Re: [PATCH 0/3] Win32: nanosecond-precision file times

From: Karsten Blees <karsten.blees@gmail.com>
Date: Feb 16, 2015, 20:18 UTC
Message-ID: <54E250A6.4070605@gmail.com>
In-Reply-To: <xmqq61b5obla.fsf@gitster.dls.corp.google.com>

Am 13.02.2015 um 20:28 schrieb Junio C Hamano:

Show 11 quoted lines

> Karsten Blees <karsten.blees@gmail.com> writes:
> 
>> Am 13.02.2015 um 00:38 schrieb Junio C Hamano:
>>>
>>> We do have sec/nsec fields in cache_time structure, so I have
>>> nothing against updating the msysGit port to fill that value.
> 
> Having said that, we do not enable the NSEC stuff by default on Unix
> for a reason.  I'd expect those who know Windows filesystems well to
> pick the default there wisely ;-)
>

Now I'm a bit confused about the discrepancy between racy-git.txt and the Makefile.

Racy-git.txt explains that the nsec-part may be dropped when an inode is flushed to disk if the file system doesn't support nsec resolution. This was supposedly an issue with the Linux kernel fixed back in 2005.

In my understanding, this means that git would see the file as changed and re-check the content (i.e. it will hurt performance).

IOW: Git may be slow if the file system cache has better file time
resolution than the on-disk file system representation.

However, the Makefile has this to say on the subject:

# Define USE_NSEC below if you want git to care about sub-second file mtimes # and ctimes. Note that you need recent glibc (at least 2.2.4) for this, and # it will BREAK YOUR LOCAL DIFFS! show-diff and anything using it will likely # randomly break unless your underlying filesystem supports those sub-second # times (my ext3 doesn't).

Am I missing something? Is there anything in Git that will actually "break" with USE_NSEC if the OS / file system doesn't support it (rather than just being slow)?

History:
* The Makefile comment was added in 2005 (bdd4da59), along with a
  comment in read-cache.c explaining the issue (i.e. flushing to disk
  will clear the nsec field).
* The comment in read-cache.c was removed in 2008 (7a51ed66),
  seemingly dropping USE_NSEC support entirely.
* USE_NSEC support was re-added (without the read-cache.c comment) in
  2009 (fba2f38a).

Regarding the Windows situation: I've just verified (on my Win7 x64 box) that file times obtained through a variety of APIs (GetFileTime, GetFileAttributesEx, GetFileInformationByHandle, FindFirstFile) are consistent and properly rounded to the file system's resolution (e.g. 10ms / 2s for FAT). This is even if the file is still open and I try to SetFileTime() to unrounded values.

So I think enabling USE_NSEC should be fine on Windows.

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