[libvirt] [RFC PATCH 00/12] Add block write threshold event

[Eric Blake]

On 02/23/2017 01:21 PM, Peter Krempa wrote:
> Since I'm a procrastinator and I wanted to do this after full node-name support
> is added I'm kind of late for this release here. Since oVirt is asking for this
> feature for a very long time I would like to deliver it ASAP though.

Thanks for picking up on this. I tried once a long time ago:
https://www.redhat.com/archives/libvir-list/2015-June/msg01111.html

and got stalled on node name weirdness, so it never went in.

> 
> This series adds a event which is fired once a guest writes beyond a configured
> offset in the backing file. The threshold offset is configurable via a new API.
> 
> This series is in RFC state since I did not manage to polish few details mostly
> connected to node names:
> 
> - currently only the top level image can be selected
> - some blockjobs may clear the remembered node names and thus break this
>   ( re-detection of the backing chain needs to be connected with node name
>     detection )
> - the node name detection code is VERY crude

Indeed, that's where I got stalled, but I think you may already be
further along than my attempt. (It may also be that the QMP side from
qemu is easier to use now than when I first tried)

> - I'm not sure how well it plays with qcow images, since I select only the top
>   level node (qcow protocol) rather than the file backing it

That may be a deal-breaker - my recollection is that you HAVE to use the
node-name of the file backing the qcow2 to get the useful threshold, and
not the node-name of the qcow2 protocol.

> - the documentation probably sucks since I did not proof-read
> 
> Any feedback is welcome.
> 
> Peter Krempa (12):
>   util: storage: Split out useful bits of virStorageFileParseChainIndex
>   util: storage: Add preliminary storage for node names into
>     virStorageSource
>   lib: Introduce event for tracking disk backing file write threshold
>   qemu: monitor: Add support for BLOCK_WRITE_THRESHOLD event
>   qemu: domain: Add helper to lookup disk by node name
>   qemu: domain: Add helper to generate indexed backing store names
>   qemu: process: Wire up firing of the
>     VIR_DOMAIN_EVENT_ID_BLOCK_THRESHOLD event
>   lib: Add API for setting the threshold size for
>     VIR_DOMAIN_EVENT_ID_BLOCK_THRESHOLD
>   virsh: Implement 'blockthreshold' command to call
>     virDomainSetBlockThreshold
>   qemu: domain: Add helper to look up disk soruce by the backing store
>     string
>   qemu: implement qemuDomainSetBlockThreshold
>   qemu: WIP: lookup nodenames

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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