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Re: Cost of XLogInsert CRC calculations

From: Heikki Linnakangas <hlinnaka(at)iki(dot)fi>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: pgsql-hackers(at)postgresql(dot)org
Subject: Re: Cost of XLogInsert CRC calculations
Date: 2005-03-06 08:24:55
Message-ID: Pine.OSF.4.61.0503060923210.1725@kosh.hut.fi (view raw or flat)
Thread:
Lists: pgsql-hackers
On Sun, 6 Mar 2005, Tom Lane wrote:

> I suppose that the bulk of the CPU cycles being attributed to XLogInsert
> are going into the inlined CRC calculations.  Maybe we need to think
> twice about the cost/benefit ratio of using 64-bit CRCs to protect xlog
> records that are often only a few dozen bytes.

Isn't the CRC quite important on recovery to recognize where the last 
valid log record is?

Is there any better implementations of CRC-64? Would using a different 
polynomial help?

Would it help to do the CRC calculation in a more wholesale fashion in 
XLogWrite?

How about switching to CRC-32 or even CRC-16? I searched the archives for 
the reason CRC-64 was chosen in the first place. It seems that the 
difference in computation time was not considered to be significant, and 
there was 8 bytes available in the record header anyway.

Just some thoughts...

- Heikki

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