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Date: Fri, 22 Jul 2011 19:13:29 -0600 From: Andreas Dilger <adilger@...ger.ca> To: NeilBrown <neilb@...e.de> Cc: "J. Bruce Fields" <bfields@...ldses.org>, Andi Kleen <andi@...stfloor.org>, Matt Mackall <mpm@...enic.com>, linux-fsdevel <linux-fsdevel@...r.kernel.org>, Linux Kernel Mailing List <linux-kernel@...r.kernel.org> Subject: Re: Nanosecond fs timestamp support: sad As an FYI, Lustre uses i_version to store the transaction number in which a file changed. It sets the i_version itself. If NFSv4 were to set i_version when it needs to transition the state of a file then it wouldn't cause overhead on filesystems that are not being used for NFS export. I don't think timestamps can ever be completely safe for distributed state management, unless the kernel bends the rules on what a timestamp IS, e.g. by never reverting the ctime when the clock moves backward and such. Cheers, Andreas On 2011-07-22, at 4:59 PM, NeilBrown <neilb@...e.de> wrote: > On Fri, 22 Jul 2011 18:31:58 -0400 "J. Bruce Fields" <bfields@...ldses.org> > 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? > > 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? > > NeilBrown > -- > To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in > the body of a message to majordomo@...r.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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