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Date:   Wed, 21 Oct 2020 17:10:44 +0300
From:   Sergei Shtepa <sergei.shtepa@veeam.com>
To:     Hannes Reinecke <hare@suse.de>
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Subject: Re: [PATCH 0/2] block layer filter and block device snapshot module
Message-ID: <20201021141044.GF20749@veeam.com>
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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.


-- 
Sergei Shtepa
Veeam Software developer.