Read Committed Snapshot Isolation and tempdb I/O Latency

Recommended tempdb configuration for RCSI workloads

First, address the likely explanation, then list confirmed facts, and finally give the concrete steps to mitigate I/O latency.

Likely explanation

When Read Committed Snapshot Isolation (RCSI) is enabled, each read operation retrieves rows from the version store in tempdb. Writes generate new versions, and long‑running transactions delay cleanup, so the version store continuously grows and drives both read and write I/O on the tempdb files.

Confirmed facts

• The version store size reported by sys.dm_tran_version_store grows proportionally to the rate of updates under RCSI.
• Elevated PAGEIOLATCH_* and WRITELOG wait types, together with increased avg_io_stall_ms from sys.dm_io_virtual_file_stats, indicate tempdb I/O pressure caused by version store activity.
• Allocating multiple equally sized data files and enabling trace flag 1118 reduces allocation contention, which is a common bottleneck when tempdb is heavily used for versioning.

1. Pre‑size tempdb data files to accommodate the expected version store (e.g., start with a size that holds at least 20‑30% of the total database size or based on observed peak version store usage).
2. Create 0.25 × number of logical cores data files, all of equal size, and place them on fast storage (SSD or NVMe).
3. Enable trace flag 1118 (or use the default behavior in SQL Server 2016+ which already applies uniform extents) to eliminate SGAM allocation contention.
4. Monitor sys.dm_tran_version_store and sys.dm_db_file_space_usage to track version store growth; if growth exceeds your pre‑allocated headroom, add another file or increase file size.
5. Consider shortening transaction duration or using indexed views to reduce the amount of versioned data, which directly lowers tempdb I/O.

To determine whether the version store overhead has offset the blocking reduction gained from RCSI, you need to know the current version store growth rate (e.g., MB per minute) under your typical workload. Please provide that measurement so a precise threshold can be advised.