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

Hi,

On 2023-01-27 14:24:51 +0900, Masahiko Sawada wrote:
> If I'm understanding this result correctly, it seems to me that your
> patch works well with the WAL DIO patch (WALDIO vs. WAL DIO & WAL
> BUFFERS READ), but there seems no visible performance gain with only
> your patch (HEAD vs. WAL BUFFERS READ). So it seems to me that your
> patch should be included in the WAL DIO patch rather than applying it
> alone. Am I missing something?

We already support using DIO for WAL - it's just restricted in a way that
makes it practically not usable. And the reason for that is precisely that
walsenders need to read the WAL. See get_sync_bit():

/*
* Optimize writes by bypassing kernel cache with O_DIRECT when using
* O_SYNC and O_DSYNC. But only if archiving and streaming are disabled,
* otherwise the archive command or walsender process will read the WAL
* soon after writing it, which is guaranteed to cause a physical read if
* we bypassed the kernel cache. We also skip the
* posix_fadvise(POSIX_FADV_DONTNEED) call in XLogFileClose() for the same
* reason.
*
* Never use O_DIRECT in walreceiver process for similar reasons; the WAL
* written by walreceiver is normally read by the startup process soon
* after it's written. Also, walreceiver performs unaligned writes, which
* don't work with O_DIRECT, so it is required for correctness too.
*/
if (!XLogIsNeeded() && !AmWalReceiverProcess())
o_direct_flag = PG_O_DIRECT;

Even if that weren't the case, splitting up bigger commits in incrementally
committable chunks is a good idea.

Greetings,

Andres Freund