Hello, On 08/17/2010 04:07 PM, Mike Snitzer wrote: >> With the patch applied, there's no second flush. Those requests would >> now be REQ_FLUSH + REQ_DISCARD. The first can't be avoided anyway and >> there won't be the second flush to begin with, so I don't think this >> worsens anything. > > Makes sense, but your patches still need to be refreshed against the > latest (2.6.36-rc1) upstream code. Numerous changes went in to DM > recently. Sure thing. The block part isn't fixed yet and so the RFC tag. Once the block layer part is settled, it probably should be pulled into dm/md and other trees and conversions should happen there. >> Yeap, I want you to be concerned. :-) This was the first time I looked >> at the dm code and there are many different disjoint code paths and I >> couldn't fully follow or test all of them, so it definitely needs a >> careful review from someone who understands the whole thing. > > You'll need Mikulas (bio-based) and NEC (request-based, Kiyoshi and > Jun'ichi) to give it serious review. Oh, you already cc'd them. Great. Hello, guys, the original thread is http://thread.gmane.org/gmane.linux.raid/29100 > NOTE: NEC has already given some preliminary feedback to hch in the > "[PATCH, RFC 2/2] dm: support REQ_FLUSH directly" thread: > https://www.redhat.com/archives/dm-devel/2010-August/msg00026.html > https://www.redhat.com/archives/dm-devel/2010-August/msg00033.html Hmmm... I think both issues don't exist in this incarnation of conversion although I'm fairly sure there will be other issues. :-) >> A related question: Is dm_wait_for_completion() used in >> process_flush() safe against starvation under continuous influx of >> other commands? > > As for your specific dm_wait_for_completion() concern -- I'll defer to > Mikulas. But I'll add: we haven't had any reported starvation issues > with DM's existing barrier support. DM uses a mempool for its clones, > so it should naturally throttle (without starvation) when memory gets > low. I see but single pending flush and steady write streams w/o saturating the mempool would be able to stall dm_wait_for_completeion(), no? Eh well, it's a separate issue, I guess. >> * Guarantee that REQ_FLUSH w/ data never reaches targets (this in >> part is to put it in alignment with request based dm). > > bio-based DM already split the barrier out from the data (in > process_barrier). You've renamed process_barrier to process_flush and > added the REQ_FLUSH logic like I'd expect. Yeah and threw in WARN_ON() there to make sure REQ_FLUSH + data bios don't slip through for whatever reason. >> * For request based dm: >> >> * The sequencing is done by the block layer for the top level >> request_queue, so the only things request based dm needs to make >> sure is 1. handling empty REQ_FLUSH correctly (block layer will >> only send down empty REQ_FLUSHes) and 2. propagate REQ_FUA bit to >> member devices. > > OK, so seems 1 is done, 2 is still TODO. Looking at your tree it seems > 2 would be as simple as using the following in Oh, I was talking about the other way around. Passing REQ_FUA in bio->bi_rw down to member request_queues. Sometimes while constructing clone / split bios, the bit is lost (e.g. md raid5). > dm_init_request_based_queue (on the most current upstream dm.c): > blk_queue_flush(q, REQ_FLUSH | REQ_FUA); > (your current patch only sets REQ_FLUSH in alloc_dev). Yeah, but for that direction, just adding REQ_FUA to blk_queue_flush() should be enough. I'll add it. Thanks. -- tejun -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
