Re: PostgreSQL's handling of fsync() errors is unsafe and risks data loss at least on XFS

On Fri, Mar 30, 2018 at 10:18:14AM +1300, Thomas Munro wrote:

> >> Yeah, I see why you want to PANIC.
> >
> > Indeed. Even doing that leaves question marks about all the kernel
> > versions before v4.13, which at this point is pretty much everything
> > out there, not even detecting this reliably. This is messy.

There may still be a way to reliably detect this on older kernel
versions from userspace, but it will be messy whatsoever. On EIO
errors, the kernel will not restore the dirty page flags, but it
will flip the error flags on the failed pages. One could mmap()
the file in question, obtain the PFNs (via /proc/pid/pagemap)
and enumerate those to match the ones with the error flag switched
on (via /proc/kpageflags). This could serve at least as a detection
mechanism, but one could also further use this info to logically
map the pages that failed IO back to the original file offsets,
and potentially retry IO just for those file ranges that cover
the failed pages. Just an idea, not tested.

Best regards,
Anthony