Subject: no universally correct setting for fsync

From: "Kevin Grittner" <[email-masked]>
Date: Fri, 07 May 2010 08:47:15 -0500

Someone just posted to the -admin list with a database corrupted
while running with fsync=off.  I was all set to refer him to the
documentation explaining why he should stop doing this, but to my
surprise the documentation waffles on the issue way past what I
think is reasonable.
 
http://www.postgresql.org/docs/8.4/interactive/runtime-config-wal.html#GUC-FSYNC
 
There are dire-sounding statements interspersed with:
 
| using fsync results in a performance penalty
 
| Due to the risks involved, there is no universally correct setting
| for fsync.
 
| If you trust your operating system, your hardware, and your
| utility company (or your battery backup), you can consider
| disabling fsync.
 
Isn't this a little too rosy a picture to paint?
 
-Kevin

---

From: Robert Haas <[email-masked]>
Date: Fri, 7 May 2010 10:00:50 -0400

> Isn't this a little too rosy a picture to paint?

I agree.  I've always thought this part of the documentation made
setting fsync=off much more reasonable than I feel it to be.

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

---

From: Magnus Hagander <[email-masked]>
Date: Fri, 7 May 2010 16:03:55 +0200

> setting fsync=off much more reasonable than I feel it to be.

+1, definitely. fsync=off should only be done if you *really*
understand what it means, and that requires a lot more explanation
than that...

-- 
 Magnus Hagander
 Me: http://www.hagander.net/
 Work: http://www.redpill-linpro.com/

---

From: Tom Lane <[email-masked]>
Date: Fri, 07 May 2010 10:24:47 -0400

> Isn't this a little too rosy a picture to paint?

I think that statement is true as far as it goes, but I agree with
rejiggering the surrounding text.  The whole thing was written back
when Postgres was by far the least reliable component of the stack.
It isn't anymore.  We should make it clear that fsync=off is not ever
recommended for production.

			regards, tom lane

---

From: Andrew Dunstan <[email-masked]>
Date: Fri, 07 May 2010 10:38:47 -0400

> Isn't this a little too rosy a picture to paint?

I think the critical question is really whether you are prepared to lose 
your database.

I have a customer who rotates databases in and out of line, and 
processes major updates on the out of line database. If they lose the 
database occasionally they are prepared to wear that risk for the 
performance gain they get from running with fsync off. It just means 
that they have to recover and so the inline database will get a bit 
staler than usual while they do.

So I think its true that there is no universally right answer. Maybe the 
criteria mentioned in the last para need tweaking some, though. It's not 
just a matter of trusting hardware etc. I have seen mishaps when idiots 
knock out power cords and the like. The unexpected does sometime happen, 
despite the best planning.

cheers

andrew

---

From: "Kevin Grittner" <[email-masked]>
Date: Fri, 07 May 2010 09:48:53 -0500

Andrew Dunstan <[email-masked]> wrote:
 
> I think the critical question is really whether you are prepared
> to lose your database.
 
Precisely; and the docs don't make that at all clear.  They mention
the possibility of database corruption, but downplay it:
 
| When fsync is disabled, the operating system is allowed to do its
| best in buffering, ordering, and delaying writes. This can result
| in significantly improved performance. However, if the system
| crashes, the results of the last few committed transactions might
| be lost in part or whole. In the worst case, unrecoverable data
| corruption might occur.

> [valid use case for fsync=off]
> 
> So I think its true that there is no universally right answer.
> Maybe the criteria mentioned in the last para need tweaking some,
> though.
 
I think it goes beyond "tweaking" -- I think we should have a bald
statement like "don't turn this off unless you're OK with losing the
entire contents of the database cluster."  A brief listing of some
cases where that is OK might be illustrative.
 
I never meant to suggest any statement in that section is factually
wrong; it's just all too rosy, leading people to believe it's no big
deal to turn it off.
 
-Kevin

---

From: Josh Berkus <[email-masked]>
Date: Fri, 07 May 2010 10:56:13 -0700

> wrong; it's just all too rosy, leading people to believe it's no big

Yeah, that section is overdue for an update.  I'll write some new text 
and post it to pgsql-docs.

-- 
                                   -- Josh Berkus
                                      PostgreSQL Experts Inc.
                                      http://www.pgexperts.com

---

From: Bernd Helmle <[email-masked]>
Date: Sat, 08 May 2010 01:32:59 +0200

> cases where that is OK might be illustrative.

