A file of 5 MB on tmpfs file system uses 5 MB of RAM or more?

[DI BACCO ANTONIO - technolabs]

I have a linux-2.6.19.2 board with an MPC880 with 16MB ram and 16 MB
flash. 
Normally I have 9-10 MB of free RAM.
 
If I allocate 5 MB of ram memory, the system continues to work normally,
but if, instead of allocating 5 MB directly, I fill the /tmp directory
(that has a tmpfs file system) with a 5MB file the system becomes slow
and after some minutes the oom_killer is invoked. Before it dies I had
the time to see that mtdblockd is taking a lot of CPU (20%), what could
be the problem?
 
Any hint will be appreciate,
Bye,
Antonio.
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