On Mon, May 14, 2012 at 02:48:02PM -0400, Jeff Layton wrote: > On Mon, 14 May 2012 14:33:17 -0400 > Josef Bacik <josef@xxxxxxxxxx> 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. 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, > > > > Well, you do incur a bit more overhead in btrfs too: > > ------------------[snip]---------------------- > 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; > ------------------[snip]---------------------- > > So you'll end up with sync_it being 0 if i_version updates are > disabled, and the mtime/ctime didn't visibly change. > > If your jiffies are coarse-grained enough, then you might get "lucky" > rather often, but is that a case worth optimizing for? How often does > it happen that you mark the inode dirty, flush it to disk and then > re-mark it dirty within the same jiffy? > Well sync_it just means do we need to call mark_inode_dirty, not necessarily do we need to write it to disk, so you are just updating a field in memory. Now if your jiffies are coarse enough for you to not notice the ctime update then yes you are incurring an extra lock/inc/unlock, but this is called in the write path where you are going to do much more latency inducting operations than locking and unlocking a generally uncontended spin lock. 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
