Jochem,
----- Original Message ----- From: "Jochem van Dieten" <[EMAIL PROTECTED]>
Newsgroups: mailing.database.myodbc
Sent: Wednesday, February 23, 2005 1:01 AM
Subject: Re: wikipedia down, slashdot covering, mysql mentioned
On Wed, 23 Feb 2005 00:22:55 +0200, Heikki Tuuri wrote:
a buggy fsync() in Linux is one of the possible reasons here. If an InnoDB
tablespace gets corrupt in a power outage, it is most probably caused by a
bad fsync() implementation or configuration in the operating system or
hardware. An fsync() call should write the data physically to disk or to a
battery-backed, non-volatile disk cache. If it fails to do so, then any
database, PostgreSQL or anything, can get corrupt.
Can they? Even if the absolute ordering of writes is maintained, only full pages are written and each page has a checksum? It seems to me that then you might lose transactions that have not yet committed to disk, but your database would not get corrupt.
you are right, if the disk subsystem writes database pages to the disk or a non-volatile cache in the same order that fsync() was called on the pages, then the database cannot get corrupt. Corruption could only result from a partial page write, and against that InnoDB uses the 'doublewrite' method:
http://dev.mysql.com/doc/mysql/en/innodb-disk-i-slash-o.html
In the Wikipedia case, the disk subsystem apparently had written partial pages for whom the good copy of the page was not present in the doublewrite buffer. That means that the writes could not physically happen in the same order that fsync() was called on them. Thus, fsync() was not a 'barrier' in disk writes.
Jochem
Regards,
Heikki
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