Thank you very match, Bastian Blank! Dracut help fix this issue: 0. add cache to root logical volume. 1. I install dracut (it remove initramfs-tools) 2. add modules lvm and mdadm to dracut 3. update all initrd-images by dracut 4. add GRUB_CMDLINE_LINUX="rd.md=1 rd.md.conf=1 rd.auto=1" to /etc/default/grub (becouse system not boot after switch to dracut in my case) for init mdadm by initramfs at boot. 5. dpgk-reconfigure grub-pc 6. reboot twice 7. after all reboot - dirty cache for root lv was normal.
dracut + some tuning = fix dirty cache "bug" on root. But I think this problem was not in lvmcache, but in initramfs-tools, which not correct unmount root? So lvmcache simply show this problem... And without lvmcache but with initramfs-tools - at any reboot root was not correctly unmount? --
