Ralf Mardorf <ralf-mardorf@xxxxxxxxxx> wrote: > Hi, > > my google search was "does linux diff compare data using a cache". > > I'm trying to figure out what's going on. The first diff of 10 GiB of > data copied from a SATA3 SSD to an USB 2 stick connected to an USB 3 > port took around a minute, right after the copy finished. A second diff > needed 3 seconds. Both returned exit status 0. > > It's impossible to read 10 GiB of data in 3 seconds from an USB 2 stick. > Does diff use cached data instead of comparing the "real" files line by > line? > > Google returned "diff isn't doing any caching. The OS is. If you are > using Linux, you can flush the disk buffers and cache". > > I expected that diff ensures to compare the "real" files line by line, > but seemingly diff isn't aimed to check integrity of data. > > Does a command exist that compares "real" files, not just cached files > by default? > To the best of my knowelsge, Google gave you the correct answer. I got many results when searching for how to clear the filesystem buffer cache in linux. I don't know which, if any, is better. One of the issues is differentiating the related cache from the other entries in the cache. It could be unmounting, and pulling out the USB device, after a sync command completed is a step towrds cleaning unwanted entries. Perhaps there is also a way from the command line to disconnet and reconnect the power to the device, as if it was physically pulled out and in. > I experience weird things with Raptor Lake hardware, especially if USB > is involved and I want to check the integrity of USB transferred, saved > files by using a tool, without manually clearing cached data manually. > It is a good aproach to verify integrity of copied data, in my opinion. Unfortunately, you can not disregrad cache when doing that. -- u34 > Regards, > Ralf
