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Re: Block-level CRC checks

From: Aidan Van Dyk <aidan(at)highrise(dot)ca>
To: Greg Stark <gsstark(at)mit(dot)edu>
Cc: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Bruce Momjian <bruce(at)momjian(dot)us>,Simon Riggs <simon(at)2ndquadrant(dot)com>,Heikki Linnakangas <heikki(dot)linnakangas(at)enterprisedb(dot)com>,Alvaro Herrera <alvherre(at)commandprompt(dot)com>,Pg Hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Block-level CRC checks
Date: 2009-12-02 04:23:29
Message-ID: 20091202042329.GI15507@oak.highrise.ca (view raw or flat)
Thread:
Lists: pgsql-hackers
* Greg Stark <gsstark(at)mit(dot)edu> [091201 20:14]:
 
> I'm not sure we're on the same page.  As I understand it there are
> three proposals on the table now:
> 
> 1) set aside a section of the page to contain only non-checksummed
> hint bits. That section has to be relocatable so the crc check would
> have to read the start and end address of it from the page header.
> 
> 2) store the hint bits in the line pointers and skip checking the line
> pointers. In that case the crc check would skip any bytes between the
> start of the line pointer array and pd_lower (or pd_upper? no point in
> crc checking unused bytes is there?)
> 
> 3) store the hint bits in the line pointers and apply a mask which
> masks out the 4 hint bits in each 32-bit word in the region between
> the start of the line pointers and pd_lower (or pd_upper again)

I'm not intimately familiar with the innards of the pages, but I had
*thought* that the original suggestion of moving the hint bits was
purely to make sure that they are in the same filesystem block/disk
sector as the CRC.  That may not be possible, but *if* that's the case,
you avoid the torn-page problem, with only 1 minimal assumption:
  - the FS-block/disk-sector will write whole "blocks" at a time, or
    likely be corrupt anyways

With my understanding of disks and platters, I'ld assume that if you got
a partial sector written, and something prevented it from being
completely written, I'ld guess the part missing would be smeared with
corruption....  And that would seem to hold with flash/SSD's too...

a.

-- 
Aidan Van Dyk                                             Create like a god,
aidan(at)highrise(dot)ca                                       command like a king,
http://www.highrise.ca/                                   work like a slave.

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