Re: Improve WALRead() to suck data directly from WAL buffers when possible

On Tue, Mar 07, 2023 at 12:39:13PM +0530, Bharath Rupireddy wrote:
> On Tue, Mar 7, 2023 at 3:30 AM Nathan Bossart <nathandbossart@gmail.com> wrote:
>> Is it possible to memcpy more than a page at a time?
> 
> It would complicate things a lot there; the logic to figure out the
> last page bytes that may or may not fit in the whole page gets
> complicated. Also, the logic to verify each page's header gets
> complicated. We might lose out if we memcpy all the pages at once and
> start verifying each page's header in another loop.

Doesn't the complicated logic you describe already exist to some extent in
the patch?  You are copying a page at a time, which involves calculating
various addresses and byte counts.

>> +    elog(DEBUG1, "read %zu bytes out of %zu bytes from WAL buffers for given LSN %X/%X, Timeline ID %u",
>> +         *read_bytes, count, LSN_FORMAT_ARGS(startptr), tli);
>>
>> I definitely don't think we should put an elog() in this code path.
>> Perhaps this should be guarded behind WAL_DEBUG.
> 
> Placing it behind WAL_DEBUG doesn't help users/developers. My
> intention was to let users know that the WAL read hit the buffers,
> it'll help them report if any issue occurs and also help developers to
> debug that issue.

I still think an elog() is mighty expensive for this code path, even when
it doesn't actually produce any messages.  And when it does, I think it has
the potential to be incredibly noisy.

-- 
Nathan Bossart
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