linux-kernel - Re: [PATCH 0/2] block layer filter and block device snapshot module

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Date:   Thu, 22 Oct 2020 12:44:02 +0300
From:   Sergei Shtepa <sergei.shtepa@...am.com>
To:     Hannes Reinecke <hare@...e.de>
CC:     "axboe@...nel.dk" <axboe@...nel.dk>,
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Subject: Re: [PATCH 0/2] block layer filter and block device snapshot module

The 10/22/2020 08:58, Hannes Reinecke wrote:
> On 10/21/20 4:10 PM, Sergei Shtepa wrote:
> > The 10/21/2020 16:31, Hannes Reinecke wrote:
> >> I do understand where you are coming from, but then we already have a
> >> dm-snap which does exactly what you want to achieve.
> >> Of course, that would require a reconfiguration of the storage stack on
> >> the machine, which is not always possible (or desired).
> > 
> > Yes, reconfiguring the storage stack on a machine is almost impossible.
> > 
> >>
> >> What I _could_ imagine would be a 'dm-intercept' thingie, which
> >> redirects the current submit_bio() function for any block device, and
> >> re-routes that to a linear device-mapper device pointing back to the
> >> original block device.
> >>
> >> That way you could attach it to basically any block device, _and_ can
> >> use the existing device-mapper functionality to do fancy stuff once the
> >> submit_io() callback has been re-routed.
> >>
> >> And it also would help in other scenarios, too; with such a
> >> functionality we could seamlessly clone devices without having to move
> >> the whole setup to device-mapper first.
> > 
> > Hm...
> > Did I understand correctly that the filter itself can be left approximately
> > as it is, but the blk-snap module can be replaced with 'dm-intercept',
> > which would use the re-route mechanism from the dm?
> > I think I may be able to implement it, if you describe your idea in more
> > detail.
> > 
> > 
> Actually, once we have an dm-intercept, why do you need the block-layer 
> filter at all?
>  From you initial description the block-layer filter was implemented 
> such that blk-snap could work; but if we have dm-intercept (and with it 
> the ability to use device-mapper functionality even for normal block 
> devices) there wouldn't be any need for the block-layer filter, no?

Maybe, but the problem is that I can't imagine how to implement
dm-intercept yet. 
How to use dm to implement interception without changing the stack
of block devices. We'll have to make a hook somewhere, isn`t it?

> 
> Cheers,
> 
> Hannes
> -- 
> Dr. Hannes Reinecke                Kernel Storage Architect
> hare@...e.de                              +49 911 74053 688
> SUSE Software Solutions GmbH, Maxfeldstr. 5, 90409 Nürnberg
> HRB 36809 (AG Nürnberg), Geschäftsführer: Felix Imendörffer

-- 
Sergei Shtepa
Veeam Software developer.