Re: [RFC PATCH 1/2] sha1-file: fsync() loose dir entry when core.fsyncObjectFiles

On Thu, Sep 17 2020, Christoph Hellwig wrote:

On Thu, Sep 17, 2020 at 01:28:29PM +0200, Ævar Arnfjörð Bjarmason wrote:

quoted

Change the behavior of core.fsyncObjectFiles to also sync the
directory entry. I don't have a case where this broke, just going by
paranoia and the fsync(2) manual page's guarantees about its behavior.

It is not just paranoia, but indeed what is required from the standards
POV.  At least for many Linux file systems your second fsync will be
very cheap (basically a NULL syscall) as the log has alredy been forced
all the way by the first one, but you can't rely on that.

Acked-by: Christoph Hellwig <hch@lst.de>

Thanks a lot for your advice in this thread.

Can you (or someone else) suggest a Linux fs setup that's as unforgiving
as possible vis-a-vis fsync() for testing? I'd like to hack on making
git better at this, but one of the problems of testing it is that modern
filesystems generally do a pretty good job of not losing your data.

So something like ext4's commit=N is an obvious start, but for git's own
test suite it would be ideal to have process A write file X, and then
have process B try to read it and just not see it if X hadn't been
fsynced (or not see its directory if that hadn't been synced).

It would turn our test suite into pretty much a 100% failure, but one
that could then be fixed by fixing the relevant file writing code.