Building fast filesystems for VMs

This post details the evolution of Modal's filesystem stack to support VM Sandboxes, focusing on the performance challenges encountered with virtio-fs and the subsequent adoption of a block-based approach (ImageFS-blk) over virtio-blk. It explains how ImageFS, originally designed for containers, was adapted for VMs by using virtio-fs, and the performance regressions observed. The solution involves creating a sparse ext4 filesystem within the guest VM, where ImageFS concepts are applied to serve file content on demand via block requests over virtio-blk, significantly improving metadata-intensive operations like `chown` and `stat`.