Pool interface questions

[David Hobach]

Dear devs,

I'm trying to understand the qubes.storage.Pool interface and its
requirements for implementations.

In particular I wonder:
Are implementations required to be fully initialized right after the
constructor is called?

I'm asking because I prefer "lazy initialization"; I don't want to
execute a lot of code for initialization and then later the user doesn't
even start any VM running on my pool.

For example, you usually wouldn't want to decrypt a pool until the user
requests some data from it.

Or do I have to implement "lazy initialization" myself?

I.e. I guess I'd then have to one-time run the initialization code on
any interface method that would require me to be initialized?

Best Regards
David

[Rusty Bird]

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David Hobach:

> I'm trying to understand the qubes.storage.Pool interface and its
> requirements for implementations.
>
> In particular I wonder:
> Are implementations required to be fully initialized right after the
> constructor is called?

You don't necessarily have to do anything in __init__(), except
forward the call to the base class so it can set some standard
attributes like 'name'.

> For example, you usually wouldn't want to decrypt a pool until the user
> requests some data from it.
>
> Or do I have to implement "lazy initialization" myself?
>
> I.e. I guess I'd then have to one-time run the initialization code on any
> interface method that would require me to be initialized?

That sounds right. It's really up to the driver which of the public
methods - e.g. vol.remove() - can operate independently on a volume,
and which need to arrange some some heavy per-pool or global state.

Rusty
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[David Hobach]

On 6/14/20 3:54 PM, Rusty Bird wrote:
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> David Hobach:

>> For example, you usually wouldn't want to decrypt a pool until the user
>> requests some data from it.
>>
>> Or do I have to implement "lazy initialization" myself?
>>
>> I.e. I guess I'd then have to one-time run the initialization code on any
>> interface method that would require me to be initialized?
>
> That sounds right. It's really up to the driver which of the public
> methods - e.g. vol.remove() - can operate independently on a volume,
> and which need to arrange some some heavy per-pool or global state.

I just noticed that Pool.init_volume() is the last Pool method to be
called before a VM starts. Unfortunately it's called during boot for all
VMs, so not exactly what I'd call "late".

So I guess I'd also have to proxy the Volume class for that feature? -_-

Just please let me know if you have a better idea.

Otherwise enjoy the weekend & Best Regards
David

[Marek Marczykowski-Górecki]

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On Sat, Jun 20, 2020 at 07:00:10PM +0200, David Hobach wrote:
> On 6/14/20 3:54 PM, Rusty Bird wrote:
> > -----BEGIN PGP SIGNED MESSAGE-----
> > Hash: SHA512
> >
> > David Hobach:
> > > For example, you usually wouldn't want to decrypt a pool until the user
> > > requests some data from it.
> > >
> > > Or do I have to implement "lazy initialization" myself?
> > >
> > > I.e. I guess I'd then have to one-time run the initialization code on any
> > > interface method that would require me to be initialized?
> >
> > That sounds right. It's really up to the driver which of the public
> > methods - e.g. vol.remove() - can operate independently on a volume,
> > and which need to arrange some some heavy per-pool or global state.
>
> I just noticed that Pool.init_volume() is the last Pool method to be called
> before a VM starts. Unfortunately it's called during boot for all VMs, so
> not exactly what I'd call "late".

This method is used just to create instance of appropriate volume
object. The actual data doesn't need to be available at this point.
See here for documentation about various methods:
https://dev.qubes-os.org/projects/core-admin/en/latest/qubes-storage.html#storage-pool-driver-api

> So I guess I'd also have to proxy the Volume class for that feature? -_-

I think so. The right place for "initialize storage before VM start" is
Volume.start().

- --
Best Regards,
Marek Marczykowski-Górecki
Invisible Things Lab
A: Because it messes up the order in which people normally read text.
Q: Why is top-posting such a bad thing?
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