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Re[2]: journal on an ssd



Just a random thought, and anticipating that the experts will say that if an entire journal is lost (not present) the main data is still accessible / recoverable (in its previous state).



Is it perhaps the case that, to maximize the integrity of the main data,  one would *want* the journal to have a different failure pattern?

That, if there were any doubt about journal integrity, it would be better (for the integrity of the main file system) to discard the journal entirely?

This would suggest the use of a robust hash / cryptographic digest of the journal contents, stored with it and checked each time the journal is about to be used. These are quite quick to compute nowadays.

Any potential in this speculation?

Chris Haynes



On Thursday, September 11, 2008 at 6:43:18 AM, Tobias Oetiker wrote:
> 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



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