Thanks! ^_^

Thanks for your advice. However i wonder why SHA2 should be a better choice which is slower than MD5 or MD4?

Thank you V. ^_^ Your advice is good. In the beginning i had not a clear understanding of this question. Now i agree I/O should be the key point, and i wish i can find a way to resolve my problem. Thanks!

yeah ,it is a good idea. Another situation is that -The contents of file is the same but names are different. And i want to know which kind of hash algorithm has the best speed? or any other way to speed up?

The current thinking is to calculate hash for every file ,and then delete them. Now we can use MD5 or SHA1 to calculate hash.From the speed point of view, MD4 is a better choice when MD4 is faster but more unsafer.But after experiment, i found even md4 is so slow that it would cost a lot of time when i dealt with large files. So any algorithm to work faster?