On 04/27/2010 11:14 AM, Avi Kivity wrote:
On 04/27/2010 01:36 AM, Anthony Liguori wrote:
A few comments:
1) The problem was not block watermark itself but generating a
notification on the watermark threshold. It's a heuristic and should
be implemented based on polling block stats.
Polling for an event that never happens is bad engineering. What
frequency do you poll? you're forcing the user to make a lose-lose
tradeoff.
Otherwise, we'll be adding tons of events to qemu that we'll struggle
to maintain.
That's not a valid reason to reject a user requirement. We may argue the
requirement is bogus, or that the suggested implementation is wrong and
point in a different direction, but saying that we may have to add more
code in the future due to other requirements is ... well I can't find a
word for it.
2) A block plugin doesn't solve the problem if it's just at the
BlockDriverState level because it can't interact with qcow2.
Why not? We have a layered model. guest -> qcow2 -> plugin (sends event)
-> raw-posix. Just need to insert the plugin at the appropriate layer.
3) For general block plugins, it's probably better to tackle userspace
block devices. We have CUSE and FUSE already, a BUSE is a logical
conclusion.
We also have an nbd client.
Here's another option: an nbd-like protocol that remotes all BlockDriver
operations except read and write over a unix domain socket. The open
operation returns an fd (SCM_RIGHTS strikes again) that is used for read
and write. This can be used to implement snapshots over LVM, for example.
Why w/o read/writes? the watermark code needs them too (as info, not the
actual buffer).
IMHO the whole thing is way over engineered:
a) Having another channel into qemu is complicating management
software. Isn't the monitor should be the channel? Otherwise we'll
need to create another QMP (or nbd like Avi suggest) for these
actions. It's extra work for mgmt and they will have hard time to
understand events interleaving of the various channels
b) How the plugins are defined? Is it scripts? Binaries? Do they open
their own sockets?
So I suggest either to stick with qmp or to have new block layer but let
qmp pass events from it - this is actually the nbd-like approach but
with qmp socket.
Thanks,
Dor
--
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
