disk tutorial #20

Job	Run time
build-darwin (arm64, aarch64-apple-darwin, bridgeos, bridgeos, 2)	24s
build-darwin (arm64, aarch64-apple-darwin, iphoneos, ios, 11)	30s
build-darwin (arm64, aarch64-apple-darwin, appletvos, tvos, 11)	24s
	1m 18s

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