> > + ti->discard_zeroes_data_unsupported = 1;
> >
> > ms->kmirrord_wq = alloc_workqueue("kmirrord",
> > WQ_NON_REENTRANT | WQ_MEM_RECLAIM, 0);
>
> This should be split out to a separate patch and properly justified in
> the patch header. Is there something unique to dm-mirror that renders
> the underlying device's zeroing unreliable?
There are two possible approaches to handling REQ_DISCARD
1. treat REQ_DISCARD as REQ_FLUSH (this is what the patch does) --- i.e.
do not synchronize it with region states, do not set mirror error on
failure. In this mode we must assume that there are uninitialized data
after a flush.
For example, if there is a region that is being resynchronized and we send
REQ_DISCARD that overlaps this region, there is no guarantee that data in
this region were zeroed.
- kcopyd reads a few blocks for resynchronization
- REQ_DISCARD is sent to both mirror legs, both disks overwrites the area
with zeroes
- kcopyd writes those blocks to the other leg => the blocks are no longer
zero despite REQ_DISCARD being sent
2. treat REQ_DISCARD as writes (i.e. synchronize it with region states,
wait until resynchronization finishes, etc.) --- it is possible to do it
this way to, but if we do it this way, we have to split REQ_DISCARD on
region boundaries (it is currently split only on target boundaries,
which is insufficient).
Mikulas
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