On 04/09/2018 09:45 AM, Robert Haas wrote:
On Mon, Apr 9, 2018 at 8:16 AM, Craig Ringer <cr...@2ndquadrant.com> wrote:
In the mean time, I propose that we fsync() on close() before we age FDs out
of the LRU on backends. Yes, that will hurt throughput and cause stalls, but
we don't seem to have many better options. At least it'll only flush what we
actually wrote to the OS buffers not what we may have in shared_buffers. If
the bgwriter does the same thing, we should be 100% safe from this problem
on 4.13+, and it'd be trivial to make it a GUC much like the fsync or
full_page_writes options that people can turn off if they know the risks /
know their storage is safe / don't care.
I have a really tough time believing this is the right way to solve
the problem.  We suffered for years because of ext3's desire to flush
the entire page cache whenever any single file was fsync()'d, which
was terrible.  Eventually ext4 became the norm, and the problem went
away.  Now we're going to deliberately insert logic to do a very
similar kind of terrible thing because the kernel developers have
decided that fsync() doesn't have to do what it says on the tin?  I
grant that there doesn't seem to be a better option, but I bet we're
going to have a lot of really unhappy users if we do this.

I don't have a better option but whatever we do, it should be an optional
(GUC) change. We have plenty of YEARS of people not noticing this issue and
Robert's correct, if we go back to an era of things like stalls it is going
to look bad on us no matter how we describe the problem.



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