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..
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 Fri, Apr 26, 2024 at 4:37 AM Riku Iki <riku.iki.x@gmail.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.