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I'm running Ubuntu Squeeze and on one of the partitions df is showing the Total size as 335G:

Filesystem            Size  Used Avail Use% Mounted on
/dev/sdb              335G  225G   94G  71% /mnt

However in the past it was showing as 360GB (which is the actual size):

 fdisk -l /dev/sdb

Disk /dev/sdb: 365.0 GB, 365041287168 bytes

lsof +L1 does not return anything (and anyway if this would be the case the Total space should not be affected.)

On this partition I'm writing (and deleting) a lot of files and this happened before in the past, but problem solved by itself.

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By default with ext*, 5% of partition space is reserved for super user / maintenance use, so that alone reduces the space you will have quite a bit. Five percent from 360 GB would be 3.6 * 5 = 18 GB.

Then, if you have lots and lots of small files around, that will reduce the space available to you even more.

What do you mean "In the past it was showing 360 GB"? What did happen between?

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After I posted this, df -H started to report back 360G in total, and now it is again down to 335G. The only changes that are done is that a lot of files are wrote and deleted on that partition. – John John Jun 8 '12 at 14:15
This is a very interesting fact that few people know about. If you want to get rid of this reserved space, you can do so with tune2fs -m 0 /dev/sdX This carries the risk of not being able to do any maintenance on it if it ever gets completely full. – Kenny Rasschaert Jul 20 '12 at 6:45

The 'df' command shows you free disk space as it is reported by the filesystem. This number is usually always smaller than the physical size of the partition. Without knowing any more details about what filesystem you are using (and not knowing much about Linux internals per se) I assume that the filesystem subsystem uses disk space for metadata dynamically.

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I agree that there is a diffrence, but in the past it was showing the TOTAL as 360 GB (vs 365 GB as reported by fdisk). So I agree that the 5 GB are the metadata space (365 vs 360), but I don't understand where 25 GB are (360 GB as shown by df in the past vs 325 GB shown now). – John John Jun 5 '12 at 15:44
P.S. I'm using ext3 – John John Jun 5 '12 at 15:45

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