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".
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