Journaling is a technique used to improve reliability in the event of a
system failure while a disk is being written. The journal is a log file
that states the *intent* to write something to the disk as well as a
note that it's not yet completed. Then the actual writing of the real
data is started. If successful, the intent stated in the log is
cleared. If the system crashes in the middle of the write of the real
data it will be detected by reading the log on restart and the system
can continue where it left off.
Journaling has no direct effect on the efficiency of storage and the
possible need for defragmentation except that a non-journaling system
could occasionally lose track of unused but allocated space. B-tree
file systems are typically very efficient at avoiding any significant
fragmentation in the first place unless the space is getting very full.
Once that happens nothing but defrag can recover what's there as
contiguous space. But that begs the question of whether it's worth it.
Probably not.
Chuck Norcutt
On 9/27/2010 1:03 PM, Chris Barker wrote:
> It is too small a space, certainly, but he won't need to defrag the
> drive. Apple, and techies, reckon that the journalling, if enabled,
> will obviate the need for moving files around.
>
> Chris
>
> On 27 Sep 2010, at 17:45, Chuck Norcutt wrote:
>
>> If you've only got 22GB left on a 500GB drive it's also likely that
>> the 22GB available is badly fragmented which means lots of head
>> motion to find and use the available space. You could defrag it
>> but that might just be a stop gap measure.
>>
>> An analogous situation might be you and 9 of your buddies are going
>> to a football game where the stadium has a 500 car parking lot.
>> You're each going to drive your own cars since you all have to go
>> different directions after the game. You'd really prefer that
>> everyone be able to park close together so you can easily find each
>> other and you'd also like to be close the exit for a fast getaway.
>> But when you get there you find there are already 480 cars in the
>> lot and the 20 available spots are scattered all over. You first
>> all have to spend a lot of time trying to find an empty spot and
>> then more time trying to get yourselves back into a group.
>
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