Re: Use pg_pwritev_with_retry() instead of write() in dir_open_for_write() to avoid partial writes?

On Fri, Feb 17, 2023 at 09:31:14AM -0800, Andres Freund wrote:
> On 2023-02-17 16:19:46 +0900, Michael Paquier wrote:
>> But it looks like I misunderstood what this quote meant compared to
>> what v3 does.  It is true that v3 sets iov_len and iov_base more than
>> needed when writing sizes larger than BLCKSZ.
> 
> I don't think it does for writes larger than BLCKSZ, it just does more for
> writes larger than PG_IKOV_MAX * BLCKSZ. But in those cases CPU time is going
> to be spent elsewhere.

Yep.

>> Seems like you think that it is not really going to matter much to track
>> which iovecs have been already initialized during the first loop on
>> pg_pwritev_with_retry() to keep the code shorter?
> 
> Yes. I'd bet that, in the unlikely case you're going to see any difference at
> all, unconditionally initializing is going to win.
> 
> Right now we memset() 8KB, and iterate over 32 IOVs, unconditionally, on every
> call. Even if we could do some further optimizations of what I did in the
> patch, you can initialize needed IOVs repeatedly a *lot* of times, before it
> shows up...
> 
> I'm inclined to go with my version, with the argument order swapped to
> Bharath's order.

Okay.  That's fine by me.
--
Michael