Hi,
On 2019-09-14 14:34:21 -0400, Tom Lane wrote:
> Amit Khandekar <amitdkhan.pg@gmail.com> writes:
> > On Fri, 13 Sep 2019 at 22:01, Robert Haas <robertmhaas@gmail.com> wrote:
> >> On Fri, Sep 13, 2019 at 12:14 PM Tom Lane <tgl@sss.pgh.pa.us> wrote:
> >>> Again, though, the advice that's been given here is that we should
> >>> fix logical decoding to use the VFD API as it stands, not change
> >>> that API. I concur with that.
>
> >> A reasonable position. So I guess logical decoding has to track the
> >> file position itself, but perhaps use the VFD layer for managing FD
> >> pooling.
>
> > Yeah, something like the attached patch. I think this tracking of
> > offsets would have been cleaner if we add in-built support in VFD. But
> > yeah, for bank branches at least, we need to handle it outside of VFD.
> > Or may be we would add it if we find one more use-case.
>
> Again, we had that and removed it, for what seem to me to be solid
> reasons. It adds cycles when we're forced to close/reopen a file,
> and it also adds failure modes that we could do without (ie, failure
> of either the ftell or the lseek, which are particularly nasty because
> they shouldn't happen according to the VFD abstraction). I do not
> think there is going to be any argument strong enough to make us
> put it back, especially not for non-mainstream callers like logical
> decoding.
Yea, I think that's the right call. Avoiding kernel seeks is quite
worthwhile, and we shouldn't undo it just because of this usecase. And
that'll become more and more important performance-wise (and has already
done so, with all the intel fixes making syscalls much slower).
I could see an argument for adding a separate generic layer providing
position tracking ontop of the VFD abstraction however. Seems quite
possible that there's some other parts of the system that could benefit
from using VFDs rather than plain fds. And they'd probably also need the
positional tracking.
Greetings,
Andres Freund
