Wednesday, September 10, 2014

Linux -- tune2fs, adjusting reserved blocks percentage

Normally, In Linux systems , %5 percentage of each filesystem is reserved for the root user , by default.
That is; if you have a mount with 100gb in size, you can fill it up to 95gb.. When the filesystem contains 95gb , you will see it as "%100 used" in df command.
Furthermore, if you have a database running on this filesystem, it will hang when it reaches the 95gb , because it will not able to extend its datafiles, or create its logfiles..
Other non-root programs also will face with troubles, as when they will not able to save their data, and typically quit or crash. 
In such scenarios, we can say that ; this is just because of the reserved block percentage, which is a reserve that can only be used by the root user..
Okay good, it s an issurance for the general system health.. But how much do we need for root files, right?
The reservation is useful for the system services and other programs, which run under the rights of root and need to write to their log files, or save their data in /var for example.
So the reservation is useful for the general system health.

On the other hand; when your filesystem is big, then %5 of it is also big and unnecessary most of the times..

The good news is that, this reserved space can be changed using the -m option of the tune2fs command..
This can be changed on the fly..

Following command can be executed with root..
It sets the reserved space percentage to be %2..

tune2fs -m 2 /dev/sdb1

Following is another command which sets the reserved space percentage to be %0.
tune2fs -r 0 /dev/sda1

When you issue these commands, you will directly see an increase in the free percentage of the relevant filesystem. 
Just execute df and see the difference.

A reliable place to use these commands would be a filesystem that contains only the files of Oracle RDBMS. 
As this kind of filesystems are only used by the Oracle Software Owner Os account, there is no need to reserve space for root user on them.. (the root user will not craete any files on it..)

As you can imagine, tune2fs can be a life saver, as it did saved one of the suddenly growing standby databases of my customers..

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