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

[Damien Le Moal <Damien.LeMoal@wdc.com>]

On 2020/10/22 18:43, Sergei Shtepa wrote:
> 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?

Once your dm-intercept target driver is inserted with "dmsetup" or any user land
tool you implement using libdevicemapper, the "hooks" will naturally be in place
since the dm infrastructure already does that: all submitted BIOs will be passed
to dm-intercept through the "map" operation defined in the target_type
descriptor. It is then that driver job to execute the BIOs as it sees fit.

Look at simple device mappers like dm-linear or dm-flakey for hints of how
things work (driver/md/dm-linear.c). More complex dm drivers like dm-crypt,
dm-writecache or dm-thin can give you hints about more features of device mapper.
Functions such as __map_bio() in drivers/md/dm.c are the core of DM and show
what happens to BIOs depending on the the return value of the map operation.
dm_submit_bio() and __split_and_process_bio() is the entry points for BIO
processing in DM.

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


-- 
Damien Le Moal
Western Digital Research