mtime (default) resolution and why is it such?

[Kalin KOZHUHAROV]

Hi all,

There were some discussions on the subversion dev list about "svn and
shell scripts: managing properties" that boiled down to very quick
changes to a file (by sed) without changing its mtime ...

So I started to dig the kernel code to see what exactly is the mtime
resolution...

I could pin it down to include/linux:824

	/* Granuality of c/m/atime in ns.
	   Cannot be worse than a second */
	u32		   s_time_gran;

*** I feel this comment is wrong, should be granuLARity, submitting
*** a patch separately: 
*** [PATCH][TRIVIAL] Fix comments in 2.6.16-rc6: s/granuality/granularity/

But looking further down, I found fs/super.c:88

		s->s_time_gran = 1000000000;

So it seems that the default s_time_gran is 1 second... I was interested
for reiserfs, as that is the main fs I use.

Some filesystems use 1ns (nfs, jfs, xfs...), some 100ns (cifs, ntfs,
smbfs)... 

So I have quite a few questions popping in my head:

1. Is there any particular design concern with this?

2. Would speed performance be drowning if I (later we) play with the
   default, or at least patch reiserfs to use say 0.001s ?

3. What could be affected and how do I measure performance drop?

I understand that is not the root couse for the subversion problem, but
it may help me understand the problem better, so if you can spare a bit
of time to answer my quiestions I'd be gratefull.

Kalin.

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