On 2015-11-12 17:44:40 +0100, Fabien COELHO wrote:
> >>To fix it, ITSM that it is enough to hold a "do not close lock" on the file
> >>while a flush is in progress (a short time) that would prevent mdclose to do
> >>its stuff.
> >
> >Could you expand a bit more on this? You're suggesting something like a
> >boolean in the vfd struct?
> Basically yes, I'm suggesting a mutex in the vdf struct.

I can't see that being ok. I mean what would that thing even do? VFD
isn't shared between processes, and if we get a smgr flush we have to
apply it, or risk breaking other things.

> >* my laptop, 16 GB Ram, 840 EVO 1TB as storage. With 2GB
> > shared_buffers. Tried checkpoint timeouts from 60 to 300s.
> Hmmm. This is quite short.

Indeed. I'd never do that in a production scenario myself. But
nonetheless it showcases a problem.

> >Well, you can't easily sort bgwriter/backend writes stemming from cache
> >replacement. Unless your access patterns are entirely sequential the
> >data in shared buffers will be laid out in a nearly entirely random
> >order.  We could try sorting the data, but with any reasonable window,
> >for many workloads the likelihood of actually achieving much with that
> >seems low.
> Maybe the sorting could be shared with others so that everybody uses the
> same order?
> That would suggest to have one global sorting of buffers, maybe maintained
> by the checkpointer, which could be used by all processes that need to scan
> the buffers (in file order), instead of scanning them in memory order.

Uh. Cache replacement is based on an approximated LRU, you can't just
remove that without serious regressions.

> >>Hmmm. The shorter the timeout, the more likely the sorting NOT to be
> >>effective
> >
> >You mean, as evidenced by the results, or is that what you'd actually
> >expect?
> What I would expect...

I don't see why then? If you very quickly writes lots of data the OS
will continously flush dirty data to the disk, in which case sorting is
rather important?


Andres Freund

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