Am 06.09.2013 um 11:18 hat Fam Zheng geschrieben: > On Fri, 09/06 10:45, Kevin Wolf wrote: > > Am 06.09.2013 um 09:56 hat Fam Zheng geschrieben: > > > Since BlockDriver.bdrv_snapshot_create() is an optional operation, > > > blockdev.c > > > can navigate down the tree from top node, until hitting some layer where > > > the op > > > is implemented (the QCow2 bs), so we get rid of this top_node_below_filter > > > pointer. > > > > Is it even inherent to a block driver (like a filter), if a snapshot is > > to be taken at its level? Or is it rather a policy decision that should > > be made by the user? > > > OK, getting the point that user should have full flexibility and fine > operation > granularity. It also stands against block_backend->top_node_below_filter. Do > we > really have the assumption that all the filters are on top of the tree and > linear? > Shouldn't this be possible? > > Block Backend > | > | > Quodrum BDS > / | \ > iot filter | \ > / | \ > qcow2 qcow2 qcow2 > > So we throttle only a particular image, not the whole device. But this will > make a top_node_below_filter pointer impossible.
I was assuming that Benoît's model works for the special case of snapshotting in one predefined way, but this is actually a very good example of why it doesn't. The approach relies on snapshotting siblings together, and in this case the siblings would be iot/qcow2/qcow2, while iot is still a filter. This would mean that either iot needs to be top_node_below_filter and throttling doesn't stay on top, or the left qcow2 is top_node_below_filter and the other Quorum images aren't snapshotted. > > In our example, the quorum driver, it's not at all clear to me that you > > want to snapshot all children. In order to roll back to a previous > > state, one snapshot is enough, you don't need multiple copies of the > > same one. Perhaps you want two so that we can still compare them for > > verification. Or all of them because you can afford the disk space and > > want ultimate safety. I don't think qemu can know which one is true. > > > Only if quorum ever knows about and operates on snapshots, it should be > considered specifically, but no. So we need to achieve this in the general > design: allow user to take snapshot, or set throttle limits on particular > BDSes, as above graph. > > > In the same way, in a typical case you may want to keep I/O throttling > > for the whole drive, including the new snapshot. But what if the > > throttling was used in order to not overload the network where the image > > is stored, and you're now doing a local snapshot, to which you want to > > stream the image? The I/O throttling should apply only to the backing > > file, not the new snapshot. > > > Yes, and OTOH, throttling really suits to be a filter only if it can be a non > top one, otherwise it's no better than what we have now. Well, it would be a cleaner architecture in any case, but having it in the middle of the stack feels useful indeed, so we should support it. > > So perhaps what we really need is a more flexible snapshot/BDS tree > > manipulation command that describes in detail which structure you want > > to have in the end. Designing the corresponding QMP command is the hard part, I guess. Kevin
