Re: Journaling: Surviving or allowing unclean shutdown?

[egger@suse.de]

On  4 Jan, Richard B. Johnson wrote:

> A mobile-phone that runs out of battery power will also  lose all the
> phone numbers you have stored, etc. The same is true for most all
> embedded systems that save data.

 In your world maybe. I would be quite pissed if my mobile phones lost 
 the stored numbers every time they run out of power. Nearly all embedded 
 devices nowadays keep their settings without power; be it a satellite
 receiver, a PBX, a fax machine or a coffee brewer. 

> This means that the data-base
> software has to roll-back and redo the temporarily-lost updates
> when it restarts. It uses the journal to accomplish this. As
> N-seconds gets smaller, the overhead necessary to maintain data
> consistency gets greater, so there are trade-offs.

 And depending on the application they may really be worth it.

> A journaling file-system also needs some number to show the
> order of operations so that roll-backs and restarts work.
> Unfortunately, the systems that I have seen all use time for
> the number! You don't want time to reconstruct 'order'. You
> need a number that represents 'order'. Time is not your friend.

 Since the metadata has to be sync anyway what about using a
 normal transaction counter?

-- 

Servus,
       Daniel

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