[ILUG] Major Boot Disaster FC1... Journalised Filesystems

[Paul Jakma]

On Mon, 26 Apr 2004, Farrell, Kevin wrote:

> Journalised filesystems work by saving filesystem "meta data" at
> periodic intervals (checkpoints). 

No, they dont work by periodic saving, they work by keeping a journal
of what they're about to do, and what they have done. Be it data or
meta-data.  (journalled writes of data lower performance by half
though, data gets written twice, once to journal, once to actual
block).

> checkpoint with a complete "journal". 

journalling != checkpointing

> It is a common misconception
> that journalised filesystems are designed to protect your ordinary
> data; i.e. your kid's school project! This is NOT the case. A
> journalised filesystem is designed to protect your filesystem!

The data=journal option of ext3 disagrees with you.

> Luckily, recovery of the filesystem has essentially nothing to do
> with a "race between the CPU, disk electronics and disk motor"!

It can do, for a drive that lies about write caching (as IDE disks
sometimes do) and depends on the momentum of the platters to write
out its cache when power fails.
 
> If it's your data you want to protect, perform regular backups. ;-)

That too.
 
> Regards,
> 
> Kevin.
> 
> --
> Dr. Kevin Farrell,

regards,
-- 
Paul Jakma	paul at clubi.ie	paul at jakma.org	Key ID: 64A2FF6A
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