What did you do?

• Downloaded the Windows release: prometheus-3.5.0.windows-amd64
• Started Prometheus with the experimental feature flag enabled: .\prometheus.exe --enable-feature=use-uncached-io --web.enable-admin-api
• Waited for “Server is ready to receive web requests.”
• Forced a block write by snapshotting the head via the admin API: PowerShell: Invoke-WebRequest -Method POST -Uri "http://localhost:9090/api/v1/admin/tsdb/snapshot?skip_head=false" (Alternatively: curl.exe -X POST "http://localhost:9090/api/v1/admin/tsdb/snapshot?skip_head=false")

What did you expect to see?

• The snapshot to succeed, or if the feature is unsupported on Windows, Prometheus to gracefully ignore the flag or fall back to buffered I/O without failing block writes.

What did you see instead? Under which circumstances?

• The snapshot failed with an internal error. The API response was: {"status":"error","errorType":"internal","error":"create snapshot: snapshot head block: write chunks: direct IO is unsupported"}
• This occurs when Prometheus is started on Windows with --enable-feature=use-uncached-io and a block write is attempted (forced by snapshot or when compaction triggers naturally).

System information

OS: Windows (amd64). Example: Windows 10/11 64-bit. uname -srm not applicable on Windows.

Prometheus version

From logs on startup: Starting Prometheus Server mode=server version="(version=3.5.0, branch=HEAD, revision=8be3a9560fbdd18a94dedec4b747c35178177202)" platform=windows/amd64
Typical CLI output: .\prometheus.exe --version prometheus, version 3.5.0 (branch: HEAD, revision: 8be3a9560fbdd18a94dedec4b747c35178177202) go version: go1.24.5 platform: windows/amd64

Prometheus configuration file

Default config scraping itself: global: scrape_interval: 15s scrape_configs:
job_name: "prometheus" static_configs:
targets: ["localhost:9090"]

Alertmanager version

Not applicable.

Alertmanager configuration file

Not applicable.

Logs

Relevant API error when forcing snapshot: {"status":"error","errorType":"internal","error":"create snapshot: snapshot head block: write chunks: direct IO is unsupported"}