{"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"},"next_reads":[{"type":"topic","path":"/topics/infrastructure/","url":"https://newruntime.com/topics/infrastructure/","title":"Infrastructure","reason":"Explore the infrastructure topic hub.","media_type":"text/html","markdown_url":"https://newruntime.com/topics/infrastructure.md","json_url":"https://newruntime.com/topics/infrastructure.json"},{"type":"topic","path":"/topics/compute/","url":"https://newruntime.com/topics/compute/","title":"Compute","reason":"Explore the compute topic hub.","media_type":"text/html","markdown_url":"https://newruntime.com/topics/compute.md","json_url":"https://newruntime.com/topics/compute.json"},{"type":"collection","path":"/posts/","url":"https://newruntime.com/posts/","title":"Field Notes","reason":"Open the latest editorial analysis.","media_type":"text/html","json_url":"https://newruntime.com/posts.json"},{"type":"collection","path":"/source-ledger.json","url":"https://newruntime.com/source-ledger.json","title":"Source Ledger","reason":"Inspect the public source evidence graph.","media_type":"application/json"}]}