy Writer is a background process responsible for managing the buffer pool's clean pages. The buffer pool is an area of memory where SQL Server caches data pages in order to reduce the need to read data from disk repeatedly.

The Lazy Writer works by moving aged and less frequently accessed data pages from the buffer pool to disk, ensuring that the buffer pool remains efficient and has space for new data pages that are more actively used. This process is essential for maintaining the overall performance of the SQL Server database.

SQL Server DBA Responsibilities

A SQL Server Database Administrator (DBA) plays a crucial role in managing and maintaining the SQL Server databases within an organization. The specific roles and responsibilities may vary depending on the organization's size and structure, but generally, a SQL Server DBA is responsible for the following tasks:

Database Installation and Configuration:

Install and configure SQL Server instances according to best practices.

Configure server and database settings for optimal performance and security.

Database Design:

Collaborate with developers and system architects to design efficient and normalized database structures.

Create and modify database objects such as tables, views, indexes, and stored procedures.

Security Management:

Implement and manage security policies, roles, and permissions at both the server and database levels.

Regularly review and audit database access for compliance and security purposes.

Backup and Recovery:

Develop and implement backup and recovery strategies to ensure data integrity and availability.

Test and document disaster recovery procedures.

Performance Monitoring and Optimization:

Monitor server and database performance using tools and logs.

Identify and resolve performance bottlenecks through indexing, query optimization, and other tuning techniques.

High Availability and Disaster Recovery:

Implement and maintain high availability solutions such as clustering, mirroring, or AlwaysOn Availability Groups.

Plan and test disaster recovery procedures to minimize data loss and downtime.

Patch Management and Upgrades:

Apply patches and updates to SQL Server to ensure security and stability.

Plan and execute version upgrades as needed.

Automation and Scripting:

Develop and maintain scripts for routine tasks, monitoring, and automation.

Use PowerShell or other scripting languages to streamline administrative tasks.

Documentation:

Maintain comprehensive documentation for databases, configurations, and procedures.

Ensure that documentation is up-to-date and accessible to relevant stakeholders.

Capacity Planning:

Monitor database growth and plan for capacity upgrades as needed.

Forecast future capacity requirements based on usage trends.

Troubleshooting and Incident Response:

Investigate and resolve database-related issues and incidents.

Provide timely and effective responses to system outages or degraded performance.

Training and Knowledge Sharing:

Stay informed about new features and best practices in SQL Server.

Provide training and knowledge sharing sessions for other team members or application developers.

The role of a SQL Server DBA is multifaceted, encompassing aspects of database design, security, performance optimization, and system maintenance to ensure the smooth and secure operation of SQL Server databases within an organization.