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

John D'Albis jdalbis at pilotsoftware.com
Fri Nov 17 13:06:10 PST 2006

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:
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?
John D'Albis

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