On Tue, 12 May 2015, 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). Interesting! I didn't realize there was already a chattr +A that disabled atime (although I suspect it doesn't do the "right now" for stat thing). This makes the nomtime-ness a bit more obscure (I don't think most users would think to check these file attributes), but it's a safer failure condition for backups at least. The fact that chattr +A (and hopefully +M) will work for non-root is a bonus, as we're also trying to get ceph daemons to drop most privileges. sage -- 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