+1

> wrong; it's just all too rosy, leading people to believe it's no big

I think one mistake in this paragraph is the passing mention of 
"performance". I've seen installations in the past with fsync=off only 
because the admin was pressured to get instantly "more speed" out of the 
database (think of "fast_mode=on"). In my opinion, phrases like 
"performance penalty" are misleading, if you need that setting in 99% of 
all use cases for reliable operation.

I've recently even started to wonder if the performance gain with fsync=off 
is still that large on modern hardware. While testing large migration 
procedures to a new version some time ago (on an admitedly fast storage) i 
forgot here and then to turn it off, without a significant degradation in 
performance.

-- 
Thanks

	Bernd

---

From: Tom Lane <[email-masked]>
Date: Fri, 07 May 2010 19:49:15 -0400

> forgot here and then to turn it off, without a significant degradation in 

That says to me either that you're using a battery-backed write cache,
or your fsyncs don't really work (no write barriers or something like
that).

			regards, tom lane

---

From: Josh Berkus <[email-masked]>
Date: Fri, 07 May 2010 17:02:59 -0700

Folks,

This is what I have to replace the current fsync entry in config.sgml.

I believe that the note about needing fsync for Warm Standby to work
correctly is true, but could someone verify it?

=========================

     <varlistentry id="guc-fsync" xreflabel="fsync">
      <indexterm>
       <primary><varname>fsync</> configuration parameter</primary>
      </indexterm>
      <term><varname>fsync</varname> (<type>boolean</type>)</term>
      <listitem>
       <para>
        If this parameter is on, the <productname>PostgreSQL</> server
        will try to make sure that updates are physically written to
        disk, by issuing <function>fsync()</> system calls or various
        equivalent methods (see <xref linkend="guc-wal-sync-method">).
        This ensures that the database cluster can recover to a
        consistent state after an operating system or hardware crash.
       </para>

       <para>
        While turning off <varname>fsync</varname> is often a performance
        benefit, this can result in unrecoverable data corruption in the
event
        of an unexpected shutdown. Thus it is only advisable to turn off
        <varname>fsync</varname> if you can easily recreate
        your entire database from external data.  <varname>fsync</varname>
        must be on for WAL archiving to work correctly
        (see <xref linkend="continuous-archiving">).
       <para>

       <para>
        In many situations, turning off <xref
linkend="guc-synchronous-commit">
        for noncritical transactions can provide much of the potential
        performance benefit of turning off <varname>fsync</varname>, without
        the attendant risks of data corruption.
       </para>

       <para>
        <varname>fsync</varname> can only be set in the
<filename>postgresql.conf</>
        file or on the server command line.
        If you turn this parameter off, also consider turning off
        <xref linkend="guc-full-page-writes">.
       </para>
      </listitem>
     </varlistentry>

-- 
                                  -- Josh Berkus
                                     PostgreSQL Experts Inc.
                                     http://www.pgexperts.com

---

From: Tom Lane <[email-masked]>
Date: Fri, 07 May 2010 20:13:32 -0400

> This is what I have to replace the current fsync entry in config.sgml.

s/unexpected shutdown/system crash/, perhaps.  The wording you have
suggests that a forced Postgres stoppage produces a problem, which it
doesn't.  It takes a failure at the OS level or below to cause a
problem.

> correctly is true, but could someone verify it?

AFAIK that's nonsense.  The filesystem state that pg_standby could see
will be updated in any case; pg_standby has no direct access to the bits
on the platters, any more than Postgres does.

			regards, tom lane

---

From: Bernd Helmle <[email-masked]>
Date: Sat, 08 May 2010 02:16:23 +0200

--On 7. Mai 2010 19:49:15 -0400 Tom Lane <[email-masked]> wrote:

>> I've recently even started to wonder if the performance gain with
>> fsync=off  is still that large on modern hardware. While testing large
>> migration  procedures to a new version some time ago (on an admitedly
>> fast storage) i  forgot here and then to turn it off, without a
>> significant degradation in  performance.
>

Well, yes, BBU present and proven storage. Maybe i'm wrong, but it seems 
battery backed write caches aren't that seldom even in low end systems 
nowadays.

