On Mon, May 14, 2012 at 02:54:00PM -0400, J. Bruce Fields wrote: > On Mon, May 14, 2012 at 02:33:17PM -0400, Josef Bacik wrote: > > On Mon, May 14, 2012 at 01:58:22PM -0400, Ted Ts'o wrote: > > > On Mon, May 14, 2012 at 11:27:42AM -0600, Andreas Dilger wrote: > > > > > And if it at all possible I'd rather have it be something that Just > > > > > Works rather than something that requires extra configuration. > > > > > > > > Sure, but this is only useful for NFSv4, but costs everyone using > > > > ext4 continuous overhead, so it isn't a clear-cut case to enable > > > > the version just on the thought that NFS might one day be used on > > > > any particular filesystem. > > > > > > It's not a matter of "NFSv4 might one day be used"; if we don't turn > > > on i_version updates until the file system is actually exported via > > > NFSv4, there would be no deleterious effects. > > > > > > I always thought that was going to be the plan; that there would be > > > some flag that would be set in struct super_block when the file system > > > was exported that would enable i_version updates. > > > > > > That way we satisfy the "no extra configuration" needed requirement, > > > which I agree is ideal, but we also don't waste any CPU overhead if > > > the file system is not exported via NFSv4. I tried to implement > > > anything along these lines because I don't care enough, and I don't > > > use NFSv4 personally.... > > > > > > > Seems like this is just a bad place to be doing inode_inc_iversion(). If > > MS_IVERSION is set we will update iversion in file_update_time() and then call > > mark_inode_dirty which will jack up the iversion again. > > Agreed, that's weird. > > > In btrfs we just change > > it wherever we change ctime and that way you don't really notice the extra > > overhead since you are doing it in paths where you are changing a bunch of stuff > > in the inode already, and mostly where you hold the i_mutex so you aren't going > > to be hitting any contention on the i_lock. Thanks, > > I don't think they're worried about the inode_inc_iversion() calls > themselves, but the behavior of file_update_time(): > > if (!timespec_equal(&inode->i_mtime, &now)) > sync_it = S_MTIME; > > if (!timespec_equal(&inode->i_ctime, &now)) > sync_it |= S_CTIME; > > if (IS_I_VERSION(inode)) > sync_it |= S_VERSION; > > if (!sync_it) > return; > ... > mark_inode_dirty_sync(inode); > > So now mark_inode_dirty_sync() is called on every update, instead of > merely on every update that sees a time change (so at most once a > jiffy). > > So mark_inode_dirty_sync (and hence ->dirty_inode = ext4_dirty_inode) > may get called more often if you're writing very frequently. > > I'm a bit surprised that's expected to add significant overhead to the > write. > > I guess I should stare at the code and try to follow Andreas's > explanation.... > It shouldn't, let's be honest, most systems aren't going to have such a coarse jiffie counter that they'll be able to get away with doing 2 calls to write() or ->page_mkwrite() in the same jiffie and skip the update to mtime/ctime anyway. If they do they are damned lucky, and again the amount of overhead added even if they are should be negligible since 99% of us all incur the overhead from having to update mtime/ctime anyway. Thanks, Josef -- 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
