1. A journaling method performed for a non-volatile buffer cache in a file system, the journaling method comprising: identifying a plurality of cache blocks in the non-volatile buffer cache; and performing in-place commit operation for the a first cache block with normal dirty state among the plurality of cache blocks in the non-volatile buffer cache, and changing a state of the first cache block from the normal dirty state to the frozen dirty state; performing write operation by updating a content of a data block into a second cache block with clean state, instead of the first cache block with the frozen dirty state which is performed in-place commit operation, so that the first cache block being performed in-place commit operation is not overwritten with the content, and changing a state of the second cache block from the clean state to the normal dirty state; performing in-place commit operation for the second cache block with normal dirty state, and changing the state of the first cache block from frozen dirty state to frozen dirty out-of-date state, and changing the state of the second cache block from normal dirty state to frozen dirty up-to-date state; and performing checkpoint operation periodically, wherein when the checkpoint operation occurs, the frozen dirty out-of-date state of the first cache block is converted into a free state, and the content cached by the first cache block is deleted, wherein when the checkpoint operation occurs, the frozen dirty up-to-date state of the second cache block is converted into the clean state, and the content cached by the second cache block is transmitted into a storage, when a problem occurs in the file system after in-place commit operation has been completed, the file system is recovered with a content which is stored in the in-place committed cache block in the non-volatile buffer cache, when the problem occurs in the file system before in-place commit operation has been completed, the file system is recovered with a content which is stored in the data block of the storage, and the content in the cache block is discarded.