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? -- Sergei Shtepa Veeam Software developer.
