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Jeffrey Ying
7775584bbf
`MergeSparseOverlay` ended with a rename onto an existing `outFile`. Renaming over an existing file makes [ext4](https://github.com/torvalds/linux/blob/v6.17/Documentation/admin-guide/ext4.rst#L317-L328) (`data=ordered`) write back the renamed file's dirty pages, and the scratch file carries the whole merged memory image, so the merge pays a full flush of it. `MergeDeltaIntoBase` (see [link](https://github.com/agent-substrate/substrate/blob/a4121a4ca438603481f1799c85928c78a4ba9a6a/cmd/ateom-microvm/internal/ch/merge.go#L150-L158)) already unlinks first for this reason. On the counter demo the merge goes from ~1140 ms to ~115 ms. This is a no-op today, `MergeSparseOverlay` only runs when base and delta land on different filesystems. It becomes the common path in the follow-up that routes hardlinked restores through it, and landing first keeps that change from carrying a 1.1 s suspend regression.