From: | Heikki Linnakangas <heikki(dot)linnakangas(at)enterprisedb(dot)com> |
---|---|
To: | Simon Riggs <simon(at)2ndQuadrant(dot)com> |
Cc: | Merlin Moncure <mmoncure(at)gmail(dot)com>, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org> |
Subject: | Re: How much do the hint bits help? |
Date: | 2010-12-22 15:42:25 |
Message-ID: | 4D121C61.4000205@enterprisedb.com |
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Lists: | pgsql-hackers |
On 22.12.2010 17:31, Simon Riggs wrote:
> On Wed, 2010-12-22 at 17:01 +0200, Heikki Linnakangas wrote:
>> There's plenty of stuff in memory that's not covered by an
>> application-level CRC. That's what ECC RAM is for.
>
> http://www.google.com/research/pubs/archive/35162.pdf
>
> Google research shows that each DIMM has an 8% chance per annum of
> uncorrectable memory errors, even on ECC.
You misread that paper. From summary:
> About a third of machines and over 8% of DIMMs in
> our fleet saw at least one *correctable* error per year.
Emphasis mine.
> Our
> per-DIMM rates of correctable errors translate to an aver-
> age of 25,000–75,000 FIT (failures in time per billion hours
> of operation) per Mbit and a median FIT range of 778 –
> 25,000 per Mbit (median for DIMMs with errors), while pre-
> vious studies report 200-5,000 FIT per Mbit. The number of
> correctable errors per DIMM is highly variable, with some
> DIMMs experiencing a huge number of errors, compared to
> others. The annual incidence of uncorrectable errors was
> 1.3% per machine and 0.22% per DIMM.
So the real figure of uncorrectable errors is 0.22% per DIMM.
Anyway, unreliable RAM calls for more ECC bits in DIMMs, not invasive
architectural changes to every single application in the system.
--
Heikki Linnakangas
EnterpriseDB http://www.enterprisedb.com
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