Re: patch to allow disable of WAL recycling

On 07/17/2018 09:12 PM, Peter Eisentraut wrote:
> On 17.07.18 00:04, Jerry Jelinek wrote:
>> There have been quite a few comments since last week, so at this point I
>> am uncertain how to proceed with this change. I don't think I saw
>> anything concrete in the recent emails that I can act upon.
>
> The outcome of this could be multiple orthogonal patches that affect the
> WAL file allocation behavior somehow. I think your original idea of
> skipping recycling on a COW file system is sound. But I would rather
> frame the option as "preallocating files is obviously useless on a COW
> file system" rather than "this will make things mysteriously faster with
> uncertain trade-offs".
>

Makes sense, I guess. But I think many claims made in this thread are
mostly just assumptions at this point, based on our beliefs how CoW or
non-CoW filesystems work. The results from ZFS (showing positive impact)
are an exception, but that's about it. I'm sure those claims are based
on real-world experience and are likely true, but it'd be good to have
data from a wider range of filesystems / configurations etc. so that we
can give better recommendations to users, for example.

That's something I can help with, assuming we agree on what tests we
want to do. I'd say the usual batter of write-only pgbench tests with
different scales (fits into s_b, fits into RAM, larger then RAM) on
common Linux filesystems (ext4, xfs, btrfs) and zfsonlinux, and
different types of storage would be enough. I don't have any freebsd box
available, unfortunately.

regards

--
Tomas Vondra http://www.2ndQuadrant.com
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