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Re: journal on an ssd



Folks,

Yesterday Andreas Dilger wrote:

> On Sep 10, 2008  18:05 +0200, Tobias Oetiker wrote:
> > I have not tested this, but since we are putting about 16 different
> > journals on this one ssd, I would assume that the loss through
> > seeking between the journals would be pretty bad, and again bring
> > back that inter-filesystem-dependency we were trying to loose with
> > this measure.
>
> The cost of putting the journals on 16 separate, relatively small
> disk devices would probably be comparable to the cost of the SSD
> and not have a single point of failure.  The journal does mostly
> linear IO, so performance is probably equal or better.

You are telling me things that I am aware of. The reason I wrote to
this group is to figure what would happen to an ext3 fs when the
external journal was lost, especially what happens when it is lost
on a filesystem where 'data=journal' is set.

Because if it is catastrophic, then it basically means that the
journal has to reside on a device that is as secure as to rest of
the data, meaning that if the data is on RAID6 then the journal
should be on RAID6 too.

What I am hoping for, is that someone tells me, that in the case of
'data=journal' the loss would only be the material that is still in
the journal (eg 30 seconds worth of data) and the rest of the fs
would have a fair chance of being recoverd with fsck.

cheers
tobi


-- 
Tobi Oetiker, OETIKER+PARTNER AG, Aarweg 15 CH-4600 Olten, Switzerland
http://it.oetiker.ch tobi oetiker ch ++41 62 775 9902 / sb: -9900


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