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由 Ilya Dryomov 提交于
A whole-object layered discard is implemented as a truncate rather than a delete: a dummy object is needed to prevent the CoW machinery from kicking in. However, a truncate on a non-existent object is a no-op. If the object doesn't exist in HEAD, a discard request is effectively ignored, which violates our "discard zeroes data" promise and breaks REQ_OP_WRITE_ZEROES implementation. A non-exclusive create on an existing object is also a no-op, so the fix is to do a compound create+truncate instead. Signed-off-by: NIlya Dryomov <idryomov@gmail.com>
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