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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: (view raw or whole 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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