Re: shared_buffers smaller than max_wal_size

Thanks for your response. Ss I understand it now the difference is that
checkpoints are synchronous but dirty pages eviction from shared buffers
are asynchronous, correct? How then Postgres ensures that OS writes data to
the disk so WAL can be deleted?

>WAL writes are asynchronous.

Is there a typo? If not then what data is written synchronously?

On Sat, Sep 23, 2017 at 6:01 PM, Tomas Vondra <tomas(dot)vondra(at)2ndquadrant(dot)com>
wrote:

> Hi,
>
> On 09/23/2017 08:18 AM, Vladimir Mihailenco wrote:
> > Hi,
> >
> > I wonder what is the point of setting max WAL size bigger than shared
> > buffers, e.g.
> >
> > shared_buffers = 512mb
> > max_wal_size = 2gb
> >
> > As I understand a checkpoint happens after 2gb of data were modified
> > (writter to WAL), but shared buffers can contain at most 512mb of dirty
> > pages to be flushed to the disk. Is it still a win or I am missing
> > something?
>
> Those are mostly unrelated things.
>
> max_wal_size determines how often you'll do checkpoints. So with a lot
> of writes you probably need high max_wal_size, otherwise you'll do
> checkpoints very often. Choose reasonable checkpoint_timeout and set
> max_wal_size based on that.
>
> Shared buffers are mostly about caching data accessed by queries. If you
> can squeeze the frequently accessed data into shared buffers (high cache
> hit ratio), great.
>
> Moreover, there's very little relation between max_wal_size and
> shared_buffers, for a number of reasons:
>
> 1) You can modify the same 8kB page repeatedly - it will still be just
> 8kB of dirty data in shared buffers, but each update will generate a
> little bit of WAL data. In an extreme case a single 8kB page might be
> responsible for most of the 2GB of WAL data.
>
> 2) When changing the data page, we only really write the minimum amount
> of data describing the change into WAL. So it's not 1:1.
>
> 3) When a page is evicted from shared buffers, we don't fsync it to disk
> immeditely. We write it out to page cache, and leave the eviction to the
> OS (with some exceptions), so it's asynchronous. WAL writes are
> asynchronous.
>
> 4) Shared buffers are not just about dirty data, it's also about caching
> reads. No one knows what is the read:write ratio, what part of the
> database will receive writes, etc.
>
>
> So there's nothing inherently wrong with (shared_buffers > max_wal_size)
> or (shared_buffers > max_wal_size), it depends on your workload.
>
> regards
>
> --
> Tomas Vondra http://www.2ndQuadrant.com
> PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services
>