Preallocation changes in Postgresql 16

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.

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.munro@gmail.com>
wrote:

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.

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.iki.x@gmail.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.munro@gmail.com> wrote:

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.