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SQL Server availability basics for Linux deployments

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Applies to: SQL Server - Linux

Starting with SQL Server 2017 (14.x), SQL Server is supported on both Linux and Windows. Like Windows-based SQL Server deployments, SQL Server databases and instances need to be highly available under Linux. This article covers the technical aspects of planning and deploying highly available Linux-based SQL Server databases and instances, as well as some of the differences from Windows-based installations. Because SQL Server might be new for Linux professionals, and Linux might be new for SQL Server professionals, the article at times introduces concepts that might be familiar to some and unfamiliar to others.

SQL Server availability options for Linux deployments

Besides backup and restore, the same three availability features are available on Linux as for Windows-based deployments:

On Windows, FCIs always require an underlying Windows Server failover cluster (WSFC). Depending on the deployment scenario, an AG usually requires an underlying WSFC, with the exception being the new None variant in SQL Server 2017 (14.x). A WSFC doesn't exist in Linux. Clustering implementation in Linux is discussed in Pacemaker for availability groups and failover cluster instances on Linux.

A quick Linux primer

While some Linux installations might be installed with an interface, most aren't, meaning that nearly everything at the operating system layer is done via command line. The common term for this command line in the Linux world is a bash shell.

In Linux, many commands need to be executed with elevated privileges, much like many things need to be done in Windows Server as an administrator. There are two main methods to execute with elevated privileges:

  1. Run in the context of the proper user. To change to a different user, use the command su. If su is executed on its own without a username, as long as you know the password, you're in a shell as root.

  2. The more common and security conscious way to run things is to use sudo before executing anything. Many of the examples in this article use sudo.

Some common commands, each of which have various switches and options that can be researched online:

  • cd - change the directory
  • chmod - change the permissions of a file or directory
  • chown - change the ownership of a file or directory
  • ls - show the contents of a directory
  • mkdir - create a folder (directory) on a drive
  • mv - move a file from one location to another
  • ps - show all of the working processes
  • rm - delete a file locally on a server
  • rmdir - delete a folder (directory)
  • systemctl - start, stop, or enable services
  • Text editor commands. On Linux, there are various text editor options, such as vi and emacs.

Common tasks for availability configurations of SQL Server on Linux

This section covers tasks that are common to all Linux-based SQL Server deployments.

Ensure that files can be copied

Copying files from one server to another is a task that anyone using SQL Server on Linux should be able to do. This task is very important for AG configurations.

Things like permission issues can exist on Linux and on Windows-based installations. However, those familiar with how to copy from server to server on Windows might not be familiar with how it's done on Linux. A common method is to use the command-line utility scp, which stands for secure copy. Behind the scenes, scp uses OpenSSH. SSH stands for secure shell. Depending on the Linux distribution, OpenSSH itself might not be installed. If it isn't, OpenSSH needs to be installed first. For more information on configuring OpenSSH, see the information at the following links for each distribution:

When using scp, you must provide the credentials of the server if it isn't the source or destination. For example, using

scp MyAGCert.cer username@servername:/folder/subfolder

copies the file MyAGCert.cer to the folder specified on the other server. You must have permissions - and possibly ownership - of the file to copy it, so chown might also need to be employed before copying. Similarly, on the receiving side, the right user needs access to manipulate the file. For example, to restore that certificate file, the mssql user must be able to access it.

Samba, which is the Linux variant of server message block (SMB), can also be used to create shares accessed by UNC paths such as \\SERVERNAME\SHARE. For more information on configuring Samba, see the information at the following links for each distribution:

Windows-based SMB shares can also be used; SMB shares don't need to be Linux-based, as long as the client portion of Samba is configured properly on the Linux server hosting SQL Server and the share has the right access. For those in a mixed environment, this would be one way to use existing infrastructure for Linux-based SQL Server deployments.

One thing that is important is that the version of Samba deployed should be SMB 3.0 compliant. When SMB support was added in SQL Server 2012 (11.x), it required all shares to support SMB 3.0. If using Samba for the share and not Windows Server, the Samba-based share should be using Samba 4.0 or later, and ideally 4.3 or later, which supports SMB 3.1.1. A good source of information on SMB and Linux is SMB3 in Samba.

Finally, using a network file system (NFS) share is an option. Using NFS isn't an option on Windows-based deployments of SQL Server, and can only be used for Linux-based deployments.

Configure the firewall

Similar to Windows, Linux distributions have a built-in firewall. If your company is using an external firewall to the servers, disabling the firewalls in Linux might be acceptable. However, regardless of where the firewall is enabled, ports need to be opened. The following table documents the common ports needed for highly available SQL Server deployments on Linux.

