Horton

Re: Write Ahead Logging for Hash Indexes

Robert Haas <robertmhaas@gmail.com>

View thread View on postgr.es
From: Robert Haas <robertmhaas@gmail.com>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Stephen Frost <sfrost@snowman.net>, 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: 2017-03-15T15:12:37Z
Lists: pgsql-hackers 
On Wed, Mar 15, 2017 at 11:02 AM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
> Robert Haas <robertmhaas@gmail.com> writes:
>> 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.
>
> See my reply to Stephen.  The fact that this fails to guarantee no
> ENOSPC on COW filesystems doesn't mean that it's not worth doing on
> other filesystems.  We're reducing the risk, not eliminating it,
> but reducing risk is still a worthwhile activity.

Well, then it would presumably be worth reducing for hash indexes, too.

-- 
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company

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.