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why does blktrace only write blocks of 8?

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http://unix.stackexchange.com – I want to understand the I/O pattern a database is writing to disk to decide how many disks to use for best performance. To analyse the I/O pattern I want to use blktrace and I have to grok it first. This is what I try here. I have a USB stick that I attach to my computer and it becomes /dev/sdd. Now I start dd if=/dev/sdd of=/dev/null and on a separate window I start blktrace -d /dev/sdd -o - | blkparse -i - and expect to see read (R) operations that get merged (M) and put into the queue (Q). That works, but to my understanding the block size is always 8: 8,48 6 15257 2.15799 (HowTos)