Row versions let readers and writers work with less blocking, but those versions need space. Watching the tempdb version store shows when normal use turns into growth that needs investigation.
Know Why Versions Exist
Snapshot isolation and read committed snapshot use row versions so readers can see a consistent earlier state. Other engine features can also create versions. On a traditional SQL Server configuration, the version store uses tempdb. The space needed depends on change rate and how long old versions must remain available. A busy update workload and a long reader can form an expensive pair.
I explain the two forces separately: how quickly versions arrive and how slowly cleanup can release them. A large store of row versions is not automatically a tempdb file-layout problem. Adding files can give space while the underlying retention cause remains. Which transaction still needs the old rows? That is the question to pursue.
Measure Per-Database Version Store Use
sys.dm_tran_version_store_space_usage reports reserved version-store space by source database. It is efficient because it uses aggregate information rather than walking every version record. Capture it over time with a sample timestamp. A single value tells you current use. A trend shows growth and cleanup.
The query converts kilobytes to megabytes for reading. The numbers will come from your instance. I do not set an arbitrary universal alarm threshold. Compare the trend with tempdb capacity and the workload’s normal cycle.
SELECT SYSDATETIME() AS sample_time,
DB_NAME(database_id) AS database_name,
reserved_page_count,
reserved_space_kb / 1024.0 AS version_store_mb
FROM sys.dm_tran_version_store_space_usage
ORDER BY reserved_space_kb DESC;Check tempdb Space
Row versioning shares tempdb with temporary objects, sorts, hashes, and other internal work. Measure total file size, allocated categories, and free space before assigning blame. A growing row-version number can be harmless when capacity and cleanup are stable. A smaller number can be urgent on a nearly full volume.
I pair row-version samples with tempdb file and volume monitoring. SQL Server knows its file allocation. Windows knows the volume’s free capacity. Both views matter. A dashboard that shows only one percentage can hide a competing load from a sort or ETL job. Find the actual consumer before changing file settings.
SELECT SUM(version_store_reserved_page_count) * 8.0 / 1024 AS version_store_mb,
SUM(unallocated_extent_page_count) * 8.0 / 1024 AS unallocated_mb
FROM tempdb.sys.dm_db_file_space_usage;Find Long Snapshot Transactions Holding the Version Store
A transaction using row versions can retain old versions until it ends. sys.dm_tran_active_snapshot_database_transactions exposes active snapshot transactions and elapsed time. Inspect the oldest ones when row-version usage keeps rising. The oldest transaction is a clue, not automatic permission to end it. Check the session and application purpose.
I ask whether a report or cursor is left open across user think time. That pattern can prevent cleanup while updates continue. If the transaction is legitimate, plan capacity and application changes. If it is abandoned, work with the owner on a safe termination. The cleanup rate should be observed afterward.
SELECT session_id,
transaction_id,
elapsed_time_seconds,
is_snapshot,
max_version_chain_traversed
FROM sys.dm_tran_active_snapshot_database_transactions
ORDER BY elapsed_time_seconds DESC;
Review Database Options
READ_COMMITTED_SNAPSHOT and ALLOW_SNAPSHOT_ISOLATION settings influence versioning behavior. Do not turn them off as a quick tempdb fix. Applications can depend on their concurrency semantics, and changing them can bring blocking back. Record current options and the reason they were enabled.
I compare option history with the start of growth. A recent feature rollout can increase update rate or reader duration without any database option change. The query below shows the settings across user databases. It is an inventory, not a recommendation to flip a switch.
SELECT name,
is_read_committed_snapshot_on,
snapshot_isolation_state_desc
FROM sys.databases
WHERE database_id > 4
ORDER BY name;Distinguish Production Patterns
A scheduled report can hold a snapshot for a long time, while a batch update generates many versions quickly. These patterns call for different actions. Shorten the reader transaction, break up the writer, tune the query, or schedule overlap differently. Measure the effect on application correctness and throughput.
I look for the point where row-version growth stops after the suspected session ends. If the store keeps rising, the hypothesis was incomplete. Other active transactions or workload changes can contribute. One screenshot of a long transaction is evidence, not the whole explanation.
Mind Newer Storage Behavior
Some modern SQL Server features can place certain version information outside the traditional tempdb version store. Confirm the database configuration and engine version before assuming every version byte appears in the same DMV. The operational question remains the same: which workload creates versions, where are they stored, and what prevents cleanup?
I keep version-specific details in the runbook for the actual server build. A generic article cannot replace that check. If a chart suddenly changes after an upgrade, inspect feature configuration before declaring the workload fixed. The data can have moved to a different accounting view. Monitoring needs to follow the feature.
Set Version Store Alerts from Capacity
Trend reserved space against tempdb file size and volume free space. Alert early enough for someone to investigate and act before allocation fails. Include source database, current trend, oldest relevant transaction, and remaining capacity in the alert when possible. Avoid a page that says only that row versioning is high.
I test that the alert arrives and that the recipient can run the diagnostic queries. A threshold with no response path is decoration. Keep a short incident procedure: identify growth, find the retaining transaction, confirm workload, choose a safe action, and watch cleanup. That process protects users better than a blind file-size increase.
Measure Again After the Fix
After changing a query or transaction pattern, collect the same interval samples. Verify that the store grows and shrinks within expected capacity under comparable workload. Check application behavior because shortening transactions can change result consistency. Record the finding and review it after major releases.
What is the longest reader that overlaps the busiest writer? Answer that from your own instance. The store of row versions is a record of that overlap. When you understand both sides, a growing tempdb file becomes a manageable workload question rather than a surprise disk alert.
I also check the oldest active snapshot transaction before blaming tempdb file size. A long reader can hold versions after writers finish. Ending that reader through the application workflow can let cleanup catch up without adding storage. Compare the transaction age with row-version growth over the same interval.
Related reading on this blog: TempDB Troubles: Identifying and Resolving TempDB Contentions and What is Stored in TempDB? Interview Question of the Week #271.
Version-store growth is not just a file problem, it is a history of writers and readers overlapping.
Published by Pinal Dave on SQLAuthority. More of my work at pinaldave.com.
