* Stefan Hajnoczi (stefa...@gmail.com) wrote: > On Tue, Sep 08, 2015 at 04:48:24PM +0200, Kevin Wolf wrote: > > Am 08.09.2015 um 16:23 hat Denis V. Lunev geschrieben: > > > On 09/08/2015 04:05 PM, Kevin Wolf wrote: > > > >Am 08.09.2015 um 13:27 hat Denis V. Lunev geschrieben: > > > >>interesting point. Yes, it flushes all requests and most likely > > > >>hangs inside waiting requests to complete. But fortunately > > > >>this happens after the switch to paused state thus > > > >>the guest becomes paused. That's why I have missed this > > > >>fact. > > > >> > > > >>This (could) be considered as a problem but I have no (good) > > > >>solution at the moment. Should think a bit on. > > > >Let me suggest a radically different design. Note that I don't say this > > > >is necessarily how things should be done, I'm just trying to introduce > > > >some new ideas and broaden the discussion, so that we have a larger set > > > >of ideas from which we can pick the right solution(s). > > > > > > > >The core of my idea would be a new filter block driver 'timeout' that > > > >can be added on top of each BDS that could potentially fail, like a > > > >raw-posix BDS pointing to a file on NFS. This way most pieces of the > > > >solution are nicely modularised and don't touch the block layer core. > > > > > > > >During normal operation the driver would just be passing through > > > >requests to the lower layer. When it detects a timeout, however, it > > > >completes the request it received with -ETIMEDOUT. It also completes any > > > >new request it receives with -ETIMEDOUT without passing the request on > > > >until the request that originally timed out returns. This is our safety > > > >measure against anyone seeing whether or how the timed out request > > > >modified data. > > > > > > > >We need to make sure that bdrv_drain() doesn't wait for this request. > > > >Possibly we need to introduce a .bdrv_drain callback that replaces the > > > >default handling, because bdrv_requests_pending() in the default > > > >handling considers bs->file, which would still have the timed out > > > >request. We don't want to see this; bdrv_drain_all() should complete > > > >even though that request is still pending internally (externally, we > > > >returned -ETIMEDOUT, so we can consider it completed). This way the > > > >monitor stays responsive and background jobs can go on if they don't use > > > >the failing block device. > > > > > > > >And then we essentially reuse the rerror/werror mechanism that we > > > >already have to stop the VM. The device models would be extended to > > > >always stop the VM on -ETIMEDOUT, regardless of the error policy. In > > > >this state, the VM would even be migratable if you make sure that the > > > >pending request can't modify the image on the destination host any more. > > > > > > > >Do you think this could work, or did I miss something important? > > > > > > > >Kevin > > > could I propose even more radical solution then? > > > > > > My original approach was based on the fact that > > > this could should be maintainable out-of-stream. > > > If the patch will be merged - this boundary condition > > > could be dropped. > > > > > > Why not to invent 'terror' field on BdrvOptions > > > and process things in core block layer without > > > a filter? RB Tree entry will just not created if > > > the policy will be set to 'ignore'. > > > > 'terror' might not be the most fortunate name... ;-) > > > > The reason why I would prefer a filter driver is so the code and the > > associated data structures are cleanly modularised and we can keep the > > actual block layer core small and clean. The same is true for some other > > functions that I would rather move out of the core into filter drivers > > than add new cases (e.g. I/O throttling, backup notifiers, etc.), but > > which are a bit harder to actually move because we already have old > > interfaces that we can't break (we'll probably do it anyway eventually, > > even if it needs a bit more compatibility code). > > > > However, it seems that you are mostly touching code that is maintained > > by Stefan, and Stefan used to be a bit more open to adding functionality > > to the core, so my opinion might not be the last word. > > I've been thinking more about the correctness of this feature: > > QEMU cannot cancel I/O because there is no Linux userspace API for doing > so. Linux AIO's io_cancel(2) syscall is a nop since file systems don't > implement a kiocb_cancel_fn. Sending a signal to a task blocked in > O_DIRECT preadv(2)/pwritev(2) doesn't work either because the task is in > uninterruptible sleep.
There are things that work on some devices, but nothing generic. For NBD/iSCSI/(ceph?) you should be able to issue a shutdown(2) on the socket that connects to the server and that should call all existing IO to fail quickly. Then you could do a drain and be done. This would be very useful for the fault-tolerant uses (e.g. Wen Congyang's block replication). There are even ways of killing hard NFS mounts; for example adding a unreachable route to the NFS server (ip route add unreachable hostname), and then umount -f seems to cause I/O errors to tasks. (I can't find a way to do a remount to change the hard flag). This isn't pretty but it's a reasonable way of getting your host back to useable if one NFS server has died. Dave > > The only way to make sure a request has finished is to wait for > completion. If we treat a request as failed/cancelled but it's actually > still pending at a layer of the storage stack: > 1. Read requests may modify guest memory. > 2. Write requests may modify disk sectors. > > Today the guest times out and tries to do IDE/ATA recovery, for example. > This causes QEMU to eventually call the synchronous bdrv_drain_all() > function and the guest hangs. Also, if the guest mounts the file system > read-only in response to the timeout, then game over. > > The disk-deadlines feature lets QEMU detect timeouts before the guest so > we can pause the guest. The part I have been thinking about is that the > only option is to wait until the request completes. > > We cannot abandon the timed out request because we'll face #1 or #2 > above. This means it doesn't make sense to retry the request like > rerror=/werror=. rerror=/werror= can retry safely because the original > request has failed but that is not the case for timed out requests. > > This also means that live migration isn't safe, at least if a write > request is pending. If the guest migrates, the pending write request on > the source host could still complete after live migration handover, > corrupting the disk. > > Getting back to these patches: I think the implementation is correct in > that the only policy is to wait for timed out requests to complete and > then resume the guest. > > However, these patches need to violate the constraint that guest memory > isn't dirtied when the guest is paused. This is an important constraint > for the correctness of live migration, since we need to be able to track > all changes to guest memory. > > Just wanted to post this in case anyone disagrees. > > Stefan > -- Dr. David Alan Gilbert / dgilb...@redhat.com / Manchester, UK
