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

[Daniel P. Berrange]

On Tue, Sep 19, 2017 at 02:57:00PM +0200, Paolo Bonzini wrote:
> 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.

If using systemd socket activation and you restart the daemon, the listening
socket should be preserved, so you shouldn't need to reconnect - the client
should get queued until it has started again (likewise on crash).

Regards,
Daniel
-- 
|: https://berrange.com      -o-    https://www.flickr.com/photos/dberrange :|
|: https://libvirt.org         -o-            https://fstop138.berrange.com :|
|: https://entangle-photo.org    -o-    https://www.instagram.com/dberrange :|