On Fri, 2011-07-22 at 21:38 -0400, J. Bruce Fields wrote: > On Fri, Jul 22, 2011 at 07:07:41PM -0500, Matt Mackall wrote: > > On Sat, 2011-07-23 at 08:59 +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? > > > > In theory, a microsecond timestamp (ie gtod) may already not be good > > enough for all applications. But i_version also doesn't allow comparing > > across files. > > > > > If not, we probably should tell NFSv4 to use timestamps and focus on making > > > them work well. > > > ?? > > > > > > 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? > > > > Sort of. > > > > Some observations: > > > > - we only need to go to higher resolution when two events happen in the > > same time quantum > > - this applies at both the level of seconds and jiffies > > - if the only file touched in a given quantum gets touched ago, we don't > > need to update its timestamp if stat wasn't also called on it in this > > quantum > > - we never need to use a higher resolution than the global > > min(s_time_gran) > > Right, so there was a rough algorithm hashed out somewhere around here: > > http://thread.gmane.org/gmane.linux.kernel/1022866/focus=1024624 > > that depended on those observations. > > NFS presents a worst-case as the standard NFSv3 read and write > operations include timestamps in the result. So every single IO comes > with a stat. So either you have a clock good enough to give a distinct > timestamp for all of those, or you fall back on a global counter that > ends up serializing all IO. I think. I admit I'm not sure I understand > your proposal below. ...or you admit that NFSv3 is no longer able to keep up with modern processing speeds and storage, and you ditch it in favour of NFSv4. Time-stamps are _not_ the optimal way to label changes in a clustered environment (or even a multi-cpu/multi-core environment): aside from the various issues involving absolute time vs. wall clock time, you also have to deal with clock synchronisation across those nodes/cpus/cores at the < microsecond resolution level. Have fun doing that... Trond -- 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
