Drew wrote:
>
>> the "Erase Disk" command . . .
>
> . . . doesn't necessarily reformat the partition. Someone more in
> the know than I will correct me if I'm wrong, but I think choosing that
> option just wipes the section of the desktop db/df that corresponds to
>the partition (essentially erasing the disk), then marks the files on the
>disk as deleted. I'm not entirely sure what happens if you choose to
>change the HFS scheme (Mac OS Standard <->
>Mac OS Extended) when you erase the disk, though. "Erase Disk..." works
>differently on floppies, of course.
>
>I'm not sure how this affects disk fragmentation. I don't really know any=
>thing about how "Erase Disk..." works. It just might be the answer to a =
>quick reformat. OTOH, you won't be able to erase the
>volume if you have VM turned on and set to use that volume... So you'll ha=
>ve to turn VM off before you erase the volume.
(Disregarding floppies) formatting and initializing (as with a Finder
"Erase Disk") aren't the same thing. Formatting magnetically lays out the
pattern of tracks and blocks on the disk (the structure), while
initializing simply indicates that an area of the disk is available for
use (the contents.) It's more complicated than that of course, but I'm
trying to keep it simple.
As Drew correctly described, initializing a Mac volume ("Erase Disk")
merely deletes the directory database files which tell the OS where files
are located in that partition. The empty replacement directory files on a
newly initialized volume tell the OS, "It's ok to write anywhere you like
in this area." As a side note, initializing does _NOT_ erase the volume
or drive, but merely marks it as available for use, the old data still
there until something else is written over it.
re: Fragmentation and VM - Since the OS sees a blank canvas (an "erased
disk") on which to write, it doesn't need to dance around a bunch of
existing files. Also, I'm pretty sure it deletes the old swap file on
shutdown and creates a new swap file on starting up. This is easy to test
of course, with VM enabled, do a proper shutdown, then start again with
extensions off and see if one's invisible VM file has disappeared from
its partition. Err, I'd do it right now but this-old-Mac has too much ram
to use VM. : >)
You'll notice that Drive Setup won't allow you to "Low-level Format" an
ATA drive, as those are "Formatted" using special equipment. Dumb drives.
OTOH, scsi disks _can_ be formatted (with DS or other utility.) Smart
drives. "Zeroing" a drive is _not_ formatting it, it's just overwriting
the drive's contents with null data to eliminate whatever data might
already have been present.
Whew, writing this was harder than I expected. I know the basics pretty
well, but it required some bit of thought (at 2 AM!) to regurgitate in a
useful way. : >)
Dan K
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