Re: [Fwd: Deferred Transactions, Transaction Guarantee and COMMITwithout waiting] - Mailing list pgsql-patches

Your patch has been added to the PostgreSQL unapplied patches list at:

    http://momjian.postgresql.org/cgi-bin/pgpatches

It will be applied as soon as one of the PostgreSQL committers reviews
and approves it.

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Simon Riggs wrote:
> Once more, with feeling.
>
> On Sun, 2007-04-01 at 12:11 +0100, Simon Riggs wrote:
> > Resending...
> >
> > -------- Forwarded Message --------
> > From: Simon Riggs <simon@2ndquadrant.com>
> > To: pgsql-patches@postgresql.org
> > Cc: pgsql-hackers@postgresql.org
> > Subject: Deferred Transactions, Transaction Guarantee and COMMIT without
> > waiting
> > Date: Sat, 31 Mar 2007 22:09:23 +0100
> >
> > Here's the next version (v10) of the patch, ready for review.
> >
> > I've struggled with what to call all of the new concepts inherent in
> > this patch, but I think I've got something now. COMMIT NOWAIT doesn't
> > describe this feature, since there is no command of that name in the
> > implementation that we've agreed. So what's it called?
> >
> > This patch implements a feature called Deferred Fsync Transactions, or
> > Deferred Transactions for short. The idea is we don't fsync at commit,
> > but we defer that briefly, letting a new WAL Writer process perform the
> > fsync at regular intervals of 50-250 ms. It's a much safer version of
> > fsync = off, yet retaining most of the speed *and* it can be used for
> > some transactions and not others.
> >
> > Deferred Transactions provide considerable additional performance in a
> > range of circumstances, but at the cost that a handful of committed
> > transactions will definitely be lost if the server crashes.
> >
> > To remind everybody of the risks, this feature is enabled using a
> > parameter named transaction_guarantee. The default mode is "on"
> > reminding us that PostgreSQL provides a strong default guarantee that if
> > a transaction is committed, it stays committed. If you prefer
> > performance at the risk of data loss, then you can opt to relax the
> > standard level of protection and request transaction_guarantee = off
> >
> > The data loss isn't random, nor is it indeterminate, but it is certain.
> > We will say that a transaction is committed, but it isn't until it has
> > reached disk. So all transactions that have reached the commit point,
> > but not yet reached disk will be certainly lost - probably best to use a
> > guidelines figure of 1000 transactions when assessing the business
> > impact of such loss. The risk is very similar to normal transactions
> > waiting to write to disk, but the important difference is we will have
> > replied to the client that the transaction is safely on disk, when it is
> > not.
> >
> > Relaxing the transaction guarantee in this way is completely
> > controllable by users. Guaranteed and Unguaranteed transactions can
> > co-exist safely without increased risk for more important data.
> >
> > v10 fixes a number of lurking bugs present in v9. There are no
> > outstanding bugs, after a range of tests, though more are needed.
> >
> > wal_writer_delay = 0 (default) ms enables this feature at server start.
> > Once enabled, individual sessions or transactions may request
> > transaction_guarantee = off, or it may be set for the whole server.
> >
> > It also provides additional instrumentation, with new parameters:
> > trace_commit = on will show details of each commit (high volume)
> > trace_bg_flush = on will give more frequent summaries of monitoring data
> >
> > The patch needs a reviewers guide, which I'll write next week.
> >
> > patch -p0 < transaction_guarantee.v10.patch
> > with additional files:
> > src/backend/postmaster/walwriter.c
> > src/include/postmaster/walwriter.c
>
> --
>   Simon Riggs
>   EnterpriseDB   http://www.enterprisedb.com
>

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--
  Bruce Momjian  <bruce@momjian.us>          http://momjian.us
  EnterpriseDB                               http://www.enterprisedb.com

  + If your life is a hard drive, Christ can be your backup. +