Re: We really ought to do something about O_DIRECT and data=journalled on ext4

Greg Smith <greg@2ndquadrant.com> writes:
> Regardless, I'm now leaning heavily toward the idea of avoiding 
> open_datasync by default given this bug, and backpatching that change to 
> at least 8.4.  I'll do some more database-level performance tests here 
> just as a final sanity check on that.  My gut feel is now that we'll 
> eventually be taking something like Marti's patch, adding some more 
> documentation around it, and applying that to HEAD as well as some 
> number of back branches.

I think we have got consensus that (1) open_datasync should not be the
default on Linux, and (2) this change needs to be back-patched.  What
is not clear to me is whether we have consensus to change the option
preference order globally, or restrict the change to just be effective
on Linux.  The various testing that's been reported so far is all for
Linux and thus doesn't directly address the question of whether other
kernels will have similar performance properties.  However, it seems
reasonable to me to suppose that open_datasync could only be a win in
very restricted scenarios and thus shouldn't be a preferred default.
Also, I dread trying to document the behavior if the preference order
becomes platform-dependent.

With the holidays fast approaching, our window to do something about
this in a timely fashion grows short.  If we don't schedule update
releases to be made this week, I think we're looking at not getting the
updates out till after New Year's.  Do we want to wait that long?  Is
anyone actually planning to do performance testing that would prove
anything about non-Linux platforms?
        regards, tom lane