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When I right click on Properties for my ext4 partition I get the following pie chart:

enter image description here

The meaning of the blue and white slices are fairly obvious, since they are in the legend to the right, but what does the grey slice represent? It's quite substantial and at least a couple of GB's in size.

user300458
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Minos
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  • Maybe it's the space reserved for trash storage? I also see the same slices on ext3 or ext4 volumes, but not on ntfs ones. – user300458 Jan 13 '15 at 11:10
  • TRASH storage will be covered inside USED space only, isn't it?? why would FS will spend alot of space for that, it'll degrade usage. – Novice Jan 13 '15 at 11:21
  • @user300458 I regularly empty the trash folder and the .Trash-1000 folder on the partition is only 13kB in size at the moment. – Minos Jan 13 '15 at 11:42
  • @Minos I mean "reserved", not "actually containing"... The percent of "grey slice" on my volumes seems to be the same as on your screenshot. My trash is empty now. – user300458 Jan 13 '15 at 11:57
  • @user300458 "reserved" is the magic word, it has however nothing to do with trash. See my answer below. – Minos Jan 13 '15 at 12:24

2 Answers2

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By default ext2, ext3 and ext4 reserve 5% of all space for the root user and to prevent fragmentation (see this mailing list). However as this particular disk is used for archiving data that doesn't change much, I have taken the liberty to change this percentage to 1%, using the command:

sudo tune2fs -m 1 /dev/sdb

the last item /dev/sdb reflecting the location of my disk. Behold:

reserved space on disk is reduced

Minos
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0

It shows the size of index table of your filesystem.

In every HDD, you can not use the whole space for yourself.
When any kind of partition is made on disk; to keep track of index of any file stored on it, parttion table is created to look up when we try to access any particular file.
Not only that much information but all other information regarding each partitions' starting sector to end sector, sector size, structure, sector skewing information, etc...

Like in FAT(File Allocation Table, you can store a file max of 4GB at a time, for greater than 4GB it won't allow you as it's capacity is of 4GB only.)

There are different types of File-Systems, which uses different techniques to hold information, so that grey mark can be vary according to File-System you're using

OR possibly it can be showing OS reserved space that used as SWAP area, for PAGING purpose, but I doubt it, if I'm wrong than correct it. Above answer is quite sure.

Novice
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    Are you sure? 25 GB for an index table?.. – user300458 Jan 13 '15 at 11:15
  • As the HDD size increases, the size of index table will get bigger, as it needs to allocate more files; More file >> More number of index >> larger index number. Won't FS need space??!!! – Novice Jan 13 '15 at 11:23
  • It seems an awful lot for an index table, I just checked and there are about 150000 files on the disk. If I estimate the grey slice to represent 10GB worth of data that corresponds to 67kB's per file! I also know that there is no OS reserved space on the disk, because swap is disabled and the disk itself is most of the time unmounted. – Minos Jan 13 '15 at 11:40
  • Can you fetch the HDD details, vendor, it's band, etc..??? – Novice Jan 13 '15 at 12:03
  • @Minos according to this [LINK 1](http://scripthacks.wordpress.com/2009/10/21/calculate-usable-hard-drive-size/), [LINK 2](http://www.ussscctv.com/harddrivesizecapacitiescalculator.aspx); your HDD should've only 465.66GB usable space only. As per details in image above Total is ~466GB, near to calculation, The ~25GB grey portion is part of FS only I guess. – Novice Jan 13 '15 at 12:10
  • @Novice it has nothing to do with the conversion of 500GB to GiB, see my answer for the real 'issue'. – Minos Jan 13 '15 at 12:29
  • @Minos okay...got the point, Thanks for the more accurate info, as per the mailing list, it is part of File-System itself, and FS itself creates that portion for better performance. Some where in my answer I've not clearly written but I meant that only. – Novice Jan 13 '15 at 13:17