If you can pause writes to the disk while reading from it (and you are using a supported filesystem, and you aren't doing or can ignore any raw device accesses)

If you can pause writes to the disk while reading from it (and you are using a supported filesystem, and you aren't doing or can ignore any raw device accesses), it would be much easier to use fsfreeze(8) instead. Caution is of course required when using this on the root filesystem, but I would trust it far more than anything using blktrace. My guess, having used it before but not closely investigated it, is that while it may appear reliable at first, it is most likely not highly tested to drop no events under load, and the same applies for this tool. On the other hand, as long as I've worked out that fsfreeze won't deadlock the whole machine, I'm reasonably confident that it will result in a correct disk image.