On Thu, May 7, 2015 at 1:02 PM, Richard Weinberger <richard@xxxxxx> wrote: > Am 07.05.2015 um 21:53 schrieb Andy Lutomirski: >> On Thu, May 7, 2015 at 12:09 PM, Richard Weinberger >> <richard.weinberger@xxxxxxxxx> wrote: >>> On Thu, May 7, 2015 at 7:20 PM, Zach Brown <zab@xxxxxxxxxx> wrote: >>>> On Thu, May 07, 2015 at 10:26:17AM +1000, Dave Chinner wrote: >>>>> On Wed, May 06, 2015 at 03:00:12PM -0700, Zach Brown wrote: >>>>>> Add the O_NOMTIME flag which prevents mtime from being updated which can >>>>>> greatly reduce the IO overhead of writes to allocated and initialized >>>>>> regions of files. >>>>> >>>>> Hmmm. How do backup programs now work out if the file has changed >>>>> and hence needs copying again? ie. applications using this will >>>>> break other critical infrastructure in subtle ways. >>>> >>>> By using backup infrastructure that doesn't use cmtime. Like btrfs >>>> send/recv. Or application level backups that know how to do >>>> incrementals from metadata in giant database files, say, without >>>> walking, comparing, and copying the entire thing. >>> >>> But how can Joey random user know that some of his >>> applications are using O_NOMTIME and his KISS backup >>> program does no longer function as expected? >>> >> >> Joey random user can't have a working KISS backup anyway, though, >> because we screw up mtime updates on mmap writes. I have patches >> gathering dust that fix that, though. > > Hmmm, I thought mtime will be updated upon msync()? > Assuming a sane application is using msync()... > So would I. Unfortunately, mtime is updated on the page fault that makes an mmapped page writeable, thus guaranteeing that the resulting mtime is stale if you mmap a file, write to it, unmap it, and close it. It's much more stale if you mmap it, write, wait for a while but not long enough that the page is automatically written back, write again, unmap, and close. --Andy -- To unsubscribe from this list: send the line "unsubscribe linux-api" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
