Horton

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

Nathan Bossart <nathandbossart@gmail.com>

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From: Nathan Bossart <nathandbossart@gmail.com>
To: Bharath Rupireddy <bharath.rupireddyforpostgres@gmail.com>
Cc: Thomas Munro <thomas.munro@gmail.com>, Michael Paquier <michael@paquier.xyz>, PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>
Date: 2022-09-20T23:00:26Z
Lists: pgsql-hackers 
On Mon, Aug 08, 2022 at 06:10:23PM +0530, Bharath Rupireddy wrote:
> I'm attaching v5 patch-set. I've addressed review comments received so
> far and fixed a compiler warning that CF bot complained about.
> 
> Please review it further.

0001 looks reasonable to me.

+        errno = 0;
+        rc = pg_pwritev_zeros(fd, pad_to_size);

Do we need to reset errno?  pg_pwritev_zeros() claims to set errno
appropriately.

+/*
+ * PWRITEV_BLCKSZ is same as XLOG_BLCKSZ for now, however it may change if
+ * writing more bytes per pg_pwritev_with_retry() call is proven to be more
+ * performant.
+ */
+#define PWRITEV_BLCKSZ  XLOG_BLCKSZ

This seems like something we should sort out now instead of leaving as
future work.  Given your recent note, I think we should just use
XLOG_BLCKSZ and PGAlignedXLogBlock and add a comment about the performance
findings with different buffer sizes.

-- 
Nathan Bossart
Amazon Web Services: https://aws.amazon.com

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.