LVM2 natively used device-mapper and its RAIDx modules.
It is a very common setup to run LVM ontop of MD raid these days. I believe the reason is simply that users do not know about the --target RAIDx option, most likely because the original documentation is very old and not very thorough (http://sourceware.org/lvm2/)
Steps to Reproduce:
For example, I have two physical disks /dev/sdb2 /dev/sdc2:
# lvcreate -n userDataRaid -L 300G --type raid1 serverStorage -v
[91006.274995] device-mapper: raid: Superblocks created for new array
[91006.286605] md/raid1:mdX: not clean -- starting background reconstruction
[91006.286613] md/raid1:mdX: active with 2 out of 2 mirrors
[91006.286619] Choosing daemon_sleep default (5 sec)
[91006.286669] created bitmap (300 pages) for device mdX
[91006.372951] mdX: bitmap file is out of date, doing full recovery
[91007.032120] mdX: bitmap initialized from disk: read 19 pages, set 614400 of 614400 bits
[91007.199941] md: resync of RAID array mdX
[91007.199956] md: minimum _guaranteed_ speed: 1000 KB/sec/disk.
[91007.199961] md: using maximum available idle IO bandwidth (but not more than 200000 KB/sec) for resync.
[91007.199971] md: using 128k window, over a total of 314572800k.
[95111.688061] md: mdX: resync done.
[95111.756975] RAID1 conf printout:
[95111.756991] --- wd:2 rd:2
[95111.757000] disk 0, wo:0, o:1, dev:dm-6
[95111.757005] disk 1, wo:0, o:1, dev:dm-8
# lvs -a -o+devices
userDataRaid serverStorage mwc-ao 300,00g 100,00 userDataRaid_rimage_0(0),userDataRaid_rimage_1(0)
[userDataRaid_rimage_0] serverStorage -wc-ao 300,00g /dev/sdc2(140801)
[userDataRaid_rimage_1] serverStorage -wc-ao 300,00g /dev/sdb2(140801)
[userDataRaid_rmeta_0] serverStorage -wc-ao 4,00m /dev/sdc2(140800)
[userDataRaid_rmeta_1] serverStorage -wc-ao 4,00m /dev/sdb2(140800)
lvcreate --type RAID1 will create a striped mirroed target, whereas --mirrror 1 will create a mirrored volyme where reads will only happen from the primary volume.
Performance with RAID1 target is much better.
RedHat LVM2 RAID1/4/5/6 documentation: https://access.redhat.com/knowledge/docs/en-US/Red_Hat_Enterprise_Linux/6/html/Logical_Volume_Manager_Administration/raid_volumes.html
This document has been marked as obsolete due to insufficient resources. However, a more active document can be found on the Gentoo Wiki at https://wiki.gentoo.org/wiki/LVM. We welcome any contributions on this guide and recommend you create an account (if you do not have one already) and make the adjustments to the article as needed. In case of doubt, use the Talk page to discuss potential changes before applying them.
If needed, the wiki-style version of the old document is available at https://wiki.gentoo.org/wiki/User:SwifT/Wikified_but_not_merged_documents/LVM2 so any previously existing content can, if needed, be reused on the wiki.