Message-ID: <500371A6.5010906@ringerc.id.au>
Date: Mon, 16 Jul 2012 09:43:02 +0800
From: Craig Ringer <ringerc@ringerc.id.au>
User-Agent: Mozilla/5.0 (X11; Linux x86_64;
	rv:13.0) Gecko/20120605 Thunderbird/13.0
MIME-Version: 1.0
To: Tom Lane <tgl@sss.pgh.pa.us>
CC: Jeff Janes <jeff.janes@gmail.com>,
	pgsql-hackers <pgsql-hackers@postgresql.org>,
	Daniel Farina <daniel@heroku.com>, =?ISO-8859-1?Q?=22Harold_A=2E_Gim?=
	=?ISO-8859-1?Q?=E9nez=22?= <harold.gimenez@gmail.com>
Subject: Re: [PERFORM] DELETE vs TRUNCATE explanation
References: <4FFCCAC4.4030503@ringerc.id.au>
	<CAAZKuFZ9tYeC1K5ZayZgztm3re_7ytbsEYFa0wwXirB6Hfm+EQ@mail.gmail.com>
	<23853.1342015548@sss.pgh.pa.us>
	<CAAZKuFYsK+ZQuC=zf0xE=M-LNa02tB7RBy3CHEUyq7LWCkK6jQ@mail.gmail.com>
	<CAMkU=1ydaypgAeZxyaMc=KhJJw5riGMBiLpvSqOO42=qArRZmQ@mail.gmail.com>
	<32078D4B40DD40ECB739D0ECD7D95996@gmail.com>
	<CAMkU=1zJK6WiS9-6eO8rNmRpKSSB4=uw2mTsD=k64mNh__=Dcg@mail.gmail.com>
	<CAMkU=1w7vLA63hf-+Uc_j61CvOdKixsOHph9d0H-aLNBYgfX=w@mail.gmail.com>
	<CAMkU=1yLXvODRZZ_=fgrEeJfk2tvZPTTD-8n8BwrAhNz_WBT0A@mail.gmail.com>
	<19949.1342376967@sss.pgh.pa.us> <50035EE3.8030607@ringerc.id.au>
	<27245.1342402661@sss.pgh.pa.us>
In-Reply-To: <27245.1342402661@sss.pgh.pa.us>
Content-Type: text/plain; charset=ISO-8859-1; format=flowed
Content-Transfer-Encoding: 7bit

On 07/16/2012 09:37 AM, Tom Lane wrote:
>> There's one way that doesn't have any housekeeping cost to Pg. It's
>> pretty bad manners if there's anybody other than Pg on the system though:
>>     sync()
> Yeah, I thought about that: if we could document that issuing a manual
> sync after turning fsync on leaves you in a guaranteed-good state once
> the sync is complete, it'd probably be fine.  However, I'm not convinced
> that we could promise that with a straight face.  In the first place,
> PG has only very weak guarantees about how quickly all processes in the
> system will absorb a GUC update.  In the second place, I'm not entirely
> sure that there aren't race conditions around checkpoints and the fsync
> request queue (particularly if we do what Jeff is suggesting and
> suppress queuing requests at the upstream end).  It might be all right,
> or it might be all right after expending some work, but the whole thing
> is not an area where I think anyone wants to spend time.  I think it'd
> be much safer to document that the correct procedure is "stop the
> database, do a manual sync, enable fsync in postgresql.conf, restart the
> database".  And if that's what we're documenting, we lose little or
> nothing by marking fsync as PGC_POSTMASTER.
Sounds reasonable to me; I tend to view fsync=off as a testing feature 
anyway. Will clone onto -general and see if anyone yells.

--
Craig Ringer