On Wed, Jun 01 2016, Song Liu wrote: >> >>It isn't at all clear to me why you need the extra page. Could you >>explain when and why the ->page and the ->orig_page would contain >>different content and both of the content would be needed? >> > > In prexor, we need read old data from the disk to perform the > substract-xor. In the meanwhile, we need a buffer for the new data. > When there is no write cache, the new data is in the page of bio. > With write cache, however, the bio might be already released. > Therefore, we will need an extra page here: orig_page for new > Cached data, while newly allocated page just hold old data for prexor. > Ahh, I get it now - thanks. The fact that you said "page" rather than "pages" threw me. I assumed it was a page for the parity block only, which is wrong. So I would write: For RMW, the code allocates an extra page for each data block being updated. This is stored in r5dev->page and the old data is read into it. Then the prexor calculation subtracts ->page from the parity block, and the reconstruct calculation adds the ->orig_page data back into the parity block. Or something like that. Thanks, NeilBrown
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