Quoting Nicolas George (geo...@nsup.org): > 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.
I see your point now. Fortunately I always put a .md5 file on CDs which contains the digests of all the files. So I'll pass on trying it. > 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've use digests for pruning identical files from backups and they're computed serially, fortunately. So by accident I hadn't run into the problem you outline. But many thanks for elaborating. Cheers, David. -- To UNSUBSCRIBE, email to debian-user-requ...@lists.debian.org with a subject of "unsubscribe". Trouble? Contact listmas...@lists.debian.org Archive: https://lists.debian.org/20150720021448.GA16059@alum