| From: | marcin mank <marcin(dot)mank(at)gmail(dot)com> |
|---|---|
| To: | Heikki Linnakangas <heikki(dot)linnakangas(at)enterprisedb(dot)com> |
| Cc: | Simon Riggs <simon(at)2ndquadrant(dot)com>, Alvaro Herrera <alvherre(at)commandprompt(dot)com>, Pg Hackers <pgsql-hackers(at)postgresql(dot)org> |
| Subject: | Re: Block-level CRC checks |
| Date: | 2009-12-01 13:38:26 |
| Message-ID: | b1b9fac60912010538w18fd2510s519d2bfd9353e83d@mail.gmail.com |
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| Lists: | pgsql-hackers |
On Mon, Nov 30, 2009 at 9:27 PM, Heikki Linnakangas
<heikki(dot)linnakangas(at)enterprisedb(dot)com> wrote:
> Simon Riggs wrote:
>> Proposal
>>
>> * We reserve enough space on a disk block for a CRC check. When a dirty
>> block is written to disk we calculate and annotate the CRC value, though
>> this is *not* WAL logged.
>
> Imagine this:
> 1. A hint bit is set. It is not WAL-logged, but the page is dirtied.
> 2. The buffer is flushed out of the buffer cache to the OS. A new CRC is
> calculated and stored on the page.
> 3. Half of the page is flushed to disk (aka torn page problem). The CRC
> made it to disk but the flipped hint bit didn't.
>
> You now have a page with incorrect CRC on disk.
>
What if we treated the hint bits as all-zeros for the purpose of CRC
calculation? This would exclude them from the checksum.
Greetings
Marcin Mańk
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