Re: Purpose of wal_init_zero

Hi,

>> I can understand that "the file space has really been allocated", but
>> why do we care about this?
>>
>> [...]
>
> Can you report the benchmark difference with false (disabled)?
> Maybe It's worth considering leaving false as the default.

Good catch. This comment is not 100% clear to me either. I think that
the actual idea of zero-filling *might* be to give the filesystem a
hint to allocate sequential blocks for the file, which may be
beneficial if WAL is stored on HDD (which arguably few people do in
2025).

This GUC was added not a long time ago in 2019, see commit
475861b2615d and discussion [1]. I didn't read the entire thread
(yet?) but it looks like the actual purpose of the GUC is to *disable*
zero-filling on the filesystems with copy-on-write such as ZFS where
it's useless. This is also said in the documentation [2]. Zero-filling
was the default behaviour before the commit.

However the documentation doesn't say why we can't just always use
wal_init_zero = off, and perhaps get rid of this GUC altogether.

In either case the referenced comment is misleading.

[1]: https://postgr.es/m/CACPQ5Fo00QR7LNAcd1ZjgoBi4y97%2BK760YABs0vQHH5dLdkkMA%40mail.gmail.com
[2]: https://www.postgresql.org/docs/current/runtime-config-wal.html
--
Best regards,
Aleksander Alekseev