Sync between volumes

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2

Well, there does have same post arguing this problem, see https://community.fly.io/t/data-not-in-new-volume-after-using-fly-scale-count-2/18919/9, so I’m confused why “Always provision at least two volumes per app. Running an app with a single Machine and volume leaves you at risk for downtime and data loss.” is urged though data between two different volumes won’t be synced? Should I do it manually? But there is no applicable command I can perform.