gjournal questions and observations

[Ross Gohlke]

KUDOS
Congratulations to all GEOM contributors. While I am new to GEOM, so far 
I am very impressed with the way it is designed and the capabilities 
(both realized and anticipated) the design offers.

QUESTIONS
1. What is the best way to journal whole disks whose slices (without 
partitions) are used by gconcat and gmirror? Does the same apply for gvinum?
The ultimate scenario seems to be journaling another GEOM class such as 
gmirror because gjournal handles the synchronization of all mirror 
consumers. You can turn off autosync on the mirror, thus saving CPU 
cycles and improving disk access. (Am I right?)

2. How should gjournal and gmirror be configured when the journal is 
outside, instead of inside, the mirror?
The above scenario only seems possible if a) you are willing to journal 
slices, which is not best practice [1] [2] or b) you use whole disks in 
your mirrors, which is not very realistic.
Further I am on PowerPC and don't even have bsdlabel, so journaling 
slices and mirroring partitions is not an option anyway.
My thought was to journal each disk separately, outside the mirror, and 
keep autosynchronization on for the mirror.

[1] 
http://lists.freebsd.org/pipermail/freebsd-questions/2008-April/173501.html
[2] 
http://unix.derkeiler.com/Mailing-Lists/FreeBSD/questions/2008-11/msg00247.html

3. What is the best way to completely remove a whole disk journal such 
that re-issuing
% gjournal label /dev/ad0
does not require -f?
I have tried gpart destroy/create and newfs -E. I have not tried 
blanking the whole disk with dd, nor have I tried newfs -E on the whole 
disk.

4. Does it matter whether gjournal is loaded when gjournal label is issued?
Originally I was journaling slices, and I was unable to properly stop a 
particular slice.
% gjournal stop ad0s6.journal
% gjournal list
Showed the slice still loaded, but under a different name:
ie, ufsid/48x6x1bxc39394x7
I don't know if it was related to the originally issued gjournal label 
command or that gjournal was loaded before the slice was labeled.
I believe what fixed it was:
% dd if=/dev/zero of=/dev/ad0s6

OBSERVATIONS
  GJOURNAL
These may seem obvious to you but were hard won by me.
1. Never try to store a whole disk's journal on a slice of the same disk.
2. When journaling a whole disk and keeping the journal on the same 
disk, not only must the very last sector of the disk be reserved, but 
also the last gig (or whatever the size of your journal). Just leave it 
as free space when you create slices with gpart.
While gjournal man page states journaling an existing file system 
REQUIRES a separate device for storing the journal, it appears to work 
without specifying a second device. At least
% gjournal label -f /dev/ad0
seems to work, using the end of /dev/ad0 to store the journal whether a 
  slice occupies those sectors or not. (Consequently, trying to gmirror 
the last slice when it occupies journal sectors will fail.)

Regards,
Ross Gohlke