Comment by Conch5606

This is not quite the same, it's for migrating from one device to another while keeping the file system writable, but it's quite neat: dm-clone[1]

I've used it before for a low downtime migration of VMs between two machines - it was a personal project and I could have just kept the VM offline for the migration, but it was fun to play around with it.

You give it a read-only backing device and a writable device that's at least as big. It will slowly copy the data from the read-only device to the writable device. If a read is issued to the dm-clone target it's either gotten from the writable device if it's already cloned or forwarded to the read-only device. Writes are always going to the writable device and afterwards the read-only device is ignored for that block.

It's not the fastest, but it's relatively easy to set up, even though using device mapper directly is a bit clunky. It's also not super efficient, IIRC if a read goes to a chunk that hasn't been copied yet, that's used to give the data to the reading program, but it's not stored on the writable device, so it has to be fetched again. If the file system being copied isn't full, it's a good idea to run trimming after creating the dm-clone target as discarded blocks are marked as not needing to be fetched.

[1] https://docs.kernel.org/admin-guide/device-mapper/dm-clone.h...