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

On Wed, Feb 15, 2023 at 01:00:00PM +0530, Bharath Rupireddy wrote:
> The v3 patch reduces initialization of iovec array elements which is a
> clear win when pg_pwrite_zeros is called for sizes less than BLCKSZ
> many times (I assume this is what is needed for the relation extension
> lock improvements feature). However, it increases the number of iovec
> initialization with zerobuf for the cases when pg_pwrite_zeros is
> called for sizes far greater than BLCKSZ (for instance, WAL file
> initialization).

It seems to me that v3 would do extra initializations only if
pg_pwritev_with_retry() does *not* retry its writes, but that's not
the case as it retries on a partial write as per its name.  The number
of iov buffers is stricly capped by remaining_size.  FWIW, I find v3
proposed more elegant.

> FWIW, I attached v4 patch, a simplified version of the v2  - it
> initializes all the iovec array elements if the total blocks to be
> written crosses lengthof(iovec array), otherwise it initializes only
> the needed blocks.

+   static size_t   zbuf_sz = BLCKSZ;
In v4, what's the advantage of marking that as static?  It could
actually be dangerous if this is carelessly updated.  Well, that's not
the case, still..
--
Michael