On Sun, 2015-12-13 at 07:10 +0000, Duncan wrote: > > So you basically mean that ro snapshots won't have their atime > > updated > > even without noatime? > > Well I guess that was anyway the recent behaviour of Linux > > filesystems, > > and only very old UNIX systems updated the atime even when the fs > > was > > set ro. > > I'd test it to be sure before relying on it (keeping in mind that my > own > use-case doesn't include subvolumes/snapshots so it's quite possible > I > could get fine details of this nature wrong), but that would be my > very > (_very_! see next) strong assumption, yes. > > Because read-only snapshots are used for btrfs-send among other > things, > with the idea being that the read-only will keep them from changing > in > the middle of the send, and ro snapshot atime updates would seem to > throw > that entirely out the window. So I can't imagine ro snapshots doing > atime > updates under any circumstance because I just can't see how send > could > rely on them then, but I'd still test it before counting on it.
For those who haven't followed up the other threads: I've tried it out and yes, ro-snapshots (as well as ro mounted btrfs filesystem/subvolumes) don't have their atimes changed on e.g. read. > AFAIK, the general idea was to eventually have all the (possible, > some > are global-filesystem-scope) subvolume mount options exposed as > properties, it's just not implemented yet, but I'm not entirely sure > if > that was all /btrfs-specific/ mount options, or included the generic > ones > such as the *atime and no* (noexec/nodev/...) options as well. In > view > of that and the fact that noatime is generic, adding it as a specific > request still makes sense. Someone with more specific knowledge on > the > current plan can remove it if it's already covered. Not sure if I'd had already posted that here, but I did write some of these ideas up and added it to the wiki: https://btrfs.wiki.kernel.org/index.php?title=Project_ideas&action=historysubmit&diff=29757&oldid=29743 Best wishes, Chris.
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