Re: smgrzeroextend clarification - Mailing list pgsql-hackers

On Wed, May 10, 2023 at 3:20 PM Peter Eisentraut
<peter.eisentraut@enterprisedb.com> wrote:
>
> I was looking at the new smgrzeroextend() function in the smgr API.  The
> documentation isn't very extensive:
>
> /*
>   *  smgrzeroextend() -- Add new zeroed out blocks to a file.
>   *
>   *      Similar to smgrextend(), except the relation can be extended by
>   *      multiple blocks at once and the added blocks will be filled with
>   *      zeroes.
>   */
>
> The documentation of smgrextend() is:
>
> /*
>   *  smgrextend() -- Add a new block to a file.
>   *
>   *      The semantics are nearly the same as smgrwrite(): write at the
>   *      specified position.  However, this is to be used for the case of
>   *      extending a relation (i.e., blocknum is at or beyond the current
>   *      EOF).  Note that we assume writing a block beyond current EOF
>   *      causes intervening file space to become filled with zeroes.
>   */
>
> So if you want to understand what smgrzeroextend() does, you need to
> mentally combine the documentation of three different functions.  Could
> we write documentation for each function that stands on its own?  And
> document the function arguments, like what does blocknum and nblocks mean?

Why not?

> Moreover, the text "except the relation can be extended by multiple
> blocks at once and the added blocks will be filled with zeroes" doesn't
> make much sense as a differentiation, because smgrextend() does that as
> well.

Not exactly. smgrextend() doesn't write a zero-ed block on its own, it
writes the content that's passed to it via 'buffer'. It's just that
some of smgrextend() callers pass in a zero buffer. Whereas,
smgrzeroextend() writes zero-ed blocks on its own, something
smgrextend() called in a loop with zero-ed 'buffer'. Therefore, the
existing wording seems fine to me.

> AFAICT, the differences between smgrextend() and smgrzeroextend() are:
>
> 1. smgrextend() writes a payload block in addition to extending the
> file, smgrzeroextend() just extends the file without writing a payload.

I think how smgrzeroextend() extends the zeroed blocks is internal to
it, and mdzeroextend() happens to use fallocate (if available). I
think the existing wording around smgrzeroextend() seems fine to me.

> 2. smgrzeroextend() uses various techniques (posix_fallocate() etc.) to
> make sure the extended space is actually reserved on disk, smgrextend()
> does not.

It's not smgrzeroextend() per se, it is mdzeroextend() that uses
fallocate() if available.

Overall, +0.5 from me if we want to avoid comment traversals to
understand what these functions do and be more descriptive; we might
end up duplicating comments. But, I'm fine with ""except the relation
can be extended by multiple...." before smgrzeroextend().

--
Bharath Rupireddy
PostgreSQL Contributors Team
RDS Open Source Databases
Amazon Web Services: https://aws.amazon.com