Modal published a technical post on how it evolved the filesystem stack underpinning its VM Sandboxes. Its first approach routed guest VMs through virtio-fs on top of ImageFS, inheriting lazy loading and sub-second cold starts but making metadata operations very slow: a recursive chown went from four seconds to over a minute, pnpm install and bun install ran about three times slower, and ripgrep performance dropped sharply. Modal then moved to virtio-blk with ext4 inside the guest plus a new host daemon, ImageFS-blk, which writes ext4 metadata itself and serves blocks on demand; the post says a chown fell from about 99 microseconds over virtio-fs to about 2.1 microseconds.

Modal·VM Sandboxes·ImageFS·+5