At 4:35 PM +1100 01/17/2006, Chris Zantides wrote:
I have this machine with 448 (3x128 1x 64) ram and a 40 gig maxtor
7200 rpm drive running 10.4.4. quite happily.
I was wondering why journalling is all about - do I need it? Do I
take a performance hit by having it on?
Short answer: No. Yes.
But IMO, it's well worth it. Here's why:
Unjournaled: Write your data to a disk cache (in RAM) then
periodically flush (write) the cache to disk. This form of virtual
writes followed by a massive physical write speeds up applications.
But it also means you can loose more data when you crash -- which
makes disk repair functions more iffy.
Journaled: Write not only to a disk cache, but also keep a "journal"
of the acutal write commands separately. You get almost the same app
speed-up; loosing only a little because of the double physical write.
Finally, when a bad crash happens, the journal is automatically used
to "play back" the disk writes .... which almost always guarentees
that perfect disk integrity is maintained.
HTH,
- Dan.
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