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.
