Re: wikipedia down, slashdot covering, mysql mentioned

[Greg Whalin]

I was under the impression that fsync() was only buggy in Linux in the 
2.4 kernels.  Is it still problematic in 2.6 series?

Greg
--
greg@stripped
Meetup.com

Heikki Tuuri wrote:
> 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
> 
> Order MySQL Network from http://www.mysql.com/network/
> 
> 
> ----- Original Message ----- From: "Peter Wilm" <listen@stripped>
> 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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> 
>