Horton

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

Michael Paquier <michael@paquier.xyz>

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From: Michael Paquier <michael@paquier.xyz>
To: Andres Freund <andres@anarazel.de>
Cc: Bharath Rupireddy <bharath.rupireddyforpostgres@gmail.com>, Alvaro Herrera <alvherre@alvh.no-ip.org>, Nathan Bossart <nathandbossart@gmail.com>, Thomas Munro <thomas.munro@gmail.com>, PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>
Date: 2023-02-17T07:19:46Z
Lists: pgsql-hackers 
On Thu, Feb 16, 2023 at 11:00:20AM -0800, Andres Freund wrote:
> I don't really understand this bit?

As of this message, I saw this quote:
https://www.postgresql.org/message-id/fCALj2ACXEBwY_bM3kmZEkYpcXsM+yGitpYHi4FdT6MSk6YRtKTQ@mail.gmail.com
"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)."

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.  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?
--
Michael

Commits

  1. Revise pg_pwrite_zeros()

  2. Use pg_pwrite_zeros() in walmethods.c

  3. Introduce pg_pwrite_zeros() in fileutils.c

  4. Move pg_pwritev_with_retry() to src/common/file_utils.c

  5. Restore pg_pread and friends.

  6. Remove dead pread and pwrite replacement code.