Horton

Re: Write Ahead Logging for Hash Indexes

Stephen Frost <sfrost@snowman.net>

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From: Stephen Frost <sfrost@snowman.net>
To: Robert Haas <robertmhaas@gmail.com>
Cc: Tom Lane <tgl@sss.pgh.pa.us>, Amit Kapila <amit.kapila16@gmail.com>, Jesper Pedersen <jesper.pedersen@redhat.com>, Jeff Janes <jeff.janes@gmail.com>, pgsql-hackers <pgsql-hackers@postgresql.org>
Date: 2017-03-15T14:55:06Z
Lists: pgsql-hackers 
Robert,

* Robert Haas (robertmhaas@gmail.com) wrote:
> On Wed, Mar 15, 2017 at 10:34 AM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
> > FWIW, I'm not certain that Stephen is correct to claim that we have
> > some concrete problem with sparse files.  We certainly don't *depend*
> > on sparse storage anyplace else, nor write data in a way that would be
> > likely to trigger it; but I'm not aware that we need to work hard to
> > avoid it.
> 
> That theory seems inconsistent with how mdextend() works.  My
> understanding is that we zero-fill the new blocks before populating
> them with actual data precisely to avoid running out of disk space due
> to deferred allocation at the OS level.  If we don't care about
> failures due to deferred allocation at the OS level, we can rip that
> logic out and improve the performance of relation extension
> considerably.  If we do care about failures due to deferred
> allocation, then leaving holes in the file is a bad idea.

That is a fantastic point.

Thanks!

Stephen

Commits

  1. Add a regression test for snapshot too old with hash indexes.

  2. hash: Add write-ahead logging support.

  3. Improve coding in _hash_addovflpage.

  4. Remove _hash_wrtbuf() in favor of calling MarkBufferDirty().

  5. Improve hash index bucket split behavior.