Am 20.01.2011 06:19, schrieb Yoshiaki Tamura: >>>>> + return; >>>>> + } >>>>> + >>>>> + bdrv_aio_writev(bs, blk_req->reqs[0].sector, blk_req->reqs[0].qiov, >>>>> + blk_req->reqs[0].nb_sectors, blk_req->reqs[0].cb, >>>>> + blk_req->reqs[0].opaque); >>>> >>>> Same here. >>>> >>>>> + bdrv_flush(bs); >>>> >>>> This looks really strange. What is this supposed to do? >>>> >>>> One point is that you write it immediately after bdrv_aio_write, so you >>>> get an fsync for which you don't know if it includes the current write >>>> request or if it doesn't. Which data do you want to get flushed to the disk? >>> >>> I was expecting to flush the aio request that was just initiated. >>> Am I misunderstanding the function? >> >> Seems so. The function names don't use really clear terminology either, >> so you're not the first one to fall in this trap. Basically we have: >> >> * qemu_aio_flush() waits for all AIO requests to complete. I think you >> wanted to have exactly this, but only for a single block device. Such a >> function doesn't exist yet. >> >> * bdrv_flush() makes sure that all successfully completed requests are >> written to disk (by calling fsync) >> >> * bdrv_aio_flush() is the asynchronous version of bdrv_flush, i.e. run >> the fsync in the thread pool > > Then what I wanted to do is, call qemu_aio_flush first, then > bdrv_flush. It should be like live migration. Okay, that makes sense. :-) >>>> The other thing is that you introduce a bdrv_flush for each request, >>>> basically forcing everyone to something very similar to writethrough >>>> mode. I'm sure this will have a big impact on performance. >>> >>> The reason is to avoid inversion of queued requests. Although >>> processing one-by-one is heavy, wouldn't having requests flushed >>> to disk out of order break the disk image? >> >> No, that's fine. If a guest issues two requests at the same time, they >> may complete in any order. You just need to make sure that you don't >> call the completion callback before the request really has completed. > > We need to flush requests, meaning aio and fsync, before sending > the final state of the guests, to make sure we can switch to the > secondary safely. In theory I think you could just re-submit the requests on the secondary if they had not completed yet. But you're right, let's keep things simple for the start. >> I'm just starting to wonder if the guest won't timeout the requests if >> they are queued for too long. Even more, with IDE, it can only handle >> one request at a time, so not completing requests doesn't sound like a >> good idea at all. In what intervals is the event-tap queue flushed? > > The requests are flushed once each transaction completes. So > it's not with specific intervals. Right. So when is a transaction completed? This is the time that a single request will take. >> On the other hand, if you complete before actually writing out, you >> don't get timeouts, but you signal success to the guest when the request >> could still fail. What would you do in this case? With a writeback cache >> mode we're fine, we can just fail the next flush (until then nothing is >> guaranteed to be on disk and order doesn't matter either), but with >> cache=writethrough we're in serious trouble. >> >> Have you thought about this problem? Maybe we end up having to flush the >> event-tap queue for each single write in writethrough mode. > > Yes, and that's what I'm trying to do at this point. Oh, I must have missed that code. Which patch/function should I look at? > I know that > performance matters a lot, but sacrificing reliability over > performance now isn't a good idea. I first want to lay the > ground, and then focus on optimization. Note that without dirty > bitmap optimization, Kemari suffers a lot in sending rams. > Anthony and I discussed to take this approach at KVM Forum. I agree, starting simple makes sense. Kevin -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
