Linux 101 : Root user Reserved Blocks

Reserved Blocks, are disk blocks that are used by the root user in case of emergency - no space on the storage, ... -

When running the "df" command, we can see clearly that some storage is missing: 

We notice that:

15G - used storage - plus 2.8G - available storage - equals 17.8G not 19G.
The missing storage are what Linux keeps as reserved blocks, in case the filesystem gets full.

We could check the reserved blocks using the below command:

Root is the only user that can have access to these reserved blocks.


For the  "extN" filesystems, the reserved blocks make up about 5 per cent of all the blocks.

We could also checks the reserved blocks using the following command:

If you divide "13107" by "262144", we will get "0.049" which translates to around 5%.

We could change the percentage of the reserved blocks using the below command:

  • -m : used for specifying the reserved blocks percentage to be used only by privileged processes
  • x : percentage of the reserved blocks - 3 for 3% for example -.


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