On Tue, May 12, 2015 at 09:54:27AM -0400, John Stoffel wrote: > >>>>> "Austin" == Austin S Hemmelgarn <ahferroin7@xxxxxxxxx> writes: > > Austin> On 2015-05-12 01:08, Kevin Easton wrote: > >> On Mon, May 11, 2015 at 07:10:21PM -0400, Theodore Ts'o wrote: > >>> On Mon, May 11, 2015 at 09:24:09AM -0700, Sage Weil wrote: > >>>>> Let me re-ask the question that I asked last week (and was apparently > >>>>> ignored). Why not trying to use the lazytime feature instead of > >>>>> pointing a head straight at the application's --- and system > >>>>> administrators' --- heads? > >>>> > >>>> Sorry Ted, I thought I responded already. > >>>> > >>>> The goal is to avoid inode writeout entirely when we can, and > >>>> as I understand it lazytime will still force writeout before the inode > >>>> is dropped from the cache. In systems like Ceph in particular, the > >>>> IOs can be spread across lots of files, so simply deferring writeout > >>>> doesn't always help. > >>> > >>> Sure, but it would reduce the writeout by orders of magnitude. I can > >>> understand if you want to reduce it further, but it might be good > >>> enough for your purposes. > >>> > >>> I considered doing the equivalent of O_NOMTIME for our purposes at > >>> $WORK, and our use case is actually not that different from Ceph's > >>> (i.e., using a local disk file system to support a cluster file > >>> system), and lazytime was (a) something I figured was something I > >>> could upstream in good conscience, and (b) was more than good enough > >>> for us. > >> > >> A safer alternative might be a chattr file attribute that if set, the > >> mtime is not updated on writes, and stat() on the file always shows the > >> mtime as "right now". At least that way, the file won't accidentally > >> get left out of backups that rely on the mtime. > >> > >> (If the file attribute is unset, you immediately update the mtime then > >> too, and from then on the file is back to normal). > >> > > Austin> I like this even better than the flag suggestion, it provides > Austin> better control, means that you don't need to update > Austin> applications to get the benefits, and prevents backup software > Austin> from breaking (although backups would be bigger). > > Me too, it fails in a safer mode, where you do more work on backups > than strictly needed. I'm still against this as a mount option > though, way way way too many bullets in the foot gun. And as someone > else said, once you mount with O_NOMTIME, then unmount, then mount > again without O_NOMTIME, you've lost information. Not good. That was me. Zach also pointed out to me that'd mean figuring out where to store that information on-disk for every filesystem you care about. I like the idea of something persistent, but maybe it's more trouble than it's worth--I honestly don't know. --b. -- 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
