in_tail: fix file_cache_advise causing high disk IOPS

[singholt]

Problem

The file_cache_advise option (added in #8422) calls posix_fadvise(fd, 0, 0, POSIX_FADV_DONTNEED) before read() in flb_tail_file_chunk(). With offset=0, len=0 this evicts the entire file from the page cache on every chunk cycle, including pages the kernel had readahead-cached for the next read. Every subsequent read() becomes a cache miss and must go to disk, causing high IOPS.

Fix

Move the posix_fadvise call to after read and processing, and scope it to only the already-consumed byte range (0 to stream_offset) so that kernel readahead pages for upcoming reads are preserved.

Before

flb_tail_file_chunk()
│
├─ check buffer capacity, resize if needed
│
├─ posix_fadvise(fd, 0, 0, DONTNEED) ← evicts ENTIRE file from page cache
│
├─ read(fd, buf, size) ← cache miss, must hit disk every time
│
├─ process_content()
│
├─ update stream_offset
│
└─ return

After

flb_tail_file_chunk()
│
├─ check buffer capacity, resize if needed
│
├─ read(fd, buf, size) ← hits page cache normally
│
├─ process_content()
│
├─ update stream_offset
│
├─ posix_fadvise(fd, 0, stream_offset, ← evicts only consumed bytes;
│ DONTNEED) readahead pages preserved
│
└─ return

Impact

• Same memory benefit: processed data no longer lingers in page cache
• No IOPS penalty: kernel readahead works as intended for sequential reads
• Customers tailing many files under high append rates should see significantly reduced disk I/O

PENDING TESTING, DO NOT MERGE

[coderabbitai]

Important

Review skipped

Draft detected.

Please check the settings in the CodeRabbit UI or the .coderabbit.yaml file in this repository. To trigger a single review, invoke the @coderabbitai review command.

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: efc0ea15-801e-4df3-8e61-58b1b73b905a

You can disable this status message by setting the reviews.review_status to false in the CodeRabbit configuration file.

Use the checkbox below for a quick retry:

• 🔍 Trigger review

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

---

Thanks for using CodeRabbit! It's free for OSS, and your support helps us grow. If you like it, consider giving us a shout-out.

❤️ Share

• X
• Mastodon
• Reddit
• LinkedIn

_{Comment @coderabbitai help to get the list of available commands and usage tips.}