Squiddy question: No space left on device

My squid.conf contains the line:
cache_dir /SQUIDcacheG 1100 32 256

which, if my maths is correct, means that the maximum cache size should
be 1126400k

Given that my log partition is 1231254k:
Filesystem kbytes used avail capacity Mounted on
/dev/dsk/c0t1d0s6 1231254 915573 254119 79% /SQUIDcacheG

Then I reckon that the disk should not get more than 91% full. So how
come I get:?

cache.log:
1999/07/03 13:55:02| diskHandleWrite: FD 44: disk write error: (28) No
space left on device
1999/07/03 13:55:02| storeSwapOutHandle: SwapOut failure (err code =
-6).
1999/07/03 13:55:02| WARNING: Shrinking cache_dir #6 to 963447 KB

/var/adm/messages:
Jul 3 13:55:02 newportal unix: NOTICE: alloc: /SQUIDcacheG: file system
full

The swap.state file on that partition is only about 4mB.

-- 
Sid Baldwin
 Web Services Administrator
 University of the West of England
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