---
title: "Block Device"
description: "1 posts about Block Device, summarised, each linking to the original."
---

# Block Device
> 1 posts about Block Device, summarised, each linking to the original.

## Articles

### [Building fast filesystems for VMs](https://yomu.fyi/post/building-fast-filesystems-for-vms.md)
- Company: [Modal](https://yomu.fyi/company/modal.md)
- Author: Nitin Maddi
- Published: Oct 16, 2026
- Type: Problem & solution

Modal’s VM Sandboxes originally used virtio-fs on top of ImageFS, a lazy-loading FUSE filesystem. This stack caused severe performance degradation: recursive chown took over a minute instead of seconds, and tools like bun install and ripgrep ran three times slower. Analysis revealed that every metadata operation like chown required three round trips from the guest VM to the host, each incurring approximately 99 µs overhead. Modal replaced virtio-fs with ImageFS-blk, a block-level solution that presents a virtual block device to the guest, letting the kernel manage its own filesystem. The new approach delivers dramatic speedups: chown dropped from 9.87 to 0.21 seconds, and bun install fell from 12.5 to 1.07 seconds, often exceeding even direct ImageFS performance.