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Cache drive used space discrepancy

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I have a 60GB SSD being used as a cache drive.  On the main page of the GUI it reports that I am using 42.1 GB:

Size	Used	Free
60GB	42.1GB	17.9GB

But du tells me it's only using 33G:

du -h -d 1 /mnt/cache
12G     /mnt/cache/appdata
0       /mnt/cache/domains
21G     /mnt/cache/system
0       /mnt/cache/transcode
33G     /mnt/cache

None of my shares, except the ones listed above, are set to use the cache drive but I ran the mover manually anyway just in case.


Is there a reason that the two don't match?

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Unlike Windows, Linux allows files to have a different number for "Space Used" and "Size on Disk"...  This is because Linux allows files filled with zeros to have a 40GB size for instance while taking up the space needed to store the file name...  Causes some issues for the humans that need to read this stuff sometimes...  The files under cache/system especially will have lots of empty space allocated to them, don't worry too much about it...  The attached is an list of my libvirt file for instance, see that it shows that the file is both 205M and 2.0G at the same time...?  The little meter bar on the "Main" page for UnRaid tends to show disk free space just fine...


/mnt/cache/system/libvirt# ls -lhsa
total 205M
   0 drwxrwxrwx 1 root   root    22 Mar  8  2018 ./
   0 drwxrwxrwx 1 nobody users  322 Nov  9 11:45 ../
205M -rw-rw-rw- 1 nobody users 2.0G Jan 11 16:10 libvirt.img


Edited by Warrentheo

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On a related note, if you have a large file that takes up lots of space, but you know most of it is zeros (most hard drive image file backups for instance) you can use the 


fallocate -d

command on it, and Linux will go through and un-map all the space that currently stores nothing but zeros from it, had it free up a bunch of hard drive space on some of my VM backups...  Just be careful with the command, it can easily accidentally delete files...


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That can happen, especially when sparse files are present, the used space on the GUI is the correct one and the one you should pay attention to.

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Thank you guys so much for the responses. If I wanted to try to clear out some space, how would I go about finding the large files that are taking up the most space?  My docker image is set to 20gb and my libvirt.img is 1gb so I'm assuming the system folder is ok. My appdata is reporting 12g but that must be inflated on the main view? 

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