[Qemu-discuss] minimum size of block commit from an qcow2 overlay file

[정복득]

Is the minimum size of block commit from an qcow2 overlay file to a base file 
equal to the cluster size of qcow2? 

I printed out the size of each write operation inside the for() clause in 
bdrv_commit()/block.c to see the write pattern . According to the bdrv_commit() 
code, during each write operation to the base file, it writes the maximum 
number of contiguous sectors which exist in the overlay file. The result shows 
that each write size in the for() clause is multiple of 128 sectors which is 
equal to the cluster size(64KB) of my qcow2 file. 

I cannot figure out which part of qemu code make qcow2 store data in the unit 
of cluster size rather than sector size. If a VM writes a 4KB page as an 
update, does some part of qemu read a whole cluster which contains the page and 
then write the whole updated cluster chunk? Or is VM aware of the minimum size 
of r/w operation?
Thanks in advance.

Bokdeuk