-- 
Thanks

	Bernd

---

From: Josh Berkus <[email-masked]>
Date: Fri, 07 May 2010 17:17:53 -0700

> doesn't.  It takes a failure at the OS level or below to cause a

I actually meant "unexpected *system* shutdown", i.e. power-out.  A lot
of people think "crash" just means kernel dump, whereas a UPS failure or
tripped power cord is a lot more likely (except maybe on Windows).

Revised:

==================

       <para>
        While turning off <varname>fsync</varname> is often a performance
        benefit, this can result in unrecoverable data corruption in the
event
        of an unexpected system shutdown or crash.  Thus it is only
advisable
        to turn off  <varname>fsync</varname> if you can easily recreate
        your entire database from external data.
       <para>

       <para>

       <para>

-- 
                                  -- Josh Berkus
                                     PostgreSQL Experts Inc.
                                     http://www.pgexperts.com

---

From: Craig Ringer <[email-masked]>
Date: Sat, 08 May 2010 16:07:01 +0800

> and post it to pgsql-docs.

It's probably worth mentioning that people who want to turn off fsync to 
gain a performance boost should instead look at a RAID controller with a 
BBU so they can safely enable write-back caching, getting most of the 
benefits of fsync=off safely.

--
Craig Ringer

---

From: Michael Tharp <[email-masked]>
Date: Mon, 10 May 2010 10:48:01 -0400

> benefits of fsync=off safely.

Which options specifically should be set if a BBU is in use? Obviously 
fsync should be on always, but can full_page_writes be disabled? Are 
there other tweaks that can be done?

It would be great to see some practical hints in the documentation while 
the fsync part is getting changed.

-- m. tharp

---

From: Bruce Momjian <[email-masked]>
Date: Mon, 10 May 2010 11:12:35 -0400 (EDT)

> the fsync part is getting changed.

Uh, our docs have:

	Turning this parameter off speeds normal operation, but might
	lead to a corrupt database after an operating system crash or
	power failure. The risks are similar to turning off
	<varname>fsync</>, though smaller.  It might be safe to turn
	off this parameter if you have hardware (such as a battery-backed
	disk controller) or file-system software that reduces the risk
	of partial page writes to an acceptably low level (e.g., ZFS).

-- 
  Bruce Momjian  <[email-masked]>        http://momjian.us
  EnterpriseDB                             http://enterprisedb.com

---

From: Robert Haas <[email-masked]>
Date: Mon, 10 May 2010 11:41:22 -0400

>        of partial page writes to an acceptably low level (e.g., ZFS).

"It might be safe" is a bit of a waffle.  It would be nice if we could
provide some more clear guidance as to whether it is or is not, or how
someone could go about testing their hardware to find out.

---

From: Bruce Momjian <[email-masked]>
Date: Mon, 10 May 2010 11:49:15 -0400 (EDT)

Robert Haas wrote:
> > ? ? ? ?Turning this parameter off speeds normal operation, but might
> > ? ? ? ?lead to a corrupt database after an operating system crash or
> > ? ? ? ?power failure. The risks are similar to turning off
> > ? ? ? ?<varname>fsync</>, though smaller. ?It might be safe to turn
> > ? ? ? ?off this parameter if you have hardware (such as a battery-backed
> > ? ? ? ?disk controller) or file-system software that reduces the risk
> > ? ? ? ?of partial page writes to an acceptably low level (e.g., ZFS).
> 

Agreed.  It is "safe" for us to be definitive here?

---

From: "Kevin Grittner" <[email-masked]>
Date: Mon, 10 May 2010 10:55:40 -0500

Robert Haas <[email-masked]> wrote:
 
> "It might be safe" is a bit of a waffle.  It would be nice if we
> could provide some more clear guidance as to whether it is or is
> not, or how someone could go about testing their hardware to find
> out.
 
I think that the issue is that you could have corruption if some,
but not all, disk sectors from a page were written from OS cache to
controller cache when a failure occurred.  The window would be small
for a RAM-to-RAM write, but it wouldn't be entirely *safe* unless
there's some OS/driver environment where you could count on all the
sectors making it or none of them making it for every single page. 
Does such an environment exist?
 
-Kevin

---

From: Greg Stark <[email-masked]>
Date: Mon, 10 May 2010 18:46:53 +0100

> Does such an environment exist?

The reason for the waffle is that the following sentence describes a
whole set of environments based the following description:

> > ? ? ? ?if you have hardware (such as a battery-backed
> > ? ? ? ?of partial page writes to an acceptably low level

Depending on which set of hardware and how low the risk is it might be safe.

