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

[Daniel P. Berrange]

On Tue, Sep 19, 2017 at 03:23:09PM +0200, Paolo Bonzini wrote:
> On 19/09/2017 15:12, Daniel P. Berrange wrote:
> > 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).
> 
> Oh, that's cool.  I didn't know that.  However, systemd socket
> activation is optional, and it's only a handful of lines so I think it's
> a bit nicer behavior (chardevs for example have options to reconnect).

The downside is that if someone forget to start the daemon, or enable
the socket,  QEMU will spin for 5 seconds trying to reconnect, instead
of reporting an error immediately. 

Regards,
Daniel
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