Re: [Qemu-devel] [Qemu-block] [PATCH COLO v3 01/14] docs: block replication's description

[Stefan Hajnoczi <stefanha@gmail.com>]

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On Tue, Apr 21, 2015 at 09:25:59AM +0800, Wen Congyang wrote:
> On 04/20/2015 11:30 PM, Stefan Hajnoczi wrote:
> > On Fri, Apr 03, 2015 at 06:01:07PM +0800, Wen Congyang wrote:
> > One general question about the design: the Secondary host needs 3x
> > storage space since it has the Secondary Disk, hidden-disk, and
> > active-disk.  Each image requires a certain amount of space depending on
> > writes or COW operations.  Is 3x the upper bound or is there a way to
> > reduce the bound?
> 
> active disk and hidden disk are temp file. It will be maked empty in
> bdrv_do_checkpoint(). Their format is qcow2 now, so it doesn't need too
> many spaces if we do checkpoint periodically.

A question related to checkpoints: both Primary and Secondary are active
(running) in COLO.  The Secondary will be slower since it performs extra
work; disk I/O on the Secondary has a COW overhead.

Does this force the Primary to wait for checkpoint commit so that the
Secondary can catch up?

I'm a little confused about that since the point of COLO is to avoid the
overheads of microcheckpointing, but there still seems to be a
checkpointing bottleneck for disk I/O-intensive applications.

> > 
> > The bound is important since large amounts of data become a bottleneck
> > for writeout/commit operations.  They could cause downtime if the guest
> > is blocked until the entire Disk Buffer has been written to the
> > Secondary Disk during failover, for example.
> 
> OK, I will test it. In my test, vm_stop() will take about 2-3 seconds if
> I run filebench in the guest. Is there anyway to speed it up?

Is it necessary to commit the active disk and hidden disk to the
Secondary Disk on failover?  Maybe the VM could continue executing
immediately and run a block-commit job.  The active disk and hidden disk
files can be dropped once block-commit finishes.

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