Microsoft Windows Server Semi-Annual Channel (SAC) is a servicing option that delivers a feature update twice a year for organizations that want the latest Windows Server functionality.
In June 2017, Microsoft announced Windows Server SAC would follow the same servicing model of the Windows 10 SAC with a release every six months -- roughly one in the spring and one in the fall -- with a number of new features and updates. Microsoft said it tailors the Windows Server SAC releases for enterprises with rapid application development cycles, such as DevOps shops that want the latest capabilities in container-based virtualization.
Semi-Annual Channel customer requirements
Businesses with Software Assurance on their Windows Server Standard or Datacenter licenses or a Microsoft Developer Network (MSDN) license can download the SAC releases from Microsoft's Volume Licensing Service Center.
Businesses without Software Assurance can use SAC releases in Azure or another cloud or hosting environment.
Differences between SAC and LTSC
For enterprises that prefer the more traditional Windows Server feature release cycle of every two to three years, Microsoft refers to this as the Long Term Servicing Channel (LTSC) -- formerly the Long Term Servicing Branch.
LTSC features two installation options: Windows Server with Desktop Experience and Server Core; Server Core and Nano Server container images are also available. The only SAC installation option is Server Core -- there is no Server with Desktop Experience mode available in this release. Microsoft also provides Nano Server and Server Core base OS container images for Windows Server SAC. While LTSC and SAC both have Server Core container images, Microsoft said they are not compatible.
The Server Core and Nano Server products in the SAC release receive 18 months of mainstream support without the option of extended support. The LTSC gets five years of mainstream support and five years extended support.
The SAC release naming convention uses the last two digits of the year and the month as the version number, such as Windows Server version 1709. The LTSC version of Windows Server retains the traditional format with Windows Server and the full year, such as Windows Server 2016.
Microsoft says releases in the SAC and LTSC channels are not interchangeable, meaning an in-place upgrade from Server Core in Windows Server 2016 to Server Core in Windows Server version 1709 is not supported. Customers must do a clean install to use the SAC release.
Microsoft plans to add most of the enhancements -- with some variations -- from the SAC releases into the next LTSC release.
Features of the Windows Server version 1709 release
Microsoft released its first SAC release -- Windows Server version 1709 -- in October 2017.
The company, which had debuted Nano Server in the Windows Server 2016 release-to-manufacturing (RTM) version as more streamlined version of Server Core, removed Nano Server's infrastructure role capabilities in SAC version 1709. Microsoft converted Nano Server to a base OS container image which shrunk its size from about 400 MB to about 80 MB. The reduced version of Nano Server has no servicing stack; when an update arrives, administrators redeploy the image through Docker.
Microsoft also reduced the install size of the Server Core from just under 15 GB in the Windows Server 2016 Server with Desktop Experience to about 6.6 GB in the 1709 release.
Microsoft added support to Linux containers with kernel isolation provided by Hyper-V.
Missing from version 1709 is the Storage Spaces Direct feature that collects server drive space into pooled storage. Microsoft said a Windows Server 2016 Storage Spaces Direct deployment will not support the use of servers that run Windows Server version 1709.