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Re: Block-level CRC checks

From: Robert Treat <xzilla(at)users(dot)sourceforge(dot)net>
To: pgsql-hackers(at)postgresql(dot)org
Cc: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, pgsql(at)mohawksoft(dot)com, "Hannu Krosing" <hannu(at)2ndquadrant(dot)com>, "Decibel!" <decibel(at)decibel(dot)org>, "Alvaro Herrera" <alvherre(at)commandprompt(dot)com>
Subject: Re: Block-level CRC checks
Date: 2008-10-02 19:37:01
Message-ID: 200810021537.01727.xzilla@users.sourceforge.net (view raw or flat)
Thread:
Lists: pgsql-hackers
On Wednesday 01 October 2008 10:27:52 Tom Lane wrote:
> pgsql(at)mohawksoft(dot)com writes:
> >> No, it's all about time penalties and loss of concurrency.
> >
> > I don't think that the amount of time it would take to calculate and test
> > the sum is even important. It may be in older CPUs, but these days CPUs
> > are so fast in RAM and a block is very small. On x86 systems, depending
> > on page alignment, we are talking about two or three pages that will be
> > "in memory" (They were used to read the block from disk or previously
> > accessed).
>
> Your optimism is showing ;-).  XLogInsert routinely shows up as a major
> CPU hog in any update-intensive test, and AFAICT that's mostly from the
> CRC calculation for WAL records.
>

Yeah... for those who run on filesystems that do checksumming for you, I'd bet 
they'd much rather see time spent in turning that off rather than 
checksumming everything else.  (just guessing) 

-- 
Robert Treat
Build A Brighter LAMP :: Linux Apache {middleware} PostgreSQL

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