Hi.

I spent the last couple of days trying to set up a new server with 16 sas drives for the main storage and 2 SSDs for caching it using lvmcache.

First I used mdadm to create raid10 on HDDs and raid1 on SSDs. Then I followed the steps from the lvmcache manpage and the lvcreate/lvconvert manpages to create the cache and the metadata LVs and convert them to a cache-pool. It went fine, "lvs -a" showed a normal cache-pool device. But the last step failed with an ambiguous error when I used lvconvert --type cache --cachepool vg/cache vg/slow. I tried playing with parameters and made sure that the error had nothing to do with them. For an unidentified reason ioctl failed saying it could not lock the device vg/slow.

Today I tried setting up the raids with lvm itself and it all worked like a charm.

So my thought is maybe manpages should actually mention something about it and say "don't try setting up caching on a bare mdadm raid - it won't work".