On Fri, Jan 29, 2016 at 6:43 PM, Sage Weil <sage@xxxxxxxxxxxx> wrote: > On Fri, 29 Jan 2016, Guang Yang wrote: >> Hi Sage, >> I found a thread discussing whether we could turn on disk cache with >> new kernel or not - >> http://www.spinics.net/lists/ceph-devel/msg06173.html >> >> From within that thread, you mentioned that since the fsync/fdatasync >> would flush the disk cache as well, we could actually turn on the >> cache without worrying about the data safety. > > Yeah, any vaguely modern kernel will do a proper flush to the device; no > need to worry about the disk cache. > >> Is that true if journal_dio is being used (by default), in which case >> O_DIRECT | O_DSYNC is being used to write journal rather than >> fsync(2)? > > Right. O_DSYNC triggers a flush at the SCSI (or virtio) level, so these > journal [A]IOs are fully synchronous all the way to the platter. The last time I looked, O_DSYNC was nothing but an implicit fdatasync after each write, triggered at the VFS layer - not that much different from the explicit fdatasync. Thanks, Ilya -- To unsubscribe from this list: send the line "unsubscribe ceph-devel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
