There’s a –mirrorlog argument which can either be disk or core.The default is disk, which uses a small LV on a third physical device to hold the log file which tracks the synchronization between the mirrors. If you specify core (“–corelog” is a deprecated synonym for “–mirrorlog core”), the log is stored in memory. The “drawback” is that there’s no persistant log, then, so every time you reboot, the mirrored drive is rebuilt from the master.

Also, be aware that doing things like mounting by label or uuid are less than fun with the mirrors, since there end up being two devices with the same id (the filesystems are mirrored, after all) and you get odd results. Not that you’d mount by uuid if you’re using lvm anyway, but it seemed worth mentioning…

1st + 2nd for the mirror

lvcreate -L 1024 -m 1 –corelog vgdata