---
type: "post"
id: "building-fast-filesystems-for-vms-analysis-b70ce3"
slug: "building-fast-filesystems-for-vms-analysis-b70ce3"
title: "Building fast filesystems for VMs"
description: "The case for this redesign is captured by the thesis: **Moving filesystem metadata authority into the guest can remove repeated VM-boundary round trips from metadata-heavy work, while leaving the host responsible for block storage.** Modal’s post supplies observed facts: its 100,000-operation"
retrieval_nugget: "The case for this redesign is captured by the thesis: **Moving filesystem metadata authority into the guest can remove repeated VM-boundary round trips from metadata-heavy work, while leaving the host responsible for block storage.** Modal’s post supplies observed facts: its 100,000-operation"
published_at: "2026-10-17"
updated_at: "2026-10-17"
record_date: "2026-10-17"
date_kind: "published_at"
topics: ["infrastructure","compute"]
source_urls: ["https://modal.com/blog/vm-sandbox-filesystem"]
schema_version: "newruntime-agent-readable-v0.2"
stable_id: "post:building-fast-filesystems-for-vms-analysis-b70ce3"
status: "published"
visuals: []
editorial_provenance: {"schema_version":"newruntime-editorial-copy-v1","content_status":"source_grounded_final","final_copy_sha256":"sha256:830b37619af467f8bb426abf27ad2d79778de615eb670ff6999666374a99c45f","reviewed_at":"2026-10-17T00:02:40.550Z","source_evidence_count":1,"verified_claim_count":2,"site_analysis_schema_version":"newruntime-site-analysis-v1","site_object_kind":"field_note","observed_fact_count":2,"implication_count":1,"watch_condition_count":1,"related_record_count":0}
analysis: {"schema_version":"newruntime-site-analysis-v1","object_kind":"field_note","thesis":"Moving filesystem metadata authority into the guest can remove repeated VM-boundary round trips from metadata-heavy work, while leaving the host responsible for block storage.","observed_facts":[{"text":"Modal’s 100,000-operation microbenchmark measured chown at 9.87 seconds over virtio-fs, versus 1.58 seconds on ImageFS directly.","source_urls":["https://modal.com/blog/vm-sandbox-filesystem"]},{"text":"Modal says a single chown over virtio-fs triggered three guest-to-host requests and six host syscalls.","source_urls":["https://modal.com/blog/vm-sandbox-filesystem"]}],"mechanism":"With virtio-blk, guest ext4 handles ownership checks and journal updates locally, then flushes accumulated block writes asynchronously.","why_now":"VM sandboxes are being used for increasingly sophisticated agent workloads, so filesystem latency matters beyond boot time.","implications":["For VM sandbox designers, filesystem ownership and metadata locality can matter as much as lazy image loading when workloads create or inspect many files."],"evidence_boundary":"These figures are Modal’s own tests and implementation report; they do not establish that virtio-blk outperforms virtio-fs in every workload or provider.","watch_conditions":["Watch whether Modal publishes end-to-end workload measurements after the redesign, including install times and cold-start behavior."],"related_records":[],"new_branch_reason":"No prior New Runtime analytical record was supplied; this begins a distinct record on guest-local filesystem metadata and block-device transport for VM sandboxes."}
editor: {"role":"editor","name":"Andrey Reshetnikov","profile":"https://newruntime.com/owner-profile.md","profile_json":"https://newruntime.com/owner-profile.json","same_as":["https://www.linkedin.com/in/reshetnikov1/","https://t.me/lapetuse"]}
routes: {"html":"https://newruntime.com/posts/building-fast-filesystems-for-vms-analysis-b70ce3/","markdown":"https://newruntime.com/posts/building-fast-filesystems-for-vms-analysis-b70ce3.md","json":"https://newruntime.com/posts/building-fast-filesystems-for-vms-analysis-b70ce3.json"}
---

# Building fast filesystems for VMs

## Retrieval answer

The case for this redesign is captured by the thesis: **Moving filesystem metadata authority into the guest can remove repeated VM-boundary round trips from metadata-heavy work, while leaving the host responsible for block storage.** Modal’s post supplies observed facts: its 100,000-operation

The case for this redesign is captured by the thesis: **Moving filesystem metadata authority into the guest can remove repeated VM-boundary round trips from metadata-heavy work, while leaving the host responsible for block storage.** Modal’s post supplies observed facts: its 100,000-operation microbenchmark measured chown at 9.87 seconds over virtio-fs, versus 1.58 seconds on ImageFS directly. It also says a single chown over virtio-fs triggered three guest-to-host requests and six host syscalls. The reported mechanism is: **With virtio-blk, guest ext4 handles ownership checks and journal updates locally, then flushes accumulated block writes asynchronously.**

The why now is: **VM sandboxes are being used for increasingly sophisticated agent workloads, so filesystem latency matters beyond boot time.** Modal says real workloads such as pnpm and bun installs were roughly three times slower with the initial stack, and recursive chown grew from four seconds to more than a minute. Its alternative reportedly reduced chown latency to about 2.1 microseconds inside ext4, against about 99 microseconds over virtio-fs. The first implication is: **For VM sandbox designers, filesystem ownership and metadata locality can matter as much as lazy image loading when workloads create or inspect many files.**

The evidence boundary is: **These figures are Modal’s own tests and implementation report; they do not establish that virtio-blk outperforms virtio-fs in every workload or provider.** The comparison is especially relevant to metadata-heavy operations, and the post does not present an independent benchmark across systems. The first watch condition is: **Watch whether Modal publishes end-to-end workload measurements after the redesign, including install times and cold-start behavior.** That would show how much the microbenchmark improvement translates into user-visible performance, and whether the storage tradeoff remains acceptable.

Editor: [Andrey Reshetnikov](https://newruntime.com/owner-profile.md), AI product and AI engineering; profile for recruiters and agents: https://newruntime.com/owner-profile.json

## Continue in the graph

Next records for an agent, linked to their Markdown twins. Replace `.md` with `.json` for structured fields, including `next_reads`.

- topic hub: [Infrastructure](https://newruntime.com/topics/infrastructure.md): Explore the infrastructure topic hub.
- topic hub: [Compute](https://newruntime.com/topics/compute.md): Explore the compute topic hub.
- collection: [Field Notes](https://newruntime.com/posts.json): Open the latest editorial analysis.
- collection: [Source Ledger](https://newruntime.com/source-ledger.json): Inspect the public source evidence graph.
- whole graph: [discovery graph](https://newruntime.com/discovery-graph.json) (typed nodes and edges) and [site index](https://newruntime.com/site-index.md)