[p4] Trying to understand the benefits of "Warning! We stronglyrecommend that you set up your system so that the journal iswritten to a filesystem other than the P4ROOT filesystem."

[Sweeney, Tony]

Having access to the journal after a db failure allows for intra-day
recovery in the three disk case (disk 1: db files; disk 2: depot files;
disk 3: checkpoints and journal).  You can recover from last night's
checkpoint, AND replay the day's journal to get complete point-in-time
recovery in the event of loss of the db files, rather than just recover
to last night's checkpoint.

Tony.

quis custodiet ipsos custodes -- Juvenal VI, 347-8  

> -----Original Message-----
> From: perforce-user-bounces at perforce.com 
> [mailto:perforce-user-bounces at perforce.com] On Behalf Of John D'Albis
> Sent: 17 November 2006 21:06
> To: perforce-user at perforce.com
> Subject: [p4] Trying to understand the benefits of "Warning! 
> We stronglyrecommend that you set up your system so that the 
> journal iswritten to a filesystem other than the P4ROOT filesystem."
> 
> Currently, we have the journal and depot files on the same physical
> disk.  We don't follow how this change would actually help in a
> restore... It is repeated several times in the admin guide making it
> appear to be an imperative:
> http://www.perforce.com/perforce/doc.061/manuals/p4sag/02_back
> up.html#10
> 47307   
>  
> The only use we can determine for having this file on a separate file
> system is that, if we were to lose only the non-journal disk - it will
> show which updates will not be included in the restore. What are we
> missing?  Here is our logic:
>  
> Assumptions:  We do a checkpoint followed by a full backup every night
> at 3 am.  If, in the case of any disk failure, we want to be able to
> restore to a known state as quickly as possible, with the least amount
> of work lost.  Journal and checkpoint are on disk A, *.db files and
> Depot are on Disk B.
>  
> Scenario a) disk A crashes, lose Journal and checkpoint.
>  
> Recovery procedure:  Install a new A disk - create new checkpoint file
> and start a new journal.  No work is lost.
>  
> Scenario b) disk B crashes, lose *.db and depot files.
>  
> Recovery procedure:  Install a new B disk - restore depot from backup
> AND recreate *.db files from checkpoint on Disk A.  (While we 
> also have
> a journal file and hence the ability to restore the metadata 
> to a state
> that is newer than the depot, doing so would be fruitless as the depot
> files themselves would only be at the state of our last 
> backup - 3 am).
> Work since 3 am is lost.
>  
> Scenario c) disk A & B crashes - lose everything!
>  
> Recovery procedure:  Similar to scenario b. Work since 3 am is lost.
>  
> Is there a scenario that we're missing?
>  
> Thanks,
>  
> John D'Albis
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