CachedPlan logs until full disk

Hello guys,

it is the second time (in two weeks), that  have a very strange Postgresql in a 8.4.22 installation (32 bit still).

Logfile grow up (in few hours) until filling the Whole disk space.
I can read infinite series of this messages:

CachedPlan: 1024 total in 1 blocks; 640 free (0 chunks); 384 used
    CachedPlanSource: 1024 total in 1 blocks; 336 free (0 chunks); 688 used
    SPI Plan: 1024 total in 1 blocks; 912 free (0 chunks); 112 used
    CachedPlan: 1024 total in 1 blocks; 200 free (0 chunks); 824 used
    CachedPlanSource: 1024 total in 1 blocks; 96 free (0 chunks); 928 used
    SPI Plan: 1024 total in 1 blocks; 928 free (0 chunks); 96 used
    CachedPlan: 1024 total in 1 blocks; 640 free (0 chunks); 384 used
    CachedPlanSource: 1024 total in 1 blocks; 336 free (0 chunks); 688 used
    SPI Plan: 1024 total in 1 blocks; 912 free (0 chunks); 112 used
    CachedPlan: 1024 total in 1 blocks; 200 free (0 chunks); 824 used

I had to stop, delete logs, and then restart again and the problems solved.
But i did not understand why this problem occurred.

Thank you for any help!
Francesco