cpc

A copy tool for incremental copies of large files, such as databases.

It's like rsync --inplace but a bit faster.

It uses a thread per core and doesn't do writes of 4KB pages that are unchanged.

Problem statement

We had two filesystems on a machine and some large SQLite databases and other files on one filesystem that we wanted to move to the other filesystem. More specifically: two ext4 filesystems on separate AWS EBS block devices, both attached to the same Linux 64-core VM with lots of memory (larger than the data to be copied).

To minimize service disruption, we wanted to do a live inconsistent copy of the data to get most of it over, then stop the service, then do another quick increment copy, then start it up again.

This tool let us do the migration with minimal downtime.