PREEMPTIVE_OS_FILEOPS is a generic wait covering a family of Windows file system functions that do not have their own dedicated preemptive wait type. When a thread calls one of them, it switches from SQL Server’s cooperative scheduling to Windows control (showing state RUNNING throughout) and banks the time here.
It is a catch-all: file attribute queries, directory operations, and assorted file-level housekeeping all land in this bucket.
Is It a Problem?
It has not been a noticeable contention point in practice. You will see it tick up around operations that touch the file system beyond plain data I/O: backups and restores, database file creation and growth, FILESTREAM housekeeping, and similar. The wait inherits whatever latency the underlying file system adds, so its interesting failure mode is slow file infrastructure rather than anything inside SQL Server.
Sustained unusual growth deserves the same checklist as any file-path slowness.
Common Causes
- Backup and restore file handling, especially to network or deduplicating targets.
- Database file operations: creation, growth, attribute changes.
- Slow file systems, SMB paths, or filter drivers (antivirus) taxing each call.
What To Do
- Correlate the wait with backup windows and file operations; most accumulation maps cleanly to scheduled activity.
- If it grows without an obvious driver, test basic file operations on the relevant volumes from the OS; slowness there is an infrastructure fix.
- Confirm antivirus exclusions for SQL Server’s data, log, and backup paths.
How To See It
Rank it against everything else with Get-WaitStatistics. Read it as file-infrastructure latency and judge it by what was running at the time.
Part of the SQL Server Wait Types Library.
Related deep dive: SOS_SCHEDULER_YIELD Wait Type.
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