[dm-devel] extracting thin mappings in real time

[Thanos Makatos]

> Instead I gave a second thought to the solution of using dm-thin's
> public functions to access it's metadata, namely dm_thin_find_block.
> The problem is that if the mapping is not in memory I'll have to read
> it from disk, outside the critical path, and this adds complexity.
> However, since the mapping was just established it should be cached so
> it should mostly work (not being able to obtain the mapping every now
> and then is acceptable). Does this make sense?

I call dm_thin_find_block right after I've received the write
completion and if I don't set can_issue_io to 1 it _always_ fails. If
I do set it to 1 then it always succeeds but reads 20 KB from the
metadata device, even when repeatedly called. I can see that
internally it uses dm-bufio, which is supposed to some caching, no? Am
I missing something?