[Devel] [PATCH RH9] dm-ploop: port the standby mode feature.

[Alexander Atanasov]

On 26.10.22 16:15, Kui Liu wrote:
> 
> 
> -----Original Message-----
> From: Alexander Atanasov <alexander.atanasov at virtuozzo.com>
> Date: Wednesday, 26 October 2022 at 5:14 PM
> To: Kui Liu <Kui.Liu at acronis.com>, Konstantin Khorenko <khorenko at virtuozzo.com>, Alexey Kuznetsov <kuznet at acronis.com>
> Cc: Devel <devel at openvz.org>
> Subject: Re: [Devel] [PATCH RH9] dm-ploop: port the standby mode feature.
> 
>      Hi,
> 
>      On 21.10.22 16:28, Kui Liu wrote:
>      >
>      >
>      > -----Original Message-----
>      > From: Alexander Atanasov <alexander.atanasov at virtuozzo.com>
>      > Date: Friday, 21 October 2022 at 7:35 PM
>      > To: Kui Liu <Kui.Liu at acronis.com>, Konstantin Khorenko <khorenko at virtuozzo.com>, Alexey Kuznetsov <kuznet at acronis.com>
>      > Cc: Devel <devel at openvz.org>
>      > Subject: Re: [Devel] [PATCH RH9] dm-ploop: port the standby mode feature.
>      >
>      >      On 21.10.22 14:28, Kui Liu wrote:
>      >      >      >      >      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.
>      >      >
>      >      >      This rised a question - how and who will pass the module parameter when
>      >      >      required ? How we will handle the existin users ? What do you think ?
>      >      >
>      >      > [LIU]: Like add a conf file ploop.conf in /etc/modprobe.d or lib/modeprobe.d? This conf file can be shipped with the release image, or installed later.  Are you referring to VZ7 as existing users? I saw you also made the changes to VZ7, but I'm not sure whether it is necessary to backport these changes to VZ7.
>      >
>      >      Yes, VZ7 users - there are issues related to it so trying to fix them.
>      >      the modprobe.d would do but do you have a package that can install the
>      >      conf tied to the iSCSI target, so it gets installed only if the iSCSI is
>      >      used ?
>      >
>      > [LIU]: Yes, we do build the our own SCST package, so we can install the conf from there.
> 
>      After discussion turns out module_param is not an option - package is
>      always installed so it will enable the key always which is not what we want.
> 
>      To solve this we can
>        struct static_key ploop_standby_check = STATIC_KEY_INIT_FALSE;
>      +EXPORT_SYMBOL(ploop_standby_check)
> 
>      and the module can enable it at use or init time.
> 
>      static int scst_vstor_pr_init(struct scst_device *dev, const char
>      *cl_dev_id)
> 
>      looks like the place to enable the static key and set the _en flag on
>      the queue.
> 
>      _en flag is a MUST static key is a SHOULD - where do you think is the
>      best place to implement them in the scst code?
> 
> [LIU]:  This would introduce explicit dependency between SCST and ploop module,  which I feel is inappropriate .
> Can we instead export the static key via a sysfs file, say /sys/kernel/ploop/ploop_standby_check?

We can export it via sysfs or proc but who and how  will enable it thru 
there ?

-- 
Regards,
Alexander Atanasov