Solution:
use linux command df -k to see where at the OS level is space is been eaten up ..
as in my case my /opt was 99% full... i just cleaned the unneccesary files , freed up space..by using the command rm -rf (this command must never be used unless absolutely needed..double care should be taken while working on Prodcuction).. and then took the backup....this time backup worked absolutely fine....
BTW the rman block I used for simple backup...
run
{
allocate channel diskchan device type disk;
configure controlfile autobackup on;
configure channel device type disk format '/opt/%U';
backup database;
backup archivelog all;
release channel diskchan;
};
where diskchan is the diskname.....