| From: | Gregory Stark <stark(at)enterprisedb(dot)com> |
|---|---|
| To: | Alvaro Herrera <alvherre(at)commandprompt(dot)com> |
| Cc: | Zdenek Kotala <Zdenek(dot)Kotala(at)Sun(dot)COM>, Pg Hackers <pgsql-hackers(at)postgresql(dot)org> |
| Subject: | Re: Block-level CRC checks |
| Date: | 2008-10-30 15:41:17 |
| Message-ID: | 87tzauay5e.fsf@oxford.xeocode.com |
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| Lists: | pgsql-hackers |
Alvaro Herrera <alvherre(at)commandprompt(dot)com> writes:
> Ah, actually there's another alternative -- leave the checksum on its
> current position (start of struct) and move other members below
> pg_pagesize_version (leaning towards pd_tli and pd_flags). That'd leave
> the page version in the same position.
I don't understand why the position of anything matters here. Look at TCP
packets for instance, the checksum is not at the beginning or end of anything.
The CRC is chosen such that if you CRC the resulting packet including the CRC
you get a CRC of 0. That can be done for whatever offset the CRC appears at I
believe.
--
Gregory Stark
EnterpriseDB http://www.enterprisedb.com
Ask me about EnterpriseDB's 24x7 Postgres support!
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