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Re: [Qemu-devel] Qemu and Changed Block Tracking

From: Peter Lieven
Subject: Re: [Qemu-devel] Qemu and Changed Block Tracking
Date: Thu, 23 Feb 2017 15:27:32 +0100
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Am 22.02.2017 um 13:32 schrieb Eric Blake:
On 02/22/2017 02:45 AM, Peter Lieven wrote:
A bit outdated now, but:

and also a summary I wrote not too far back (PDF):

and I'm sure the Virtuozzo developers could chime in on this subject,
but basically we do have something similar in the works, as eblake says.
Hi John, Hi Erik,
It's Eric, but you're not the first to make that typo :)

thanks for your feedback. Are you both the ones working primary on this topic?
If there is anything to review or help needed, please let me know.

My 2 cents:
I thing I had in mind if there is no image fleecing available, but fetching the 
dirty bitmap
from external would be a feauture to put a write lock on a block device.
The whole idea is to use a dirty bitmap coupled with image fleecing,
where the point-in-time of the image fleecing is done at a window where
the guest I/O is quiescent in order to get a stable fleecing point.  We
already support write locks (guest quiesence) using qga to do fsfreeze.
You want the time that guest I/O is frozen to be as small as possible
(in particular, the Windows implementation of quiescence will fail if
you hold things frozen for more than a couple of seconds).

Right now, the qcow2 image format does not track write generations, and
I don't think we plan on adding that directly into qcow2.  However, you
can externally simulate write generations by keeping track of how many
image fleecing points you have created (each fleecing point is another
write generation).

In this case something like this via QMP (and external software) should work:
  gen =  write generation of last backup (or 0 for full backup)
  do {
      nextgen = fetch current write generation (via QMP)
      dirtymap = send all block whose write generation is greater than 'gen' 
(via QMP)
No, we are NOT going to send dirty information via QMP.  Rather, we are
going to send it via NBD's extension NBD_CMD_BLOCK_STATUS.  The idea is
that a client connects and asks which qemu blocks are dirty, then uses
that information to read only the dirty blocks.

I understand, that for the case of local storage connecting via NBD to Qemu to 
grep a snapshot
might be a good idea, but consider that you have a NAS for your vServer images. 
May it be NFS,
iSCSI, CEPH or whatever. In an enterprise scenario I would generally except to 
have a NAS rather
than local storage.

When you are going to backup your vServer (full or incremental) you shuffle all 
the traffic through
Qemu and your Node running the vServer. In this case you run all the traffic 
over the wire twice.

NAS -> Node -> Qemu - > Backup Server

But the Backup Server could instead connect to the NAS directly avoiding load 
on the frontent LAN
and the Qemu Node.

I would like to find a nice solution for this scenario. If not in the first 
step it would maybe be good to
have this in mind when implementing a dirty block tracking.


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