On Thu, Jan 5, 2017 at 12:33 AM, Robert Haas <robertmh...@gmail.com> wrote:
> On Wed, Jan 4, 2017 at 9:47 AM, Simon Riggs <si...@2ndquadrant.com> wrote:
>> On 4 January 2017 at 13:57, Robert Haas <robertmh...@gmail.com> wrote:
>>> On Wed, Jan 4, 2017 at 3:05 AM, Simon Riggs <si...@2ndquadrant.com> wrote:
>>>> Strange response. Nothing has been assumed. I asked for tests and you
>>>> provided measurements.
>>> Sure, of zero-filling a file with dd.  But I also pointed out that in
>>> a real PostgreSQL cluster, the change could actually *reduce* latency.
>> I think we are talking at cross purposes. We agree that the main
>> change is useful, but it causes another problem which I can't see how
>> you can characterize as reduced latency, based upon your own
>> measurements.
> Zero-filling files will take longer if the files are bigger.  That
> will increase latency.  But we will also have fewer forced
> end-of-segment syncs.  That will reduce latency.  Which effect is
> bigger?

It depends on if the environment is CPU-bounded or I/O bounded. If CPU
is at its limit, zero-filling takes time. If that's the I/O, fsync()
would take longer to complete.

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