-----Original Message-----
From: Alexander Atanasov <alexander.atana...@virtuozzo.com>
Date: Wednesday, 19 October 2022 at 4:02 PM
To: Kui Liu <kui....@acronis.com>, Konstantin Khorenko
<khore...@virtuozzo.com>, Alexey Kuznetsov <kuz...@acronis.com>
Cc: Devel <devel@openvz.org>
Subject: Re: [Devel] [PATCH RH9] dm-ploop: port the standby mode feature.
On 19.10.22 9:58, Kui Liu wrote:
>
>
> > >
> > > + if
(blk_queue_standby_en(ploop_blk_queue(ploop))) {
> > > +
blk_queue_flag_clear(QUEUE_FLAG_STANDBY, ploop_blk_queue(ploop));
> > > + if
(!static_key_enabled(&ploop_standby_check)) {
> > > +
static_key_enable(&ploop_standby_check);
> > > + pr_info("ploop: standby mode
check enabled\n");
> > > + }
> > > + }
> > > +
> > >
> > > [LIU]: This may not work, because normally a ploop device
is created first before being attached to SCST, which means 'ploop_ctr' is
called before QUEUE_FLAG_STANDBY_EN can be set. We need to put "
static_key_enable(&ploop_standby_check))" somewhere else.
> >
> > I am not sure i undestand that correctly but dm-ploop maps
the ploop
> > device over existing device. It first opens the files that
are on the
> > existing device then passes the fds to dm-ploop. To do this
the target
> > must be up and running. device mapper setups the table before
calling
> > the ploop ctr. You already checked the queue lifecycle.
> > Is there any other precodition for the target to set the EN
flag?
> >
> >
> > There is one more alternative to pass the stanby enable flag
enable from
> > userspace .
> >
> > [LIU]: well, you are right about the dm-ploop setup process.
However SCST can only set the EN flag after the ploop device is assigned to
> > SCST, before which the table could have been loaded. So if setup
sequence is 1) create ploop device -> 2) load table -> 3) assign ploop
> > device to SCST, then the outer if condition won't be true, hence
not triggering static_key_enable().
> > My concern is this should not be the only place where the static
key can be enabled. Now that there is one more alternative in user space
> > , above code is OK.
>
> Ok - for the user space there are two options:
> 1. add an argument to enable at creation time
> 2. command standby_enable /if using the enable it is possible to
> implement a disable command too/.
>
> I prefer the second approach but let's hear other opinions too.
> And yes we can possibly do both but is there a use case ?
>
> [LIU]: Since the static key is a global option, I also think the second
approach is better.
> In case of nodes with vStorage + ploop + iSCSI setup, we just need to
enable the static
> key early in system init or iSCSI service init. I don't think it is
necessary to implement both.
You are right. It will need at least one ploop device created to issue a
device mapper command to enable standby. Would a module parameter work?
[LIU] Yes, I think it is better to make the static key a module parameter.
--
Regards,
Alexander Atanasov
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