I think with WAFL or ZFS it's entirely safe. There may be other
filesystems with similar guarantees. With a BBU the risk might be very
low -- but it might not, it would be hard to determine without a
detailed analysis of the entire stack from the buffer cache,
filesystem, lvm, hardware drivers, BBU design, etc.

-- 
greg

---

From: "Joshua D. Drake" <[email-masked]>
Date: Mon, 10 May 2010 11:42:57 -0700

> filesystem, lvm, hardware drivers, BBU design, etc.

The answer to this is:

PostgreSQL.org recommends that this setting be left on at all times.
Turning it off, may lead to data corruption.

Anything else is circumstantial and based on knowledge and facts we
don't have about environmental factors. 

Joshua D. Drake

-- 
PostgreSQL.org Major Contributor
Command Prompt, Inc: http://www.commandprompt.com/ - 503.667.4564
Consulting, Training, Support, Custom Development, Engineering

---

From: "Kevin Grittner" <[email-masked]>
Date: Mon, 10 May 2010 14:00:37 -0500

"Joshua D. Drake" <[email-masked]> wrote:
 
> The answer to this is:
> 
> PostgreSQL.org recommends that this setting be left on at all
> times.  Turning it off, may lead to data corruption.
> 
> Anything else is circumstantial and based on knowledge and facts
> we don't have about environmental factors. 
 
Perhaps Josh's language for fsync could be modified to work here
(we're now talking about full_page_writes, for anyone who's lost
track):
 
| it is only advisable to turn off fsync if you can easily recreate
| your entire database from external data.
 
That covers bulk loads to an empty or just-backed-up database and
entirely redundant databases.  Saying it should never be turned off
would tend to make one wonder why we have the setting at all.
 
-Kevin

---

From: Tom Lane <[email-masked]>
Date: Mon, 10 May 2010 15:57:42 -0400

> would tend to make one wonder why we have the setting at all.

+1.  Perhaps for both of them, we should specify that the intended
use-case is for improving performance during initial database load
and similar cases.

			regards, tom lane

---

From: Cédric Villemain <[email-masked]>
Date: Mon, 10 May 2010 22:22:05 +0200

2010/5/8 Bernd Helmle <[email-masked]>:
>
>
> --On 7. Mai 2010 09:48:53 -0500 Kevin Grittner <[email-masked]>
> wrote:
>
> database (think of "fast_mode=on"). In my opinion, phrases like "performance
> penalty" are misleading, if you need that setting in 99% of all use cases
> for reliable operation.
>

On a recent pg_restore -j 32, with perc 6i with BBU, RAID10 8 hd,
results were not so bas with fsync turn on. (XFS with nobarrier su and
sw)
-- deactivate fsync
time pg_restore -U postgres -d foodb -j 32 foo.psql
real    170m0.527s
user    43m12.914s
sys     1m56.499s
-- activate fsync
time pg_restore -U postgres -d foodb -j 32 foo.psql
real    177m0.121s
user    42m54.581s
sys     2m0.452s

>
>
> --
> Thanks
>
>        Bernd
>
> --
> Sent via pgsql-hackers mailing list ([email-masked])
> To make changes to your subscription:
> http://www.postgresql.org/mailpref/pgsql-hackers
>

-- 
Cédric Villemain

---

From: Josh Berkus <[email-masked]>
Date: Mon, 10 May 2010 13:35:32 -0700

All,

Updated docs based on tracking this discussion.  fsync through full page
writes recorded below.

============

       <para>

       <para>
	Examples of safe times to turn off <varname>fsync</varname> would be
        when initially loading a new database from a backup file, on a
database which is
	only used for processing statistics on an hourly basis and is then
deleted,
        or on a reporting read-only clone of your database which gets
recreated very
        night and is not used for failover.  High quality hardware alone
is not a
        sufficient justification for turning off <varname>fsync</varname>.
       </para>

