[dm-devel] scheduling/merging on top of a DM target?

[Peter Desnoyers]

We’re emulating some in-disk algorithms in a device mapper target, and would like to be able to do a fair comparison between the emulated and real versions; however they see radically different I/O streams, evidently because there’s no scheduling / request merging / etc. on top of the bio-based device mapper target.

It appears that if our device mapper were request-based instead of bio-based we wouldn’t have this issue, but after reading a bunch of history on the mailing list archives I don’t think this is going to happen easily.

Does anyone have any thoughts on quick-and-dirty ways to approximate this so we can get some preliminary measurements? dm-multipath won’t stack on a bio-based target, and attempts to stack loopback and various iscsi targets on top haven’t achieved the desired results. (in general these have resulted in our dm target seeing individual 4K I/Os, instead of the larger I/Os at the higher layer)

Thanks,
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 Peter Desnoyers                                  pjd at ccs.neu.edu
 Northeastern Computer & Information Science