--- title: "Uncached I/O in Prometheus" url: https://daily.dev/posts/uncached-i-o-in-prometheus-oquzws311 source_url: https://prometheus.io/blog/2026/03/05/uncached-io/ type: article source: "Prometheus" published: 2026-03-20T13:55:00.805Z updated: 2026-03-20T13:55:27.437Z tags: ["prometheus"] reading_time: 4 upvotes: 2 comments: 0 language: en --- > ## Documentation Index > Fetch the complete documentation index at: https://daily.dev/llms.txt > Use this file to discover all available pages before exploring further. # Uncached I/O in Prometheus **[Prometheus](https://daily.dev/sources/prometheus)** · 4 min read · 2 upvotes · 0 comments ## Summary Prometheus v3.5.0 introduces a `use-uncached-io` feature flag (Linux-only) that uses direct I/O for compaction chunk writes, bypassing the page cache. This reduces Prometheus's page cache footprint by 20–50% as measured by `container_memory_cache`, making memory usage more predictable and reducing pressure on shared cache. The implementation required a custom `directIOWriter` to satisfy direct I/O's strict alignment requirements (file offset, buffer address, and transfer size must align to the storage device's logical sector size). Future work includes extending direct I/O to index files and WAL writes, experimenting with the newer `RWF_DONTCACHE` flag in Linux v6.14, and adding support for non-Linux platforms. ## Full article daily.dev links to this article rather than hosting it. Read it at the original source: ## Similar posts on daily.dev - [Linux 6.19 Enables Per-CPU BIO Caching By Default For Helping Performance](https://daily.dev/posts/linux-6-19-enables-per-cpu-bio-caching-by-default-for-helping-performance-zd5bw0cf9) · Phoronix · 0 upvotes · 0 comments - [Common Performance Pitfalls of Modern Storage I/O](https://daily.dev/posts/common-performance-pitfalls-of-modern-storage-i-o-ucpqaclej) · P99 Conf · 7 upvotes · 0 comments - [Btrfs Ready With More Performance Improvements For Linux 7.3: Some ~3x To ~5x Wins](https://daily.dev/posts/btrfs-ready-with-more-performance-improvements-for-linux-7-3-some-3x-to-5x-wins-b4x0ludw1) · Phoronix · 1 upvotes · 0 comments --- Tags: [#prometheus](https://daily.dev/tags/prometheus) [View this post on daily.dev](https://daily.dev/posts/uncached-i-o-in-prometheus-oquzws311)