when there is already a old os.snap file in the source file and there is a new one on the server.there will be an err that the checksum of os.snap is wrong. and i found the os.snap size is the double size to about 150m while normal one is around 75m.
so my guess is the code first check the local os.snap if it is the lastest. when the checksum is wrong.it just open os.snap and add file stream to it.problem is that it does not delete the old one.so after download.the size is double and checksum is wrong