Message-ID: <4BF5B1E6.1020701@2ndquadrant.com>
Date: Thu, 20 May 2010 18:04:22 -0400
From: Greg Smith <greg@2ndquadrant.com>
User-Agent: Thunderbird 2.0.0.24 (X11/20100317)
MIME-Version: 1.0
To: Jesper Krogh <jesper@krogh.cc>
CC: Balkrishna Sharma <b_ki@hotmail.com>, scott.marlowe@gmail.com,
	pgsql-admin@postgresql.org
Subject: Re: Asynchronous commit | Transaction loss at server crash
References: <BAY149-w7E2586A2E044680536BD8F0E30@phx.gbl>,
	<AANLkTimxy4_ecy3MnHYTNeWcVOrIQdJIm1_nutxHEBd4@mail.gmail.com>,
	<BAY149-w47C0EFBE3720C0DA258E4AF0E30@phx.gbl>,
	<AANLkTinT3Tx3nNb4YME6j58Edp45Q9pgCyVRkZYTnXU1@mail.gmail.com>
	<BAY149-w464B3415293316B1A6739AF0E30@phx.gbl>
	<4BF59CAC.9040200@krogh.cc>
In-Reply-To: <4BF59CAC.9040200@krogh.cc>
Content-Type: text/plain; charset=ISO-8859-1; format=flowed
Content-Transfer-Encoding: 7bit

Jesper Krogh wrote:
> A Battery Backed raid controller is not that expensive. (in the range 
> of 1 or 2 SSD disks).
> And it is (more or less) a silverbullet to the task you describe.

Maybe even less; in order to get a SSD that's reliable at all in terms 
of good crash recovery, you have buy a fairly expensive one.  Also, and 
this is really important, you really don't want to deploy onto a single 
SSD and put critical system files there.  Their failure rates are not 
that low.  You need to put them into a RAID-1 setup and budget for two 
of them, which brings you right back to

Also, it's questionable whether a SSD is even going to be faster than 
standard disks for the sequential WAL writes anyway, once a non-volatile 
write cache is available.  Sequential writes to SSD are the area where 
the gap in performance between them and spinning disks is the smallest.

> Plugging your system (SSD's) with an UPS and trusting it fully
> could solve most of the problems (running in writeback mode).

UPS batteries fail, and people accidentally knock out over server power 
cords.  It's a pretty bad server that can't survive someone tripping 
over the cord while it's busy, and that's the situation the "use a UPS" 
idea doesn't improve.

-- 
Greg Smith  2ndQuadrant US  Baltimore, MD
PostgreSQL Training, Services and Support
greg@2ndQuadrant.com   www.2ndQuadrant.us