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

[Darrick J. Wong]

On Thu, Oct 22, 2020 at 04:52:13PM +0300, Sergei Shtepa wrote:
> The 10/22/2020 13:28, Damien Le Moal wrote:
> > On 2020/10/22 18:43, Sergei Shtepa wrote:
> > > 
> > > 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.
> > 
> 
> Is there something I don't understand? Please correct me.
> 
> Let me remind that by the condition of the problem, we can't change
> the configuration of the block device stack.
> 
> Let's imagine this configuration: /root mount point on ext filesystem
> on /dev/sda1.
> +---------------+
> |               |
> |  /root        |
> |               |
> +---------------+
> |               |
> | EXT FS        |
> |               |
> +---------------+
> |               |
> | block layer   |
> |               |
> | sda queue     |
> |               |
> +---------------+
> |               |
> | scsi driver   |
> |               |
> +---------------+
> 
> We need to add change block tracking (CBT) and snapshot functionality for
> incremental backup.
> 
> With the DM we need to change the block device stack. Add device /dev/sda1
> to LVM Volume group, create logical volume, change /etc/fstab and reboot.
> 
> The new scheme will look like this:
> +---------------+
> |               |
> |  /root        |
> |               |
> +---------------+
> |               |
> | EXT FS        |
> |               |
> +---------------+
> |               |
> | LV-root       |
> |               |
> +------------------+
> |                  |
> | dm-cbt & dm-snap |
> |                  |
> +------------------+
> |               |
> | sda queue     |
> |               |
> +---------------+
> |               |
> | scsi driver   |
> |               |
> +---------------+
> 
> But I cannot change block device stack. And so I propose a scheme with
> interception.
> +---------------+
> |               |
> |  /root        |
> |               |
> +---------------+
> |               |
> | EXT FS        |
> |               |
> +---------------+   +-----------------+
> |  |            |   |                 |
> |  | blk-filter |-> | cbt & snapshot  |
> |  |            |<- |                 |
> |  +------------+   +-----------------+
> |               |
> | sda blk queue |
> |               |
> +---------------+
> |               |
> | scsi driver   |
> |               |
> +---------------+
> 
> Perhaps I can make "cbt & snapshot" inside the DM, but without interception
> in any case, it will not work. Isn't that right?

Stupid question: Why don't you change the block layer to make it
possible to insert device mapper devices after the blockdev has been set
up?

--D

> 
> -- 
> Sergei Shtepa
> Veeam Software developer.