[pve-devel] [PATCH v2 qemu-server 1/4] migration: avoid crash with heavy IO on local VM disk

[Fiona Ebner]

Am 03.07.24 um 15:15 schrieb Fabian Grünbichler:
> On May 28, 2024 10:50 am, Fiona Ebner wrote:
>> +	eval {
>> +	    mon_cmd(
>> +		$vmid,
>> +		"block-job-change",
>> +		id => $job,
>> +		type => 'mirror',
>> +		'copy-mode' => 'write-blocking',
>> +	    );
>> +	    $switching->{$job} = 1;
>> +	};
>> +	die "could not switch mirror job $job to active mode - $@\n" if $@;
>> +    }
>> +
>> +    while (1) {
>> +	my $stats = mon_cmd($vmid, "query-block-jobs");
>> +
>> +	my $running_jobs = {};
>> +	$running_jobs->{$_->{device}} = $_ for $stats->@*;
>> +
>> +	for my $job (sort keys $switching->%*) {
>> +	    if ($running_jobs->{$job}->{'actively-synced'}) {
>> +		print "$job: successfully switched to actively synced mode\n";
>> +		delete $switching->{$job};
>> +	    }
>> +	}
>> +
>> +	last if scalar(keys $switching->%*) == 0;
>> +
>> +	sleep 1;
>> +    }
> 
> so what could be the cause here for a job not switching? and do we
> really want to loop forever if it happens?
> 

That should never happen. The 'block-job-change' QMP command already
succeeded. That means further writes will be done synchronously to the
target. Once the remaining dirty parts have been mirrored by the
background iteration, the actively-synced flag will be set and we break
out of the loop.

We got to the ready condition already before doing the switch, getting
there again is even easier after the switch:
https://gitlab.com/qemu-project/qemu/-/blob/stable-9.0/block/mirror.c?ref_type=heads#L1078