Re: Free Space Monster (episode 2)

In article
<noneof-85C006.06150612062007@xxxxxxxxxxxxxxxxxxxxxxx>,
Mark Conrad <noneof@xxxxxxxxxxxxxxxxxx> wrote:

Geeky System Stuff,

-or-

Kicking a Dead Horse by Wiping a Partition with Hex Zeroes


Did you ever try to compress a backup file, then wonder why the
compressed backup still gobbled up more disk space than it should?

The culprit could be the dreaded Free Space Monster.

Just for fun, I decided to create a backup of an EMPTY partition, a very
small 164 MB partition named "Pismo Notes Partition".

Looked at the backup I just created, it was 156.2 MB, so decided to
compress it using the regular "Create Archive" in OSX's File menu.

Darn compressed file was still over 137 MB !!!

Did not compress much at all.


Remember, this partition is EMPTY, nothing in it except the file system,
which is about 3 MB in size.

Get Info sez there is 161.4 MB of available space in the 164 MB
partition.



What I did to remedy this nasty situation -
*******************************************

I merely wiped all the free space in the partition, using TechTool.

Created a new backup file, it was 156.2 MB, same size as original backup
file was.

Compressed this new file, the resulting compressed file was 0.15625 MB,
almost 880 times smaller than my first compression attempt.



Only thing I did different was to wipe all the free space in the
partition with hex zeroes.


Any additional details will be gladly furnished, for those interested.


Practical application would be to allow Mac users to create a bootable
small image backup of a Vista partition on their Intel Macs.

Conventional backup methods like using the Vista version of Ghost will
not work, for various reasons.

Unfortunately, I have not been able to find a Vista utility that is
capable of wiping the free space of a Vista partition. (on a Mac)

Mark-

But if I recall correctly, you prefer to use 'dd' as your backup
utility, so you are backing up all sectors whether the file system
is currently using the sectors or not.

A file system backup utility only backs up the file data and
associated metadata for the files. This should have generated a
backup more in line with a 3MB file system.

File systems DO NOT zero freed sectors. It is a waste of time and
I/O bandwidth, since the next time the file system needs to use
those sectors, it is just going to write something on top of the
old data anyway. (The exception to not zero'ing freed sectors
would be if you use a secure erase, and even then they are more
likely to leave a totally random pattern on the erased sectors to
make it even more difficult to read back the old data; but that
random pattern would make it even more difficult to compress when
read by 'dd').

Bob Harris
.

Relevant Pages

  • Free Space Monster (episode 2)
    ... Kicking a Dead Horse by Wiping a Partition with Hex Zeroes ... Did you ever try to compress a backup file, ... compress it using the regular "Create Archive" in OSX's File menu. ... I merely wiped all the free space in the partition, ...
    (comp.sys.mac.apps)
  • Re: How to "wipe" ALL Mac files slack space?
    ... You have a partition with a filesystem where the available space ... You want to make a backup of this filesystem but for some bizarre ... backup is going to be nearly 100 times bigger than the filesystem. ... I will just compress it. ...
    (comp.sys.mac.system)
  • Re: Equivalent Replacement Windows Apps for a "Switcher"
    ... partition to a single image file. ... want to clone my startup disk to another physical disk. ... That's a very silly backup strategy IMO. ... I see no reason to bother cloning disk sectors. ...
    (comp.sys.mac.apps)
  • Re: How to "wipe" ALL Mac files slack space?
    ... You have a partition with a filesystem where the available space ... You want to make a backup of this filesystem but for some bizarre ... backup is going to be nearly 100 times bigger than the filesystem. ... I will just compress it. ...
    (comp.sys.mac.system)
  • Re: Save entire hard disks
    ... Others use acronis true image. ... entire partition to a second partition and do incremental backups and ... compress the image. ... it was when you made the backup so any installed programs will work the same ...
    (microsoft.public.windowsxp.general)
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