A few days ago, I ran
I found the answer after consulted Google using columns of
The discrepancy comes from Reserved Blocks. From what I read, Extended File System (ext) since ext2, has such feature and the default reserved blocks are 5% of total blocks, marked by
5% (3.6G) is really a lot, even for my tiny harddrive. Imagine a 1TB harddrive, that's 50G, almost as big as my harddrive. There are two ways to set the reserved amount of blocks using
For percentage using
df -h
for no reasons as I usually did, the output as follows:$ df -h Filesystem Size Used Avail Use% Mounted on rootfs 72G 49G 20G 72% / [...]To my amaze or not to, the part of my brain which manages mathematics finally told me that 72G != 49G + 20G. I have been using Linux full-time for more than five years and probably have run
df
for a thousand times if not less. Finding the inconsistency is not really what confused me but why it took me so long to see the numbers. I found the answer after consulted Google using columns of
df
, ie. "df size used avail"
. Yes, that's how I found the answer at my first try. I am still a master of Google.The discrepancy comes from Reserved Blocks. From what I read, Extended File System (ext) since ext2, has such feature and the default reserved blocks are 5% of total blocks, marked by
mke2fs
.-m reserved-blocks-percentage Specify the percentage of the filesystem blocks reserved for the super-user. This avoids fragmentation, and allows root-owned daemons, such as syslogd(8), to continue to function correctly after non-privileged processes are prevented from writing to the filesystem. The default percentage is 5%.To see how many blocks are reserved, you can use
tune2fs
:$ sudo tune2fs -l /dev/sda3 | grep -i block Block count: 19037025 Reserved block count: 951851 Free blocks: 6019654 [...] Block size: 4096 [...]In my case, it's 19,037,025 * 0.05 = 951,851.25. Round to 951,851 blocks as you see in the output above. To do the math correctly, use 1K-blocks for calculations:
$ df | head -2 Filesystem 1K-blocks Used Available Use% Mounted on rootfs 74953252 50727968 20417880 72% /The reserved size is 951,851 * 4 (Block size, 4096 = 4*1K-blocks) + 507,279,68 + 204,178,880 = 74,953,252. Mystery solved!
5% (3.6G) is really a lot, even for my tiny harddrive. Imagine a 1TB harddrive, that's 50G, almost as big as my harddrive. There are two ways to set the reserved amount of blocks using
tune2fs
, one by percentage, another by number of blocks.For percentage using
-m
, the following code set to 1%, you can assign a float number.$ sudo tune2fs -m 1 /dev/sda3 tune2fs 1.42 (29-Nov-2011) Setting reserved blocks percentage to 1% (190370 blocks)Or, the specific amount of block using
-r
, the following code set to around 500MB in 4K block size.$ sudo tune2fs -r 129008 /dev/sda3 tune2fs 1.42 (29-Nov-2011) Setting reserved blocks count to 129008When creating an ext2/ext3/ext4 filesystem using
mke2fs
/mkfs.ext2
/mkfs.ext3
/mkfs.ext4
, you can only use -m
to specify a reversed percentage.
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