Port Number Type Description
111 TCP/UDP NFS - rpcbind/sunrpc
135 TCP Samba (if used) - Endpoint Mapper
137 UDP Samba (if used) - NetBIOS Name Service
138 UDP Samba (if used) - NetBIOS Datagram
139 TCP Samba (if used) - NetBIOS Session
445 TCP Samba (if used) - SMB over TCP
1433 TCP SQL Server - default port; if desired, can change with mssql-conf set network.tcpport <portnumber>
2049 TCP, UDP NFS (if used)
2224 TCP Pacemaker - used by pcsd
3121 TCP Pacemaker - Required if there are Pacemaker Remote nodes
3260 TCP iSCSI Initiator (if used) - Can be altered in /etc/iscsi/iscsid.config (RHEL), but should match port of iSCSI Target
5022 TCP SQL Server - default port used for an AG endpoint; can be changed when creating the endpoint
5403 TCP Pacemaker
5404 UDP Pacemaker - Required by Corosync if using multicast UDP
5405 UDP Pacemaker - Required by Corosync
21064 TCP Pacemaker - Required by resources using DLM
Variable TCP AG endpoint port; default is 5022
Variable TCP NFS - port for LOCKD_TCPPORT (found in /etc/sysconfig/nfs on RHEL)
Variable UDP NFS - port for LOCKD_UDPPORT (found in /etc/sysconfig/nfs on RHEL)
Variable TCP/UDP NFS - port for MOUNTD_PORT (found in /etc/sysconfig/nfs on RHEL)
Variable TCP/UDP NFS - port for STATD_PORT (found in /etc/sysconfig/nfs on RHEL)

For additional ports that might be used by Samba,see Samba Port Usage.

Conversely, the name of the service under Linux can also be added as an exception instead of the port; for example, high-availability for Pacemaker. Refer to your distribution for the names if this is the direction you wish to pursue. For example, on RHEL the command to add in Pacemaker is

sudo firewall-cmd --permanent --add-service=high-availability

Firewall documentation

Install SQL Server packages for availability

On a Windows-based SQL Server installation, some components are installed even in a basic engine install, while others aren't. Under Linux, only the SQL Server engine is installed as part of the installation process. Everything else is optional. For highly available SQL Server instances under Linux, two packages should be installed with SQL Server:

  • SQL Server Agent (mssql-server-agent)
  • the high availability (HA) package (mssql-server-ha)

While SQL Server Agent is technically optional, it's the SQL Server scheduler for jobs and is required by log shipping, so installation is recommended.

On SQL Server 2017 (14.x) with CU 4 and later versions, SQL Server Agent is included in the Database Engine package, but you still need to enable it. On Windows-based installations, SQL Server Agent isn't optional.

Note

For those new to SQL Server, SQL Server Agent is SQL Server's built-in job scheduler. You can schedule things like backups and other SQL Server maintenance. Unlike a Windows-based installation of SQL Server where SQL Server Agent is a completely different service, on Linux, SQL Server Agent runs in context of SQL Server itself.

When AGs or FCIs are configured on a Windows-based configuration, they are cluster-aware. Cluster awareness means that SQL Server has specific resource DLLs that a WSFC knows about (sqagtres.dll and sqsrvres.dll for FCIs, hadrres.dll for AGs) and are used by the WSFC to ensure that the SQL Server clustered functionality is up, running, and functioning properly. Because clustering is external not only to SQL Server but Linux itself, Microsoft had to code the equivalent of a resource DLL for Linux-based AG and FCI deployments. This is the mssql-server-ha package, also known as the SQL Server resource agent for Pacemaker. To install the mssql-server-ha package, see Deploy a Pacemaker cluster for SQL Server on Linux.

The other optional packages for SQL Server on Linux, SQL Server Full-Text Search (mssql-server-fts) and SQL Server Integration Services (mssql-server-is), aren't required for high availability, either for an FCI or an AG.

SQL Server high availability and disaster recovery partners

To provide high availability and disaster recovery for your SQL Server services, choose from a wide variety of industry-leading tools. This section highlights Microsoft partner companies with high availability and disaster recovery solutions supporting SQL Server.

Partner Description
DH2i DxEnterprise is Smart Availability software for Windows, Linux & Docker that helps you achieve the nearest-to-zero planned and unplanned downtime, unlocks huge cost savings, drastically simplifies management, and gets you both physical and logical consolidation.

- Deploy availability groups with DH2i DxEnterprise on Kubernetes
- Tutorial: Set up a three node Always On availability group with DH2i DxEnterprise
HPE Serviceguard HPE SGLX offers context-sensitive monitoring and recovery options for Failover Cluster Instance and Always On Availability Groups. Maximize uptime with HPE SGLX without compromising data integrity and performance.

- Tutorial: Set up a three node Always On availability group with HPE Serviceguard for Linux.
Pacemaker Pacemaker is an open source high-availability cluster resource manager. With Corosync, an open source group communication system, Pacemaker can detect component failures and orchestrate necessary failover procedures to minimize interruptions to applications.

- Pacemaker for availability groups and failover cluster instances on Linux
- Deploy a Pacemaker cluster for SQL Server on Linux

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