On Oct 12, 2006, at 10:46:14, Alasdair G Kergon wrote:
On Wed, Oct 11, 2006 at 10:33:14AM -0500, Jon Brassow wrote:
This patch adds read balancing. The round-robin method is the
first to be implemented, but provisions are made for others to be
implemented in the future.
My first thought in looking over this is that it ought to share the
existing dm-round-robin module used by multipath (which is already
more flexible than this as paths can be weighted). It looks fairly
straightforward to do this. (And we certainly shouldn't confuse
people by having 'roundrobin' in one part of dm and 'round-robin'
in another!)
When we implement more complex load balancing policies, there seems
little point doing everything twice, once for multipath and once
for mirroring:-)
Well, actually it would seem to me that mirroring and multi-path are
both special-cases of a "multi-device" design. If you can specify
differing device selection algorithms for read and write paths, then
you can implement the above as follows:
multipath:
read = round-robin
write = round-robin
mirroring:
read = round-robin
write = all devices
Then new read or write algorithms can be written for different
situations. If you need real-time verification of mirrored data, you
could do "read = all devices". If you have a 5-disk mirror you might
write an algorithm which reads all disks and choses the "correct"
data by voting (then rewrites the sector if any device was wrong).
Cheers,
Kyle Moffett
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