Introduction
The SQL Server design implies a one-to-many mapping between the database engine (instance) and the databases hosted on the instance. It means that you can deploy several databases on one instance of the SQL server. According to the Microsoft documentation, you can have up to 32767 databases on a single instance of SQL Server. Of course, there will be limitations, like the resources on the server, managing concurrency on TempDB, network traffic, etc.
Databases deployed on a SQL Server instance can either be System Databases or User Databases. System Databases come installed with the instance. In this article, we will discuss the purpose of each System database. Also, we’ll clarify what you need to care for when managing system databases on SQL Server.
Overview of System Databases
System databases are a part of many processes taking place when you install an instance of SQL Server. By default, these databases are created in the following paths:
%programfiles%\Microsoft SQL Server\MSSQL15.MSSQLSERVER\MSSQL\DATA
and
%programfiles%\Microsoft SQL Server\MSSQL15.MSSQLSERVER\MSSQL\Log
The path can be different. During the installation of SQL Server, you can specify the location of the system database files.
To list all the system databases in an instance, you can invoke the code in Listing 1. Listing 2 can be used to determine the location of the datafiles associated with the system databases. Note that in both scripts, we use a filter returning databases with database_id of 5 or higher than 5.
The essential “visible” system databases have database_ids 1,2,3,4 – they refer to master, tempdb, model, and msdb, respectively. There is also an “invisible” database called the resource databases and other system databases created when you enable features like replication.
-- Listing 1: System Databases
select
name
,database_id
,create_date
,state_desc
,recovery_model_desc
,log_reuse_wait_desc
,physical_database_name
,is_db_chaining_on
,is_broker_enabled
,is_mixed_page_allocation_on
from sys.databases
where database_id<5;
-- Listing 2: System Database Files
select
name
,database_id
,DB_NAME(database_id)
,name
,physical_name
,type_desc
from sys.master_files
where database_id<5;
Figure 1: System Databases
The master Database
The master database is the first database open on the start of SQL Server, containing the following data:
- The records of the structure/configuration of the instance and all other databases.
- The most dynamic management views that are necessary for monitoring the instance.
Thus, it has the information necessary for opening all other databases. That’s why it has to be first to open. The question is how to do it.
The SQL Server startup parameters contain two entries, which define the locations of the master database data and log files. The default startup parameters include only three lines – the third one is the error log file location. When SQL Server starts up, it must be able to write to that error log file.
The master database opens first. The information stored in the master database, including the configurations defined using sp_configure, applies to open other databases and complete the instance startup process.
Figure 3: SQL Server Startup Parameters
There are several ways to learn about useful SQL Server system objects, like Dynamic Management Views and Functions.
For instance, expand the views or programmability nodes for the master database on object explorer. There, review these objects’ names and get more details from Books Online.
You can also migrate logins from one instance to another. For that, restore a backup of the master database to the destination instance. We’ll describe the specific technique in a separate article.
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