PB et al I'm being lazy and not looking up a term.
"Journaling" what effect does it have. It certainly seems to keep drive speeds consistent over time on the G5/RAID or is that a factor of daily cleanups. TIA
On a quick lookup it appears that journaling offers significant stability and recovery advantages and apparently IS backward compatible to aid HFS+ volumes in a similar manner.
Maybe that's part of why Panther seems so bulletproof. The underlying system structure is better protected.
My guess this is an advantage for quick boxes like the G5 and may be a drag on slower systems adding significant overhead.
Journaling offers no real speed enhancements, but it does offer mush in the way of stability. basically, journaling keeps track of what changes are about to be made and what changes have been made to files.
To paraphrase another source, Imagine a library, and each night a librarian goes around with a stack of books and puts them back on the shelves. when she is done she writes down in the log all the books she put back.
One night she is in the middle of putting the books back and she dies of a heart attack.
Now what is the replacement supposed to do? How is she supposed to find all of the books that were put back? She would have to check every book against the checked in and put back log, eventually finding that all those not on the list were the ones that her predecessor done the night she died.
Now imagine that the first librarian had carried a notebook around with her (a journal, if you will) that had a list of all the books to be put back and she marked each one off as she put it back. Now when she has that heart attack the new librarian just has to look at the notebook to see where to continue.
That's a journaled filesystem. It keeps detailed track of changes to files before and after they occur.
Filesystem corruption is very hard to have happen with a journaled file system (although not impossible), and if any does occur it is very easy to fix. Also, because there are less problems like corruption associated with journaled filesystems your productivity can and probably will go up because you don't have to worry as much about problems happening and they are faster to fix if they do.
With a journaled file system you should be able to start a huge disk write procedure involving gigs worth of files and then literally pull the power cord half way through and not see any issues, and on top of that under some implementations the disk write will start up again from where it left off when the system is rebooted (although I am not sure if Apple's implementation does this).
Some implementations also offer a history function where you can roll back changes to a file, which is very cool.
One thing to note about Journaling (at least under OS X 10.2.x) is that it does break FSCK. Probably though you wont need it anymore anyway. Also, it willl add some overhead to your write speeds but quicker systems probably wont be affected and from what I have read even older systems don't see that much of a lag.
If you have a RAID setup, journaling could very easily save your butt (especially on mirrored setups) because afterall, two drives are twice as likely to become corrupt as one.
Oh, and Apple did write it on top of HFS+ as well.
PosterBoy's report on journaling is essencially correct.
I enable Journaling on just about all the systems I install these days. I definetly use it on any servers and workstations under heavy load or with lots of files.
Here's a case in point:
A couple of years ago I was maintaining a network of 40 Macs in a design sweatshop on Netware 4 (don't ask, I couldn't understand it either). ANyhow, that Netware server would crash at least a couple of times a week resulting in about 2 HOURS of downtime while the file system went through a consistency check upon a reboot.
My linux servers (formatted as ext3 journaled) on the other hand contain way more files and data that the Netware server ever did and if I pull the plug or have a power outage those machines reboot, catch the filesystem problem, read the journal and keep booting in a matter of SECONDS. HUGE difference.
Same thing happens on the Xserves I setup. Journaling ON, always.