Le decadi 30 messidor, an CCXXIII, David Wright a écrit : > > And of course (unless the files are large (unlikely for .forward) and on the > > same mechanical drive), cmp file1 file2 is much simpler. > I may've missed something here. I can't think why computing the > md5/sha-2 digest would ever be better or simpler than cmp, even > if the files are large and/or on the same spindle). You missed the end of the parenthesized text. Try this: cmp /cdrom/300_megs_file_1 /cdrom/300_megs_file_2 ... and when you are done buying a replacement for your optical drive, you can tell me if cmp was really better than a hash. The explanation is: If the files are large, then neither the application nor the kernel will read them at once. Therefore, with cmp, read will happen alternatively on each file until the end. If the file are not already present in the cache and are on the same mechanical drive, that means moving the read head hundreds of time. Even if it does not kill your drive, it will be awfully slow. With hashes, unless you make the mistake of running the hashes in parallel thinking you will save time, the first file is read in full and then the second, and everything goes as fast as sequential reads. I gave the example with a very slow device, but the effect is quite noticeable even with internal hard drives. Of course, SSD drives do not cause this problem. Regards, -- Nicolas George
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