[libvirt-users] Using virsh blockcopy -- what's it supposed to accomplish?

[Gary R Hook]

On 12/22/14 4:50 PM, Eric Blake wrote:
> On 12/22/2014 03:27 PM, Gary R Hook wrote:
>> I am experimenting with the blockcopy command, and after figuring out
>> how to integrate qemu-nbd, nbd-client and
>> dumpxml/undefine/blockcopy/define/et. al. I have one remaining question:
>>
>> What's the point?
>
> Among other uses, live storage migration.

There is so very much to say here, but I will endeavor to be brief and 
to the point:

And then what?

Please note that I am working with libvirt 1.2.2, as stated in my OP. 
And up front: I'm pretty sure I'm missing data points and working from a 
position of ignorance and unreasonable expectation. Your bearing with me 
is much, much appreciated.

> Let's say you are running on a cluster, where your VM is running locally
> but was booted from network-accessed storage.  You don't want any guest
> downtime, but you want to have the faster performance made possible by
> accessing local storage instead of the network-accessed storage.  virsh
> blockcopy can be used to change qemu's notion of where the active layer
> of the disk lives without any guest time, by copying then pivoting to a
> local file.

I think I totally understand that. But I don't care about the old file, 
I care about the new one. And also, once you've moved to a block device 
(post-pivot) there's no going back, is there? The old qcow2 file can be 
an old snapshot, but that's about it. And the new file is not, in and of 
itself, usable as it stands. I tested a pivot and redefine of my domain 
using the new block device (/dev/nbd2, e.g.) and I was able to shut down 
and then start the domain successfully.

Which is no help whatsoever without forever including the NBD device. 
Or: what am I missing here?

I would expect that the domain, no matter where its disk files are 
located, may at some point need to be shutdown, then restarted. If I 
don't have an actual "mirror" (actually, a replication is what I want) 
then I'm still missing the point of this feature. Although I guess I 
could block copy and then migrate with non-shared storage to get 
something as usable and flexible as the original. Except that requires 
another host.

Let me summarize: I want to mirror a disk and at any point in time 
switch over and use the copy qcow2 file directly, in every way possible, 
as my new backing file. Including defining, shutting down and starting 
up domains arbitrarily. The old file is old news; forget about it. I 
want to move on. How does one accomplish that?

>> The "replication" disk file is not, from what I can ascertain, bootable.
>
> Correct in the current implementation, if you don't manually freeze
> guest I/O prior to the point where you abort the copy (whether you do a
> straight abort, leaving the copy as the point in time, or whether you do
> a pivot, leaving the original as the point in time).  But I would like
> to add a --quiesce option to blockcopy, similar to what is already
> available for snapshot-create --quiesce.  The idea is that just before
> breaking sync, you tell the guest to freeze all I/O, so that when you do
> break sync, the disk you are no longer using _is_ a consistent image
> (and depending on how well your guest is able to freeze I/O, it may well
> be bootable).  But until that is implemented, you can use 'virsh
> domfsfreeze' as the manual access to freezing guest I/O, if you have new
> enough qemu and also have qemu-guest-agent wired up in your guest.

Wow. Clearly something past 1.2.2. So my first step is to update to a 
current level? Dang it.


>> I expect this operation to create a pristine copy of my source qcow2
>> file (at a given point in time) which implies that I can swap that copy
>> in and use it just like the original.
>>
>> Neither using --finish nor --pivot (both appear successful) give me a
>> mirror that seems to serve any purpose. It seems especially pointless if
>> I use --pivot because anything that happens after the pivot ends up lost
>> if I don't actually have a usable qcow2 file.
>
> How is it not usable?  When you break sync at the conclusion of a
> blockcopy, the image that you no longer use is an accurate snapshot of
> the state of the disk at the time you broke sync; but whether or not
> that is useful to a guest depends on how much influence unflushed I/O
> that was still in guest memory at the time you broke sync will have on
> your data.

As stated above, I don't want the old file, I want the _new_ one. If I 
am running a block copy to a remote system (because, you know, that's 
one of the cool things about NBD) and my local system dies or otherwise 
becomes unrecoverable, I'd like to go to the remote system and bring up 
the guest there with minimal downtime in the event of a catastrophic 
failure. Right now that appears to be impossible. And I'm the only 
person to think this is reasonable.

For _me_, if I shut down my local guest, which causes the nbd-client 
process to exit, I expect the far end's server to flush blocks and 
create a disk file with integrity. What could it possibly be waiting for 
to prevent that from happening almost immediately? (He asks, without 
looking at the code...)

>> libvirt (1.2.2) and qemu (2.2.0) as distributed with Ubuntu Trusty.


-- 
Gary R Hook
Senior Kernel Engineer
NIMBOXX, Inc