Re: [Qemu-devel] [PATCH 4/4] scsi: add persistent reservation manager using qemu-pr-helper

[Paolo Bonzini <pbonzini@redhat.com>]

On 19/09/2017 14:53, Daniel P. Berrange wrote:
>> +    /* Try to reconnect while sending the CDB.  */
>> +    for (attempts = 0; attempts < PR_MAX_RECONNECT_ATTEMPTS; attempts++) {
>
> I'm curious why you need to loop here. The helper daemon should be running
> already, as you're not spawning it on demand IIUC. So it shoudl either
> succeed first time, or fail every time.

You're focusing on the usecase where the helper daemon is spawned per-VM
by the system libvirtd, which I agree is the most important one.
However, the other usecase is the one with a global daemon, access to
which is controlled via Unix permissions.  This is not SELinux-friendly,
but it is nicer for testing and it is also the only possibility for user
libvirtd.

In that case, upgrading QEMU on the host could result in a "systemctl
restart qemu-pr-helper.service" (or, hopefully unlikely, a crash could
result in systemd respawning the daemon).  Reconnect is useful in that case.

Paolo