       <para>

       <para>

     <varlistentry id="guc-synchronous-commit"
xreflabel="synchronous_commit">
      <term><varname>synchronous_commit</varname>
(<type>boolean</type>)</term>
      <indexterm>
       <primary><varname>synchronous_commit</> configuration
parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Specifies whether transaction commit will wait for WAL records
        to be written to disk before the command returns a <quote>success</>
        indication to the client.  The default, and safe, setting is
        <literal>on</>.  When <literal>off</>, there can be a delay between
        when success is reported to the client and when the transaction is
        really guaranteed to be safe against a server crash.  (The maximum
        delay is three times <xref linkend="guc-wal-writer-delay">.)  Unlike
        <xref linkend="guc-fsync">, setting this parameter to
<literal>off</>
        does not create any risk of database inconsistency: a crash might
        result in some recent allegedly-committed transactions being
lost, but
        the database state will be just the same as if those
transactions had
        been aborted cleanly.  So, turning
<varname>synchronous_commit</> off
        can be a useful alternative when performance is more important than
        exact certainty about the durability of a transaction.  For more
        discussion see <xref linkend="wal-async-commit">.
       </para>
       <para>
        This parameter can be changed at any time; the behavior for any
        one transaction is determined by the setting in effect when it
        commits.  It is therefore possible, and useful, to have some
        transactions commit synchronously and others asynchronously.
        For example, to make a single multi-statement transaction commit
        asynchronously when the default is the opposite, issue <command>SET
        LOCAL synchronous_commit TO OFF</> within the transaction.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-wal-sync-method" xreflabel="wal_sync_method">
      <term><varname>wal_sync_method</varname> (<type>enum</type>)</term>
      <indexterm>
       <primary><varname>wal_sync_method</> configuration
parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Method used for forcing WAL updates out to disk.
        If <varname>fsync</varname> is off then this setting is irrelevant,
        since WAL file updates will not be forced out at all.
        Possible values are:
       </para>
       <itemizedlist>
        <listitem>
        <para>
         <literal>open_datasync</> (write WAL files with
<function>open()</> option <symbol>O_DSYNC</>)
        </para>
        </listitem>
        <listitem>
        <para>
         <literal>fdatasync</> (call <function>fdatasync()</> at each
commit)
        </para>
        </listitem>
        <listitem>
        <para>
         <literal>fsync_writethrough</> (call <function>fsync()</> at
each commit, forcing write-through of any disk write cache)
        </para>
        </listitem>
        <listitem>
        <para>
         <literal>fsync</> (call <function>fsync()</> at each commit)
        </para>
        </listitem>
        <listitem>
        <para>
         <literal>open_sync</> (write WAL files with <function>open()</>
option <symbol>O_SYNC</>)
        </para>
        </listitem>
       </itemizedlist>
       <para>
        Not all of these choices are available on all platforms.
        The default is the first method in the above list that is supported
        by the platform.
        The <literal>open_</>* options also use <literal>O_DIRECT</> if
available.
        The utility <filename>src/tools/fsync</> in the PostgreSQL
source tree
        can do performance testing of various fsync methods.
        This parameter can only be set in the <filename>postgresql.conf</>
        file or on the server command line.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-full-page-writes" xreflabel="full_page_writes">
      <indexterm>
       <primary><varname>full_page_writes</> configuration
parameter</primary>
      </indexterm>
      <term><varname>full_page_writes</varname>
(<type>boolean</type>)</term>
      <listitem>
       <para>
        When this parameter is on, the <productname>PostgreSQL</> server
        writes the entire content of each disk page to WAL during the
        first modification of that page after a checkpoint.
        This is needed because
        a page write that is in process during an operating system crash
might
        be only partially completed, leading to an on-disk page
        that contains a mix of old and new data.  The row-level change data
        normally stored in WAL will not be enough to completely restore
        such a page during post-crash recovery.  Storing the full page image
        guarantees that the page can be correctly restored, but at the price
        of increasing the amount of data that must be written to WAL.
        (Because WAL replay always starts from a checkpoint, it is
sufficient
        to do this during the first change of each page after a checkpoint.
        Therefore, one way to reduce the cost of full-page writes is to
        increase the checkpoint interval parameters.)
       </para>

       <para>
        Turning this parameter off speeds normal operation, but
        might lead to either unrecoverable data corruption, or silent
        data corruption, after a system failure. The risks are similar
to turning off
        <varname>fsync</varname>, though smaller, and it should be
turned off
        only based on the same circumstances recommended for that parameter.
       </para>

       <para>
        Turning off this parameter does not affect use of
        WAL archiving for point-in-time recovery (PITR)
        (see <xref linkend="continuous-archiving">).
       </para>

       <para>
        This parameter can only be set in the <filename>postgresql.conf</>
        file or on the server command line.
        The default is <literal>on</>.
       </para>
      </listitem>
     </varlistentry>

-- 
                                  -- Josh Berkus
                                     PostgreSQL Experts Inc.
                                     http://www.pgexperts.com

---

From: "Kevin Grittner" <[email-masked]>
Date: Mon, 10 May 2010 15:59:34 -0500

Cédric Villemain<[email-masked]> wrote:
 
> results were not so bas with fsync turn on. (XFS with nobarrier su
> and sw)
> -- deactivate fsync
 
Wow.  In a situation where you save seven minutes (4%), it's hardly
worth turning off.
 
