Re: [Patch] Make block and file size for WAL and relations defined at cluster creation

Hi,

On 2018-01-03 21:43:51 +0100, Remi Colinet wrote:
> - we may test different combinations of file and block sizes, for the
> relation and the WAL in order to have the better performances of the server.
> Avoiding a compilation for each combination of values seems to make sense.

That's something you need to proof to beneficial *before* we make this
change.

> - Selecting the correct values for file and block sizes is a DBA task, and
> not a developer task.
> For instance, when someone wants to create a Linux filesystem with a given
> block size, he is not forced to accept a given value chosed by the
> developer of the filesystem driver when this later was compiled.

I'm unconvinced there's as much value syncing up fs in pg as some
conventional wisdom says.

> - The file and block sizes should depend mostly of the physical server and
> physical storage.
> Not of the database software itself.

Citation needed.

> Regarding the cost of using run-time configurable values for file and block
> sizes of the WAL and relations, this cost is low both :
>
> - from a developer point of view: the source code changes are spread in
> many files but only a few one have significant changes.
> Mainly the tidbitmap.c is concerned the change. Other changes are minor
> changes.
>
> - from a run-time point of view. The overhead is only at the start of the
> database instance.
> And moreover, the overhead is still very low at the start of the server,
> with only a few more dynamic memory allocations.

That's to some degree because you rely on stack allocation of variable
sided amounts of data - we can't rely on that. E.g. you allocate stack
variables sized by rel_block_size, that's unfortunately not
ok. Additionally some of the size calculations will have some
performance impact.

- Andres