Re: Streaming base backups

On Tue, Jan 11, 2011 at 11:39:20AM -0500, Cédric Villemain wrote:
> 2011/1/11 Garick Hamlin <ghamlin(at)isc(dot)upenn(dot)edu>:
> > On Mon, Jan 10, 2011 at 09:09:28AM -0500, Magnus Hagander wrote:
> >> On Sun, Jan 9, 2011 at 23:33, Cédric Villemain
> >> <cedric(dot)villemain(dot)debian(at)gmail(dot)com> wrote:
> >> > 2011/1/7 Magnus Hagander <magnus(at)hagander(dot)net>:
> >> >> On Fri, Jan 7, 2011 at 01:47, Cédric Villemain
> >> >> <cedric(dot)villemain(dot)debian(at)gmail(dot)com> wrote:
> >> >>> 2011/1/5 Magnus Hagander <magnus(at)hagander(dot)net>:
> >> >>>> On Wed, Jan 5, 2011 at 22:58, Dimitri Fontaine <dimitri(at)2ndquadrant(dot)fr> wrote:
> >> >>>>> Magnus Hagander <magnus(at)hagander(dot)net> writes:
> >> >>>>>> * Stefan mentiond it might be useful to put some
> >> >>>>>> posix_fadvise(POSIX_FADV_DONTNEED)
> >> >>>>>>   in the process that streams all the files out. Seems useful, as long as that
> >> >>>>>>   doesn't kick them out of the cache *completely*, for other backends as well.
> >> >>>>>>   Do we know if that is the case?
> >> >>>>>
> >> >>>>> Maybe have a look at pgfincore to only tag DONTNEED for blocks that are
> >> >>>>> not already in SHM?
> >> >>>>
> >> >>>> I think that's way more complex than we want to go here.
> >> >>>>
> >> >>>
> >> >>> DONTNEED will remove the block from OS buffer everytime.
> >> >>
> >> >> Then we definitely don't want to use it - because some other backend
> >> >> might well want the file. Better leave it up to the standard logic in
> >> >> the kernel.
> >> >
> >> > Looking at the patch, it is (very) easy to add the support for that in
> >> > basebackup.c
> >> > That supposed allowing mincore(), so mmap(), and so probably switch
> >> > the fopen() to an open() (or add an open() just for mmap
> >> > requirement...)
> >> >
> >> > Let's go ?
> >>
> >> Per above, I still don't think we *should* do this. We don't want to
> >> kick things out of the cache underneath other backends, and since we
> >> can't control that. Either way, it shouldn't happen in the beginning,
> >> and if it does, should be backed with proper benchmarks.
> >
> > Another option that occurs to me is an option to use direct IO (or another
> > means as needed) to bypass the cache.  So rather than kicking it out of
> > the cache, we attempt just not to pollute the cache by bypassing it for cold
> > pages and use either normal io for 'hot pages', or use a 'read()' to "heat"
> > the cache afterward.
>
> AFAIR, even Linus is rejecting the idea to use it seriously, except if
> I shuffle in my memory.

Direct IO is generally a pain.

POSIX_FADV_NOREUSE is an alternative (I think). Realistically I wasn't sure which
way(s) actually worked. My gut was that direct io would likely work right on Linux
and Solaris, at least. If POSIX_FADV_NOREUSE works than maybe that is the answer
instead, but I haven't tested either.

Garick

>
> >
> > Garick
> >
> >>
> >> I've committed the backend side of this, without that. Still working
> >> on the client, and on cleaning up Heikki's patch for grammar/parser
> >> support.
> >>
> >> --
> >>  Magnus Hagander
> >>  Me: http://www.hagander.net/
> >>  Work: http://www.redpill-linpro.com/
> >>
> >> --
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>
>
>
> --
> Cédric Villemain               2ndQuadrant
> http://2ndQuadrant.fr/     PostgreSQL : Expertise, Formation et Support