Performance Tuning with SQL Server Dynamic Management Views

Chapter 4: Transactions
Version store usage
SQL Server provides two DMVs that store data with regard to version store usage on a
given instance, as follows (definitions taken from Books Online).
• sys.dm_tran_version_store – returns a virtual table that displays all version
records in the version store. sys.dm_tran_version_store is inefficient to run
because it queries the entire version store, and the version store can be very large.
Each versioned record is stored as binary data together with some tracking or status
information. Similar to records in database tables, version store records are stored
in 8,192-byte pages. If a record exceeds 8,192 bytes, the record will be split across two
different records. Because the versioned record is stored as binary, there are
no problems with different collations from different databases. Use sys.dm_tran_
version_store to find the previous versions of the rows in binary representation as
they exist in the version store.
• sys.dm_tran_top_version_generators – returns a virtual table for the objects
that are producing the most versions in the version store. sys.dm_tran_top_
version_generators returns the top 256 aggregated record lengths that are
grouped by the database_id and rowset_id. sys.dm_tran_top_version_
generators retrieves data by querying the dm_tran_version_store virtual table.
sys.dm_tran_top_version_generators is an inefficient view to run because this
view queries the version store, and the version store can be very large. We recommend
that you use this function to find the largest consumers of the version store.
Both are aimed at allowing us to investigate the highest consumers of space within the
version store. Heed the warning in each case that, if the version store is large, queries
against these DMVs can be expensive, and may have performance implications (a classic
case of the "watcher" effect).
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