[linux-lvm] lvmcache and bare mdadm arrays

[Eugene Vikhman]

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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