Re: [patch] pg_copy - a command for reliable WAL archiving

Andres Freund wrote:
> On 2014-08-19 17:42:06 -0400, Alvaro Herrera wrote:
> > MauMau wrote:
> >
> > > With that said, copying to a temporary file like <dest>.tmp and
> > > renaming it to <dest> sounds worthwhile even as a basic copy
> > > utility. I want to avoid copying to a temporary file with a fixed
> > > name like _copy.tmp, because some advanced utility may want to run
> > > multiple instances of pg_copy to copy several files into the same
> > > directory simultaneously. However, I'm afraid multiple <dest>.tmp
> > > files might continue to occupy disk space after canceling copy or
> > > power failure in some use cases, where the copy of the same file
> > > won't be retried. That's also the reason why I chose to not use a
> > > temporary file like cp/copy.
> >
> > Is there a way to create a link to a file which only exists as an open
> > file descriptor? If there was, you could create a temp file, open an
> > fd, then delete the file. That would remove the issue with files being
> > leaked due to failures of various kinds.
>
> Isn't this a solution looking for a problem? We're using tempfiles in
> dozens of other places and I really don't see why this is the place to
> stop doing so. Just copy to <dest>.tmp and move it into place. If things
> crash during that, the command will be repeated shortly afterwards again
> *anyway*. Let's not get into platform specific games here.

The issue is what happens if there's a crash while the temp file is in
the middle of being filled. I agree there are bigger problems to solve
in this area though. Yes, I agree that a fixed name such as _copy.tmp
is a really bad choice for several reasons.

--
Álvaro Herrera http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Training & Services