-Kevin

---

From: Josh Berkus <[email-masked]>
Date: Mon, 10 May 2010 14:20:18 -0700

> Wow.  In a situation where you save seven minutes (4%), it's hardly

I've had it be much higher, especially for really large databases.

-- 
                                  -- Josh Berkus
                                     PostgreSQL Experts Inc.
                                     http://www.pgexperts.com

---

From: "Ross J. Reedstrom" <[email-masked]>
Date: Mon, 10 May 2010 16:21:35 -0500

>         night and is not used for failover.  High quality hardware alone

s/very/every/
or 
s/very night/periodically/

Ross
-- 
Ross Reedstrom, Ph.D.                                 [email-masked]
Systems Engineer & Admin, Research Scientist        phone: 713-348-6166
The Connexions Project      http://cnx.org            fax: 713-348-3665
Rice University MS-375, Houston, TX 77005
GPG Key fingerprint = F023 82C8 9B0E 2CC6 0D8E  F888 D3AE 810E 88F0 BEDE

---

From: Greg Smith <[email-masked]>
Date: Mon, 10 May 2010 19:03:34 -0400

> I've had it be much higher, especially for really large databases.

Cedric's system had a non-volatile write cache in it.  In that case, a 
few percentage points of improvement is normal--the overhead of fsync is 
very low.  In the case where you don't have one of those, and the write 
cache on the drives are turned off for safety too, I've seen turning 
fsync off be a 40X speedup--100 inserts/second jumping to 4000TPS.  
(This was before synchronous_commit).

The real question is how much of a speed-up fsync provides compared to 
the same workload with synchronous_commit disabled.  The only case for 
fsync=off is one where that number is much faster.  That's the case on 
some low-level operations (I seem to recall there is no async commit 
speedup for CREATE DATABASE for example).  But for most of what people 
want to speed, just killing sync commit while keeping fsync is on is 
good enough.  I suspect there are still some bulk-load workloads where 
fsync=off helps beyond just going for async commit, but they're tougher 
to find and the difference isn't huge relative to total load times.

-- 
Greg Smith  2ndQuadrant US  Baltimore, MD
PostgreSQL Training, Services and Support
[email-masked]   www.2ndQuadrant.us

---

From: Josh Berkus <[email-masked]>
Date: Mon, 10 May 2010 17:23:10 -0700

> fsync=off is one where that number is much faster.  

I can't say I've tested this.  Most of my head-to-heads on fsync were
before asych existed.

-- 
                                  -- Josh Berkus
                                     PostgreSQL Experts Inc.
                                     http://www.pgexperts.com

---

From: Josh Berkus <[email-masked]>
Date: Mon, 10 May 2010 17:26:54 -0700

> s/very night/periodically/

"frequently" I think.  Periodically could mean once a year.

-- 
                                  -- Josh Berkus
                                     PostgreSQL Experts Inc.
                                     http://www.pgexperts.com

---

From: Greg Smith <[email-masked]>
Date: Mon, 10 May 2010 21:04:29 -0400

> I can't say I've tested this.  Most of my head-to-heads on fsync were

Ditto for me.  Curious about that, and I'd like to help work on 
improving this chunk of the docs too.  I don't know about you guys, but 
I'm swamped until after PGCon though.

I have some hardware testing stuff planned anyway later this month, can 
check exactly where this situation truly stands on a couple of common 
pieces of hardware (next system has one of the LSI controllers Dell 
rebrands too).  I'll have the systems setup for something similar 
anyway--can certainly see fsync differences with pgbench--easy to throw 
this test into the mix too.

With that report, we should have the info needed to really nail this 
down accurately.  I can make my own proofreading pass of what Josh has 
already been doing that also reflects the new data, and then we can 
commit something that's good and well reviewed for 9.0 here.

