UBIFS size issue

Hi,

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When I am giving 55MiB of vol size, by following command

ubimkvol /dev/ubi0 -N vol -s 55MiB

I could able to see only, 49 MiB of space when Kernel boot-up.

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Can't you get the answer from here ?

http://www.linux-mtd.infradead.org/faq/ubifs.html#l_df_report

Kazu