Physical disk IO size smaller than fragment block filesystem size ?

Hello,
in one default UFS filesystem we have 8K block size (bsize) and 1K fragmentsize (fsize). At this scenary I thought all "FileSytem IO" will be 8K (or greater) but never smaller than the fragment size (1K). If a UFS fragment/blocksize is allwasy several ADJACENTS sectors on disk (in a disk with sector=512B), all "physical disk IO" it will allways, like "Filesystem IO", greater than 1K.
But with dtrace script from DTrace Toolkit (bitesize.d) I can see IO with 512B size.
¿What is wrong in my assumptions or what is the explanation?
Thank you very much in advance!!

rar wrote:
Like Jim has indicated me in unix.com forum, That cross-post thread happens to be:
http://www.unix.com/unix-advanced-expert-users/215823-physical-disk-io-size-smaller-than-fragment-block-filesystem-size.html
You could have pasted the URL to be polite ...

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