>>>>> "NeilBrown" == NeilBrown <neilb@xxxxxxx> writes: NeilBrown> On Mon, 11 Nov 2013 16:23:49 -0500 "John Stoffel" <john@xxxxxxxxxxx> wrote: >> >> I thought you could also get a mis-match count from open mmap'd files, >> which aren't completely written to one disk or another? >> NeilBrown> The only cause I can imagine for the mismatch count NeilBrown> increasing is for a page of memory to be change while it is NeilBrown> being written out (so each device sees a different value) NeilBrown> and then for the page to be invalidated (so the dirty page NeilBrown> never gets written out again). NeilBrown> The only way to change a page while it is being written out NeilBrown> is (I think) through memory mapping (though this could have NeilBrown> change, "write" might achieve it). NeilBrown> But normally if you change a memory mapped page while it is NeilBrown> being written it will be marked 'dirty' and so will be NeilBrown> written out again - the same to both devices. NeilBrown> You could possibly modify a mem-mapped file, and then NeilBrown> delete it before the latest changes were written, but think NeilBrown> that would be unlikely to do on purpose. NeilBrown> Swap is the most likely cause. If some pages in a process NeilBrown> were written out and changed during the writeout, and then NeilBrown> the process was killed, you could easily get a mismatch NeilBrown> persisting. I think it is swap in my case, it's a RHEL 5.6 box being used to host VNC sessions for my users. They do weekly checks on the mirrored disks, and I think I get mis-match counts because swap is on MD too. John -- To unsubscribe from this list: send the line "unsubscribe linux-raid" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
