Re: [RFC][PATCH] make file's timestamp more accurate

[john stultz <johnstul@us.ibm.com>]

On Tue, 2010-08-31 at 17:42 +0900, Satoru Takeuchi wrote:
> linux has supported nanosecond order file's timestamp since 2.5.48.
> However current file timestamp is got by current_fs_time() and
> is only updated once a tick. It can't say true nanosecond accuracy.
> In addition, gettimeofday() before a file operation updating
> {a,c,m}time would outstrip file's timestamp because of the difference
> about time source between gettimeofday() and file's timestamp.
> A certain kind of application would corrupted by this problem.

Applications mixing gettimeofday and filesystem timesamps can currently
use clock_gettime(CLOCK_REALTIME_COARSE,...) - which returns tick
granular timestamps, the same as the filesystem timestamps - method to
avoid this issue.

However, Patrick LoPresti (cc'ed) was working on a similar issue here
connected to nfs.  

> I attached a most simple patch fixing this problem here. However
> it has several problems and I don't say it can be applied as is.
> The most big two problems is the following:
> 
>  - It would cause performance regression, especially in
>    not TSC capable system.
>  - Is gettimeofday()'s monotonicity reliable on all systems?

It *should* be. But hardware issues can cause trouble here.

> The relative discussion:
> http://lkml.org/lkml/2010/7/13/443
> 
> Does anybody have good idea? Should it be tunable, for example?

I think the discussion from earlier suggested that this be configurable
from a mount option so the performance/granularity trade-off can be
managed there. 

Potential pot-holes on the road here: Although I guess doing this on a
per-mount basis in the future could make it difficult for apps that use
CLOCK_REALTIME_COARSE to function if fs granularity is increased. Some
sort of CLOCK_REALTIME_FS could be introduced to map to whichever
granularity is right, but that can only be done on a global basis.
Hrm...

thanks
-john