[lustre-devel] Request arc buffer, zerocopy

Anna Fuchs anna.fuchs at informatik.uni-hamburg.de
Thu Jun 13 04:54:47 PDT 2019

Dear all,

in osd-zfs/osd_io.c:osd_bufs_get_write you can find a comment regarding 

	 * currently only full blocks are subject to zerocopy approach:
	 * so that we're sure nobody is trying to update the same block

Whenever a block to be written is full, an arc buffer is requested, 
otherwise alloc_page.

I do not really understand the conclusion. Why and how do full blocks 
prevent updates?
And put it differently - why not to try zerocopy for not full blocks?
What could happen if I tried to request an arc buffer for e.g. a block 
with missing last page?

I would be greateful for details.
Best regards

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