IIRC, SpiderOak doesn't encrypt your files "immediately after uploading". Files are encrypted on the client side even before they're uploaded. Therefore SpiderOak never even sees unencrypted data. That's their "zero knowledge" policy. They think it's better to make it impossible for themselves ever to have any idea what files they're hosting, rather than saying "We used to know X seconds ago, but we don't know anymore."
The blog doesn't mention whether or not SpiderOak uses block-level deduplication. Maybe it's part of their storage infrastructure, maybe isn't. But all that client-side encryption would severely reduce the number of duplicate blocks even if everyone uploaded the same file.
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IIRC, SpiderOak doesn't encrypt your files "immediately after uploading". Files are encrypted on the client side even before they're uploaded. Therefore SpiderOak never even sees unencrypted data. That's their "zero knowledge" policy. They think it's better to make it impossible for themselves ever to have any idea what files they're hosting, rather than saying "We used to know X seconds ago, but we don't know anymore."
The blog doesn't mention whether or not SpiderOak uses block-level deduplication. Maybe it's part of their storage infrastructure, maybe isn't. But all that client-side encryption would severely reduce the number of duplicate blocks even if everyone uploaded the same file.