On Sat, Jul 23, 2011 at 08:59:15AM +1000, NeilBrown wrote: > On Fri, 22 Jul 2011 18:31:58 -0400 "J. Bruce Fields" <bfields@xxxxxxxxxxxx> > wrote: > > > On Fri, Jul 22, 2011 at 06:10:39PM -0400, bfields wrote: > > > On Fri, Jul 22, 2011 at 11:47:32PM +0200, Andi Kleen wrote: > > > > On Fri, Jul 22, 2011 at 04:11:42PM -0500, Matt Mackall wrote: > > > > > On Fri, 2011-07-22 at 22:59 +0200, Andi Kleen wrote: > > > > > > > Indeed. Only usefully exists on ext4 and requires extra system calls. > > > > > > > > > > > > Not sure what you mean? It's in stat(2), just like the timestamps. > > > > > > > > > > I don't see anything that looks like a version or generation number in > > > > > either the man pages, the asm-generic/stat.h, or glibc's asm/stat.h. > > > > > Pointer? > > > > > > > > Hmm you're right. I thought it was in there, but apparently not. > > > > I think it should be added there though. We still have some unused > > > > fields. > > > > > > But last I checked I thought it was only ext4 that actually incremented > > > the i_version on IO, and even then only when given a (non-default) mount > > > option. > > > > > > My notes on what needs to be done there: > > > > > > - collect data to determine whether turning on i_version causes > > > any significant performance regressions. > > > - Last I talked to him, Ted Tso recommended running > > > Bonnie on a local disk, since it does a lot of little > > > writes, which is somewhat of a worst case, as it will > > > generate extra metadata updates for each write. > > > Compare total wall-clock time, number of iops, and > > > number of bytes (using some kind of block tracing). > > > - If there aren't any problems, turn it on by default, and we're > > > done. > > > > (Well,and talk the other filesystem implementors into doing it.) > > > > But does anyone apart from NFSv4 actually *want* i_version as opposed to the > more-generally-useful precise timestamps? It *seems* like a generally useful idea, but I don't know of any other users. > If not, we probably should tell NFSv4 to use timestamps and focus on making > them work well. > ?? Well, sure, I couldn't complain about that if that proved possible. --b. > > The timestamp used doesn't need to update ever nanosecond. I think if it > were just updated on every userspace->kernel transition (or effective > equivalents inside kernel threads) that would be enough capture all > causality. I wonder how that would be achieved.. I wonder if RCU machinery > could help - doesn't it keep track of when threads schedule ... or something? > > NeilBrown -- 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
