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

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Date:   Thu, 22 Oct 2020 07:58:22 +0200
From:   Hannes Reinecke <hare@...e.de>
To:     Sergei Shtepa <sergei.shtepa@...am.com>
Cc:     "axboe@...nel.dk" <axboe@...nel.dk>,
        "viro@...iv.linux.org.uk" <viro@...iv.linux.org.uk>,
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        "linux-mm@...ck.org" <linux-mm@...ck.org>
Subject: Re: [PATCH 0/2] block layer filter and block device snapshot module

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?

Cheers,

Hannes
-- 
Dr. Hannes Reinecke                Kernel Storage Architect
hare@...e.de                              +49 911 74053 688
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