Re: Preallocation changes in Postgresql 16

Hello,

It seems that I am running into this issue as well.
Is it likely that this would ever be a config option?

Best,
Pierre Barre

On Fri, May 3, 2024, at 05:11, Riku Iki wrote:
> I did the testing and confirmed that this was the issue.
>
> I run following query:
>
> create table t as select '1234567890' from generate_series(1, 1000000000);
>
> I commented if (numblocks > 8) codeblock, and see the following results from "compsize /dbdir/" command.
>
>
> Before my changes:
>
> Processed 1381 files, 90007 regular extents (90010 refs), 15 inline.
> Type Perc Disk Usage Uncompressed Referenced
> TOTAL 97% 41G 42G 42G
> none 100% 41G 41G 41G
> zstd 14% 157M 1.0G 1.0G
> prealloc 100% 16M 16M 16M
>
>
>
> After the changes:
>
> Processed 1381 files, 347328 regular extents (347331 refs), 15 inline.
> Type Perc Disk Usage Uncompressed Referenced
> TOTAL 3% 1.4G 42G 42G
> none 100% 80K 80K 80K
> zstd 3% 1.4G 42G 42G
>
> It is clearly visible that files created with fallocate are not compressed, and disk usage is much larger.
> I am wondering if there is a way to have some feature request to have this parameter user configurable..
>
> On Fri, Apr 26, 2024 at 4:15 PM Riku Iki <riku(dot)iki(dot)x(at)gmail(dot)com> wrote:
>> Thank you, I have such a system. I think my task would be to compile PG from sources(need to learn this), and see how it works with and without that code block.
>>
>> On Thu, Apr 25, 2024 at 2:25 PM Thomas Munro <thomas(dot)munro(at)gmail(dot)com> wrote:
>>> On Fri, Apr 26, 2024 at 4:37 AM Riku Iki <riku(dot)iki(dot)x(at)gmail(dot)com> wrote:
>>> > I am wondering if there were preallocation related changes in PG16, and if it is possible to disable preallocation in PostgreSQL 16?
>>>
>>> I have no opinion on the btrfs details, but I was wondering if someone
>>> might show up with a system that doesn't like that change. Here is a
>>> magic 8, tuned on "some filesystems":
>>>
>>> /*
>>> * If available and useful, use posix_fallocate() (via
>>> * FileFallocate()) to extend the relation. That's often more
>>> * efficient than using write(), as it commonly won't cause the kernel
>>> * to allocate page cache space for the extended pages.
>>> *
>>> * However, we don't use FileFallocate() for small extensions, as it
>>> * defeats delayed allocation on some filesystems. Not clear where
>>> * that decision should be made though? For now just use a cutoff of
>>> * 8, anything between 4 and 8 worked OK in some local testing.
>>> */
>>> if (numblocks > 8)
>>>
>>> I wonder if it wants to be a GUC.