Or: **How to throttle down one LVM mirror leg lowering its assumed synchronization cpu load?**

Is it possible to tweak a LVM mirror configuration so it takes it easy on mirroring files?

Whenever there is some big write, the machine hangs for some seconds (to many).

If LVM could mirror such files in a slower pace, I don't really care having to wait some minutes so the full mirroring completes; LVM could even copy big files little by little.

**As an alternative**: I am thinking on daemonising unison to backup all small files every minute, and big files I could write a script to copy in a less intensive way.

Any tip?

PS.: I am using KVPM to manage it.

**CGroup (how?):** 
Currently I am trying to use CGroup io throttle, so I can keep LVM mirror active. 
I found some info [here](https://www.redhat.com/archives/dm-devel/2016-February/msg00192.html).
I am quite scared of make things stop working also, so I am trying to be sure on what to do... 
The lvm mirror leg I want to slow down is at `/dev/mapper/GRP-NM_mimage_1 -> /dev/dm-1 ` from `/dev/sda4` (the fast one is at /dev/sdb2). 

`lvdisplay -v /dev/GRP/NM` shows a block device without specifying what mirror leg it is related to... 

`pvdisplay -mv` shows `Physical extent 102839 to 117077, Logical volume /dev/GRP/NM_mimage_1, Logical extents 0 to 14238`

basically, from [this](https://serverfault.com/a/563173/163750) answer, I miss what major:minor to use.

[this](https://serverfault.com/a/747103/163750) command `sudo lvdisplay -am` let me determine the correct major:minor for the mirror leg I need to throttle writes down, now lets test it!