What is the version store?

The version store maintains previous versions of rows required by row-versioning features such as RCSI and SNAPSHOT isolation.

Long-running transactions can force older row versions to be retained for longer periods, increasing version-store space usage. Version-store pressure should therefore be monitored in systems using row-versioning heavily.

The exact storage architecture can vary with SQL Server features and versions, including Accelerated Database Recovery, but the core concept is that readers may access retained row versions rather than blocking writers.