>
Does that mean that RRW (as the name seems to imply) would need to read the
entire stripe before it could write back the updated stripe compared to a
partial read with RMW?
Sort of. If you have, say, seven drives total: D D D D D P Q, the access pattern look like:
D D D D D P Q
^
Write this drive... then RCW needs to read:
D D - D D - -
... and RMW needs to read:
- - D - - P Q
Now, if this is a "small" write (smaller than a page) then the stripe being modified always needs to be written, which helps RMW. On the other hands, large writes really help RCW:
D D D D D P Q ^^^^^^^ Write these four drives
... then RCW only needs to read one drive, whereas RMW needs to read six.
Now, you'd never use RMW only; it's only used when it should be faster than RCW, which is why I'm a bit surprised about the performance degradation. With only six drives in my test system it's pretty clear that RMW should very rarely be invoked, however, the performance ought to have stayed the same rather than slip.
-hpa
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