From: Greg Smith <greg@2ndquadrant.com>
To: Jesper Krogh <jesper@krogh.cc>
Cc: Balkrishna Sharma <b_ki@hotmail.com>
Cc: scott.marlowe@gmail.com
Cc: pgsql-admin@postgresql.org
Subject: Re: Asynchronous commit | Transaction loss at server crash
Date: Thu, 20 May 2010 18:04:22 -0400
Message-ID: <4BF5B1E6.1020701@2ndquadrant.com> (raw)
In-Reply-To: <4BF59CAC.9040200@krogh.cc>
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>
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
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To: pgsql-admin@postgresql.org
Cc: greg@2ndquadrant.com, jesper@krogh.cc, b_ki@hotmail.com, scott.marlowe@gmail.com
Subject: Re: Asynchronous commit | Transaction loss at server crash
In-Reply-To: <4BF5B1E6.1020701@2ndquadrant.com>
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