[SQU] unbalanced cache directories

Hi all,
After so many benchmarking tests with Polygraph, my squid cache dirs
is not balanced. They were being used equally upto first crash of kernel.
Now after many hours tests the df output is:

Filesystem 1k-blocks Used Available Use% Mounted on
/dev/sda1 13108468 9690892 2751684 78% /cache1
/dev/sdb1 13108468 5441744 7000832 44% /cache2
/dev/sdc1 13108468 9663652 2778924 78% /cache3
/dev/sdd1 13108468 10903172 1539404 88% /cache4

Is Squid load balancing between disks round-robin?
Why cache2 is small?
How can I balance them?

Thanks in advance,
--Hamid

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