[Yaffs] Some problems on yaffs free space

I meet some problems in using the yaffs as the root file system.

For example the size of root division is 59MB but only 4600KB free spaces left.
When copying a file of 4000KB to the root division(600KB space left)
yaffs will print some error messages(bad block) and then restart,
leaving the system unable to start normally any more.
Sometimes it may print more error information of the bad block.

Software and hardware environment : 
linux kernel 2.6.19 ,  
nand flash samsung k9f1208, 
Freescale MX27 ARM926EJS.         

        
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bentley
2008-10-15