On Thu, May 07, 2015 at 12:53:46PM -0700, Andy Lutomirski wrote: > 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. They are close enough to be good for backup purposes. The mtime only need change once per backup period - it doesn't need to be millisecond accurate. Yes, I know you needed that changed for different reasons (avoid variable page fault latency), but it doesn't matter for once-a-day or even once-an-hour incremental backup scans. Besides, anyone who cares about accurate backups is doing a backup from a snapshot so they data and metadata is consistent across the entire backup. And that makes worries about mmap and mtime completely irrelevant because a snapshot freezes the filesystem and hence cleans all the mapped pages. Once the snapshot is taken the next mmap write will trigger a page fault and so change the mtime and it will be picked up in the next backup scan... Cheers, Dave. -- Dave Chinner david@xxxxxxxxxxxxx -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