---

From: Yeb Havinga <[email-masked]>
Date: Tue, 11 May 2010 10:00:21 +0200

> would tend to make one wonder why we have the setting at all.
Would the term "entirely redundant databases" include (synchronously) 
replicated databases? (ps: I did indeed lose track about whether this is 
about fsync or full_page_writes and did not get on the track again)

regards,
Yeb Havinga

---

From: Bruce Momjian <[email-masked]>
Date: Mon, 31 May 2010 11:52:31 -0400 (EDT)

> Updated docs based on tracking this discussion.  fsync through full page

I have applied this doc update with the attached patch.

I added the change from "every night" to "frequently", and reworded it
slightly so it was clear it affects the entire cluster, not just a
single database.

-- 
  Bruce Momjian  <[email-masked]>        http://momjian.us
  EnterpriseDB                             http://enterprisedb.com
Index: doc/src/sgml/config.sgml
===================================================================
RCS file: /cvsroot/pgsql/doc/src/sgml/config.sgml,v
retrieving revision 1.279
diff -c -c -r1.279 config.sgml
*** doc/src/sgml/config.sgml	26 May 2010 23:49:18 -0000	1.279
--- doc/src/sgml/config.sgml	31 May 2010 15:44:36 -0000
***************
*** 1413,1446 ****
         </para>
  
         <para>
!         However, using <varname>fsync</varname> results in a
!         performance penalty: when a transaction is committed,
!         <productname>PostgreSQL</productname> must wait for the
!         operating system to flush the write-ahead log to disk.  When
!         <varname>fsync</varname> is disabled, the operating system is
!         allowed to do its best in buffering, ordering, and delaying
!         writes. This can result in significantly improved performance.
!         However, if the system crashes, the results of the last few
!         committed transactions might be completely lost, or worse,
!         might appear partially committed, leaving the database in an
!         inconsistent state. In the
!         worst case, unrecoverable data corruption might occur.
!         (Crashes of the database software itself are <emphasis>not</>
!         a risk factor here.  Only an operating-system-level crash
!         creates a risk of corruption.)
         </para>
  
         <para>
!         Due to the risks involved, there is no universally correct
!         setting for <varname>fsync</varname>. Some administrators
!         always disable <varname>fsync</varname>, while others only
!         turn it off during initial bulk data loads, where there is a clear
!         restart point if something goes wrong.  Others
!         always leave <varname>fsync</varname> enabled. The default is
!         to enable <varname>fsync</varname>, for maximum reliability.
!         If you trust your operating system, your hardware, and your
!         utility company (or your battery backup), you can consider
!         disabling <varname>fsync</varname>.
         </para>
  
         <para>
--- 1413,1435 ----
         </para>
  
         <para>
!         While turning off <varname>fsync</varname> is often a performance
!         benefit, this can result in unrecoverable data corruption in
!         the event of an unexpected system shutdown or crash.  Thus it
!         is only advisable to turn off  <varname>fsync</varname> if
!         you can easily recreate your entire database from external
!         data.
         </para>
  
         <para>
!         Examples of safe circumstances for turning off
!         <varname>fsync</varname> include the initial loading a new
!         database cluster from a backup file, using a database cluster
!         for processing statistics on an hourly basis which is then
!         recreated, or for a reporting read-only database clone which
!         gets recreated frequently and is not used for failover.  High
!         quality hardware alone is not a sufficient justification for
!         turning off <varname>fsync</varname>.
         </para>
  
         <para>
***************
*** 1572,1583 ****
  
         <para>
          Turning this parameter off speeds normal operation, but
!         might lead to a corrupt database after an operating system crash
!         or power failure. The risks are similar to turning off
!         <varname>fsync</>, though smaller.  It might be safe to turn off
!         this parameter if you have hardware (such as a battery-backed disk
!         controller) or file-system software that reduces
!         the risk of partial page writes to an acceptably low level (e.g., ZFS).
         </para>
  
         <para>
--- 1561,1570 ----
  
         <para>
          Turning this parameter off speeds normal operation, but
!         might lead to either unrecoverable data corruption, or silent
!         data corruption, after a system failure. The risks are similar to turning off
!         <varname>fsync</varname>, though smaller, and it should be turned off
!         only based on the same circumstances recommended for that parameter.
         </para>
  
         <para>