.truncate extending a file

All,

I'm trying to understand what the .truncate method is supposed to do when the file size is extended. (Using 2.5.68-rmk1)

dd if=/dev/zero count=1 seek=3 of=tempfile
In my filesystem, if I imediately read it back, I get the garbage from the disk for the first 3 blocks, but if I umount/mount the filesystem, the data is the 4 blocks of zeros expected. So it looks like there is a mapping of some sort that I'm missing, but I'm at a loss.

Any pointers to docs, examples, etc? Is there something I can read to understand what is going on with buffer heads in a filesystem context? Something I should grep or Google for?

TIA,

Eli
--------------------. "If it ain't broke now,
Eli Carter \ it will be soon." -- crypto-gram
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