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Re: journal on an ssd



On Wed, 10 Sep 2008 13:30:45 +0200, Tobias Oetiker wrote:

> What happens if the disk hosting an external journal of a filesytem
> running with data=journal goes bust.

Probably the same as if the journal was on the same disk, going bust. :-)
Or rather :-( as this can indeed get pretty ugly.
With ext3 you can always fall back to mounting as ext2 and at least try to
recover as much as possible.

> Recently we added an SSD to our setup and have moved all the journals to
> this ssd. This has dramatically improved performance and especially
> reduced the interdependence between performance of different partitions
> hosted on the same RAID.

That is one of the great SSD uses, yes.

>  http://insights.oetiker.ch/linux/external-journal-on-ssd.html

Very interesting, thanks! I was planning to do the same but waiting for
the Intel SSDs to come to market or the large OZCs to come down in price,
whatever happened first..

> I realy like the performance of this new setup, but I am not all that sure
> about the data security aspects of it. Especially after reading
> 
>  http://www.cs.wisc.edu/adsl/Publications/sfa-dsn05.pdf
> 
> which suggests that damaged journals are the worst that can happen to
> ext3.

True, a borked journal is bad but with the SSD you should actually have
*less* chance of corruption (of the type mentioned in the paper), since
the wear-leveling should keep the journal blocks alive without the
file system/block layer noticing. At least in theory.. :-D

You may also find this interesting:
http://labs.google.com/papers/disk_failures.html

Holger



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