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Linux deleted log file doesn't free up the disk space

TL DR;

Kill or restart the process which writes the log file. Even though the file is deleted, it won't free up the disk space if the process is still writing to the log file. Only after stopping the process, it will actually be deleted.

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Usually, disk spaces of servers are getting cluttered and filled by application log files when a proper log rotation mechanism is not implemented. 

In case if you find such a log file causing the disk space to fill up 100%, it will probably impact any application which trying to use the disk. 

You can see the disk space by the following command.

df -h

If the root dir ("/") usage is 100% the impact may be high for the system. Or else, if there is more than one disk attached then any application that uses that disk can be impacted. 

In the above image, mine is not filled yet. I have only used 25% of the disk.


How to find what files are using the disk space more?

You can use the "du" command to do that. 
Go to the root dir ("/") in the terminal
cd /
Then use the below command to figure out the directory which uses the most disk space.
sudo du -d 1 -h .

This command gives an estimate of the file space usage of the directories.

Since we are scanning from the root dir, you will need the sudo powers to access some directories.

-d is used to mention the depth. When scanning the files how deep it should scan over the directory structure. -d 1 means you should scan the immediate directories only.

-h says show the output in the human-readable figures. So, it will show the size in Gb or Mb values.

Finally, the directory which we tell it to scan, we have given a dot (".") to say "current directory".



It may provide some errors saying cannot access some directory or files, because we are trying to scan some paths where they are being actively used by the OS. Therefore some paths, it won't allow you to scan just like that, but don't worry about it, our cluttering files won't be there anyway.

When you find the directory which has the most usage, then go inside it, in my case /opt is taking more space. Then run the same "du" command once again. 
You can find the directories like that and go inside them. If you can't find any other directories inside a directory, then you have to use the ls -alh command to see the files and their size.

If you find the log file which clutters the disk space, you can just delete it using an "rm" command.

Problem: deleted log file still doesn't free up the disk space

Let us say you already found the problematic file and deleted it using the rm command.
Sometimes even after that, if you check the df -h it still shows 100% usage.

Solution

Kill or restart the process which writes the log file. Even though the file is deleted, it won't free up the disk space if the process is still writing to the log file. Only after stopping the process, it will actually be deleted.

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