Peter,
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.
In the Wikipedia case, James Day posted an error message that InnoDB printed. It showed that the page checksum was wrong. Probably fsync() had failed to flush some part of a 16 kB page to disk.
Unfortunately, fsync() is badly implemented in many, or most, OS/hardware configurations. Some ways to improve the situation:
- use hdparm, or proprietary disk configuration tools of the disk vendor to switch off write caching in the SCSI controller and the disk itself;
- use a battery-backed disk cache in the SCSI disk controller;
- use certified hardware from one vendor: for example, Solaris/Sparc might work more reliably than an arbitrary Linux/x86 configuration;
- do not use disk striping, software RAID, NFS, or anything exotic;
- use the MySQL replication to keep a backup server.
None of the above is guaranteed to remove corruption problems, and switching off write caching can seriously lower performance. In the Wikipedia case, battery-backed SCSI controllers did not help at all. Thus, using an Uninterrupted Power System is probably the best way to prepare against power outages.
It is possible to implement fsync() correctly and efficiently in the operating system and hardware. There are no physical limitations that would make it impossible. But, unfortunately, in practice, many implementations are seriously flawed.
Best regards,
Heikki Tuuri
Innobase Oy
Foreign keys, transactions, and row level locking for MySQL
InnoDB Hot Backup - a hot backup tool for InnoDB which also backs up MyISAM tables
http://www.innodb.com/order.php
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----- Original Message ----- From: "Peter Wilm" <[EMAIL PROTECTED]>
Newsgroups: mailing.database.myodbc
Sent: Tuesday, February 22, 2005 12:40 PM
Subject: Re: wikipedia down, slashdot covering, mysql mentioned
Could this possibly be a problem with a bad fsync implementation in linux (< 2.6.5)? See: http://www.ussg.iu.edu/hypermail/linux/kernel/0403.2/0527.html
Scott Haneda schrieb:There is a thread over at /. about WikiMedia being out due to a power outage
<http://slashdot.org/articles/05/02/22/0151213.shtml?tid=95>
MySql is getting bashed pretty hard in some cases as apparently, in power
failures, you get database corruption. (this is all from the posts, not my
opinion at all, I am asking only to learn more) Also, it is mentioned the
MySql devs just do not have data corruption from power failure a high
priority.
Many are leaning on Postgres. For me, MySql has never let me down (Knock on
PSU) but I have to ask, for more data on this matter. I run MySql on OS X,
I sometimes have to reboot, it comes back up fine, I do however, do nothing
special to shut down mysql, should I be?
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