| From: | Jim Nasby <jim(at)nasby(dot)net> |
|---|---|
| To: | Bruce Momjian <bruce(at)momjian(dot)us> |
| Cc: | Daniel Farina <daniel(at)heroku(dot)com>, Heikki Linnakangas <hlinnakangas(at)vmware(dot)com>, Jeff Davis <pgsql(at)j-davis(dot)com>, Simon Riggs <simon(at)2ndquadrant(dot)com>, Greg Smith <greg(at)2ndquadrant(dot)com>, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org> |
| Subject: | Re: Enabling Checksums |
| Date: | 2013-03-13 22:24:54 |
| Message-ID: | 5140FCB6.5020709@nasby.net |
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| Lists: | pgsql-hackers |
On 3/7/13 9:31 PM, Bruce Momjian wrote:
> 1 storage
> 2 storage controller
> 3 file system
> 4 RAM
> 5 CPU
I would add 2.5 in there: storage interconnect. iSCSI, FC, what-have-you. Obviously not everyone has that.
> My guess is that storage checksums only cover layer 1, while our patch
> covers layers 1-3, and probably not 4-5 because we only compute the
> checksum on write.
Actually, it depends. In our case, we run 512GB servers and 8GB shared buffers (previous testing has shown that anything much bigger than 8G hurts performance).
So in our case, PG checksums protect a very significant portion of #4.
> If that is correct, the open question is what percentage of corruption
> happens in layers 1-3?
The last bout of corruption we had was entirely coincident with memory failures. IIRC we had 3-4 corruption events on more than one server. Everything was running standard ECC (sadly, not 4-bit ECC).
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