Re: what's the current wisdom on git over NFS/CIFS?

[Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>]

On Fri, 3 Jul 2009, Johannes Sixt wrote:
> 
> Do we ever call fsync on a pipe? In this case, fsync should fail with
> EINVAL, but your implementation would wait until the reader has drained
> all data.

I'm pretty sure we never call fsync() on anything but a file. So I suspect 
the patch for windows is fine - it only needs to care about regular files.

That said, there's a reason we don't enable fsync by default - it's 
generally not needed. I'm not sure what the NTFS crash semantics are, but 
I _think_ NTFS does everything with a proper log, and then fsync probably 
doesn't matter on windows.

The "fsync on CIFS" was not about "windows is crap, and because it's a 
windows filesystem we need to fsync" - it was literally because there was 
a bug in the Linux CIFS implementation that meant that pending writes 
could be lost if the file was made read-only while they were still 
pending.

And on non-journaling filesystems, fsync is obviously a good idea, but 
even then the state is often recoverable even in the face of a crash. Now, 
without fsync, you might have to work at it a bit and know what to do - 
like _manually_ resetting to the previous state and re-doing the last 
corrupt commit, throwing out any corrupt objects etc.

So enabling fsync can make things _much_ easier for recovery (to the point 
of making it possible at all for somebody who doesn't know the git object 
model), but is generally not a hard requirement.

Giving windows git the capability to do it sounds like a good idea, 
though.

Keep in mind: enabling fsync _will_ make object creation slower. Most 
people probably won't care much, though. It really only matters under some 
fairly rare circumstances - I care, beause it matters for things like 
"apply 200 patches in one go as fast as you can", but for most 'normal' 
workflows you'd probably never even notice.

			Linus
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