The OCI manifest references the hashes of these compressed layers, and re-compressing them does not guarantee obtaining the same hash

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mort96 an hour ago | parent | next [-]

If that's the purpose, couldn't you store the hash and throw away the compressed image?

(As others said, compression is deterministic for the same algorithm, parameters and input data)

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a_t48 27 minutes ago | parent [-]

Zstd for example only promises determinism on the same version of the library. I've personally seen the hashes mutate between pull and export. Things like tar padding also make a difference. Really, the thing to do is to hash on the _uncompressed_ data and let compression be a transport/registry detail. That's what I've done, at least.

	▲	mort96 6 minutes ago \| parent [-]
		I didn't know that about zstd, that's a bit unfortunate. Tar isn't related here though, we're talking about compression not archival formats

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flakes 2 hours ago | parent | prev [-]

Recompressing should be guaranteed deterministic. It’s the packing/unpacking of tar archives to/from directories on disk that leads to the non-determinism (such as timestamps and ownership metadata). If the tar is left intact, both zstd and gzip should produce byte for byte identical outputs given the same compression parameters.

	▲	XYen0n 36 minutes ago \| parent [-]
		You are correct; I confused archiving with compression. However, even considering only the compression process, same compression parameters cannot be guaranteed, as it is unknown which compression parameters the image publisher used.