file size on disk vs used drastically different

[Shawn Badger]

this is on an ext3 filesystem

On Thu, Apr 15, 2010 at 1:16 PM, Eric Shubert <ejs at shubes.net> wrote:

> der.hans wrote:
>
>> Am 15. Apr, 2010 schwätzte Shawn Badger so:
>>
>>  I came across a weird problem this morning. What would cause a file to be
>>> reported as 251M for used space and 1.3G for size on disk?
>>>
>>> [root at cc1lnx5 axprac]# ls -sh cafrap_1.dbf; ls -lh cafrap_1.dbf
>>> *251M* cafrap_1.dbf
>>> -rw-r----- 1 oraxprac axprac *1.3G* Apr 15 09:47 cafrap_1.dbf
>>> [root at cc1lnx5 axprac]#
>>>
>>
>> A bunch of nulls.
>>
>> Essentially, the file has allocated 1.3GB of space, but since a bunch of
>> what it's storing are nulls the filesystem cheats and doesn't use space
>> for them.
>>
>> That space can expand out during backups and other operations, so be
>> careful copying it around.
>>
>>  I have seen this to a smaller extent with some files but never a variance
>>> of
>>> this size.
>>> This file happens to be an Oracle 11G database table file.
>>>
>>
>> I believe Oracle allots a configured amount of space for DB tables. The
>> space alloted but not used should be nulls.
>>
>> $ qemu-img create /tmp/Beispiel.img 10G
>> Formatting '/tmp/Beispiel.img', fmt=raw size=10737418240
>>
>> $ ls -sh /tmp/Beispiel.img ; ls -lh /tmp/Beispiel.img 0 /tmp/Beispiel.img
>> -rw-r--r-- 1 lufthans lufthans 10G 2010-04-15 11:06 /tmp/Beispiel.img
>>
>> It is to filesystem allocation what ticket overselling is to airlines :).
>>
>> ciao,
>>
>> der.hans
>>
>>
> Interesting.
>
> I wonder if VMware thick virtual disks (vmdk files) exhibit this same kind
> of behavior. Anyone know?
>
> Shawn, which type of filesystem is this using?
>
> hans, which filesystem types (do you know of that) do this sort of 'cheat'?
>
> --
> -Eric 'shubes'
>
>
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