On Thu, Jul 12, 2018 at 07:32:19AM +0000, Petr Pisar wrote:
> On 2018-07-11, Zbigniew Jędrzejewski-Szmek <zbys...@in.waw.pl> wrote:
> > The effects of fsync are impossible to see unless you hard-reboot the
> > machine.
> 
> Are you sure non-fsynced changes are are guaranteed to be visible on
> block cache level? E.g. if you mix read/write and mmaped I/O from
> different processes?

Block cache — no, I don't think so. But do we have packages that do
anything like this during build? It'd require low-level fs support
and would be probably pretty fragile anyway.

> > I wonder if it wouldn't be more robust to use nspawn's syscall filter
> > to filter the fsync calls.
> 
> Can the syscall filter fake a success of the syscall return value?
> Correctly written applications check fsync() return value and forward
> the error.

It can, e.g. something like system-nspawn --system-call-filter='~sync:0 fsync:0'
should be a good start.

Zbyszek
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