Disk Fills Up and fsck "Compresses" it

Anyone seen PG filling up a 66 GB partition from say 40-ish percentage to 60-ish percentage in a manner of minutes. When I run a 'fsck' the disk usage comes down to 40-ish percentage. That's about 10+ GB's variance.

This is a FreeBSD 6.2 RC2, 4GB memory, Xeon 3.2 GHz '4' of the '8' CPUs in use - dual cpu, dual core with HTT turned off in the sense that the other 4 cpu's have been masked out. The drive is a Western Digital 70 GB SATA.

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Yudhvir Singh Sidhu
408 375 3134 cell