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Reclaim lost space with tune2fs

File system Reserved Block Count is supposed to reduce Linux file system defragmentation, to allow root user login for maintenance or to allow Linux system logging facility to function properly in case file system running low of free disk space.

File system utility called tune2fs can be used to tune Linux ext2 / ext3 file system parameters, such as adjusting file system reserved block count, frequency of file system force-check based, maximal time between two file system checks, behavior of the kernel code when errors are detected, overriding the default ext3 journal parameters, etc.

tune2fs -l /dev/hda

or

dumpe2fs -h /dev/vg0/lvol1

could be used to inspect a file system superblock information. Take note on these fields of both tune2fs and dumpe2fs command output:

Reserved block count: 3399024
Free blocks: 4997248
Free inodes: 42473835
First block: 0
Block size: 4096

The block size is measured in byte unit. In this case, it’s 4,096 byte or 4KB. So, the Reserved Block Count: 3399024 is equal to 13,922,402,304 byte or roughly equivalent to 14GB.

Now, if execute the command

tune2fs -m 0 /dev/vg0/lvol1

will set the percentage of file system reserved block count to 0% for /dev/vg0/lvol1 file system (the first column of df command output). It’s OK to totally disable file system reserved block count, if the file system is not used by root user account or storing system log and system/program temp files (e.g. /var and /tmp). For example, file system dedicated to Oracle datafiles are safe to disable file system reserved block count.

So, if you’re running a Linux machine with 320GB storage array and haven’t tune the file system reserved block count, you may tune it now to claim back as much as 16GB of free disk space!