[dm-devel] questions about dm-thin and discard

[Mikulas Patocka]

Hi Joe

I would like to ask you about this code path: In process_discard, there is 
a branch with a comment "This path is hit if people are ignoring 
limits->discard_granularity." It trims the discard request so that it 
doesn't span a block boundary and submits it.

The question is: what if the block is shared? In this case, we can't 
submit discard to the block, because it would damage the other snapshot 
that is sharing this block. Shouldn't there be shomething like this?
if ((!lookup_result.shared) & pool->pf.discard_passdown) {
	remap_and_issue(tc, bio, lookup_result.block);
} else {
	bio_endio(bio, 0) 
} 
... or is it tested elsewhere and am I missing something?


Another question is about setting "ti->discards_supported = 1" in 
pool_ctr. ti->discards_supported means that the target supports discards 
even if the underlying disk doesn't. Since the pool device is passing 
anyth I/O unchanged to the underlying disk, ti->discards_supported 
shouldn't be set. Or is there any other reason why is it set?


Mikulas