difference between mount point and physical drive?

[Geert Janssens]

Tim Seufert wrote:
> On Friday, October 25, 2002, at 11:49  AM, Michael George wrote:
> 
>>
>> On Friday, October 25, 2002, at 02:37  PM, Albrecht Dreß wrote:
>>
>>> Am 25.10.02 19:45:05 schrieb(en) Stephen Lewis:
>>>
>>>> Sometimes it is faster than reading the
>>>> filesystem, but because it copies empty
>>>> blocks also it might be slower too. If your
>>>
>>>
>>> It's sometimes nice to burn complete partitions compressed on a 
>>> cd/dvd. To avoid that you have random data on the unused space, you 
>>> can use e.g.
>>>
>>> dd if=/dev/zero of=zerofile bs=1M count=1
>>
>>
>> This will fill the disk with data (even if it is all 0's).  I am 
>> guessing that one would want to make sure that nothing else is writing 
>> to the partition and you don't want to do this to your / partition 
>> (which would be a bad idea anyhow)?
> 
> 
> Doesn't matter whether anything else is writing to the partition, and 
> it's perfectly safe to do it to the root FS.  He's using dd to generate 
> a regular file (named "zerofile"), not to write directly on a 
> partition.  The idea is simply to create a file occupying all the free 
> space on the target FS, filling it with 0 rather than the leftover 
> random contents from old file deletes.  Later on, gzip will get enormous 
> compression ratios on the long strings of zeroes this generates, 
> dramatically reducing the size of the partition image if there is much 
> free space on the partition.
> 

Also note that Albrecht's explanation stated:
- create a zerofile occupying the partition's free space
- remove the zerofile
- ... rest of the backup stuff

This means that all free space is set to zero's, but eventually remains 
free space. In other words the partition is only completely full for a 
very short time (I believe that this full partition state is what 
Michael is worried about).

For the backup it doesn't really matter if the partition is completely 
full (since the compression takes care of reducing the zerofile), but 
the live system may choke on a full / partition.

Cheers,
Geert