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@xxxxxxx +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
