Обсуждение: Re: [Fwd: Deferred Transactions, Transaction Guarantee and COMMITwithout waiting]

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

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.

---------------------------------------------------------------------------


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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>
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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. +

Simon Riggs wrote:
>> 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?

Hmm. Deferred commit? Asynchronous commit?

>> v10 fixes a number of lurking bugs present in v9. There are no
>> outstanding bugs, after a range of tests, though more are needed.

We don't need any more bugs, thank you ;-).

>> 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.

This error message talks about commit_fsync_delay:
ERROR:  cannot set transaction guarantee when server commit_fsync_delay = 0

Should be wal_writer_delay, I suppose.

The HeapTupleSetVisibilityInfo function with it's action-argument looks
funny to me.

You don't need to check if a transaction has been flushed when setting
the HEAP_XMAX/XMIN_INVALID flags for aborted transactions.

--
   Heikki Linnakangas
   EnterpriseDB   http://www.enterprisedb.com

On Tue, 2007-04-03 at 15:04 +0100, Heikki Linnakangas wrote:
> Simon Riggs wrote:
> >> Here's the next version (v10) of the patch, ready for review.

Thanks for reviewing.

> >> 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?
>
> Hmm. Deferred commit? Asynchronous commit?

I'll change everything to whatever we agree. Other ideas?

> >> 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.
>
> This error message talks about commit_fsync_delay:
> ERROR:  cannot set transaction guarantee when server commit_fsync_delay = 0
>
> Should be wal_writer_delay, I suppose.

Thanks, will change.

> The HeapTupleSetVisibilityInfo function with it's action-argument looks
> funny to me.

Lets see what others think as well.

> You don't need to check if a transaction has been flushed when setting
> the HEAP_XMAX/XMIN_INVALID flags for aborted transactions.

I was after as little change as possible in the code, which is why I
left that as it was. But I think you're right and that's a one line
change to avoid that is possible. Will do.

--
  Simon Riggs
  EnterpriseDB   http://www.enterprisedb.com