what will happen while update partial block？

[wangtuanjie]

通过 writeBlock(ws io.WriteSeeker, index, bs int64, p []byte)，更新一个block。
当len(p)小于block size时，是什么语义？

1. 算错误，直接返回
2. 更新前len(p)字节，并重新计算整个block的crc

[yutongp]

@yunxing @xiang90 @yichengq Let's revisit and settle on an agreement. At block level, do we allow partly write/read on blocks except the very last block in the file?

I thought our assumption is ReadAt and WriteAt always use readBlock and writeBlock with len(p) == bsize and this is exactly how our implementation currently works.

My opinion is allowing len(p) < bsize make readBlock and writeBlock more complicated and we don't even use this logic flow in practice... So maybe a simple solution which just return error and don't handle this case is better. My concern also applies on #52

[yunxing]

@yutongp
If we don't allow partial block write, we need to keep the length of the file somewhere. The write path for the last block becomes:

1. Create a block with payload
2. Fill the rest of the block with zeros
3. Append the block to the end
4. Update the file length in some databases (which we haven't figure out yet).

The problem is step 3 and 4 are not atomic, which 3 succeeds and 4 fails, we have no way to know what the length of the payload is.

However, I guess in that case, we could just use the old value in the database as its file length.