Re: Fsync (flush) all inserted WAL records

On Wednesday, August 07, 2024 16:55 MSK, Aleksander Alekseev <aleksander(at)timescale(dot)com> wrote:
 
Perhaps you could give more context on the use cases for this
function? The value of it is not quite clear. What people typically
need is making sure if a given LSN was fsync'ed and/or replicated
and/or applied on a replica. Your case(s) however is different and I
don't fully understand it.
I use asynchronous commit (without XLogFlush/fsync at commit). At some moment I would like to XLogFlush (fsync) all already asynchronously committed transactions (inserted but not flushed/fsynced yet WAL records). Assume, that there is no any active transactions at this moment, no any potential race conditions. My problem is to find a proper LSN which I can use as a parameter for XLogFlush. The problem is that I can't use GetXLogInsertRecPtr() because it may be "in the future" due to some reasons (added page header size). XLogFlush will fail in this case.
In any case you will need to implement an SQL-wrapper in order to make
the function available to DBAs, cover it with tests and provide
documentation.Well, I would like to use such function in C language code, in some solution, not as a function to be used by users. 

With best regards,
Vitaly
​​​
 Hi Vitaly,

> I would propose a new function to fulfill my requirements like this (see below) but I prefer not to create new functions unreasonably:
>
> XLogRecPtr
> GetXLogLastInsertEndRecPtr(void)
> {
> XLogCtlInsert *Insert = &XLogCtl->Insert;
> uint64 current_bytepos;
> SpinLockAcquire(&Insert->insertpos_lck);
> current_bytepos = Insert->CurrBytePos;
> SpinLockRelease(&Insert->insertpos_lck);
> return XLogBytePosToEndRecPtr(current_bytepos);
> }
>
> This function differs from the existing GetXLogInsertRecPtr() by calling XLogBytePosToEndRecPtr instead of XLogBytePosToRecPtr.

​​​​
In any case you will need to implement an SQL-wrapper in order to make
the function available to DBAs, cover it with tests and provide
documentation.

--
Best regards,
Aleksander Alekseev