The operation of writing dirty data pages (pages containing changed data) to the database as part of query processing. A synchronous write occurs when the query needs a free page in the buffer pool during processing, but none is available. A database agent must write a dirty page to the database before execution of the query can continue.
Compare synchronous data writes with asynchronous data writes, in which page cleaners monitor the buffer pool and write dirty pages to the database before clean pages are actually needed by the query. These writes are asynchronous because they are performed outside of actual query processing. Asynchronous writes do not block query processing while data pages are written to disk.