So I've used lvmcache on two different hosts and so far the result haven't been what I expected.
Both machines were using the same settings: OS installed on SSD's with 50% of the SSD's for the raid1 of the OS and 50% for a raid0 dedicated to lvmcache. Also the filesystem cached was XFS. the mode was writethrough.
The first host was a fileserver running centos 7. I expected a lot of regular traffic of all kind (small files, huge files). One day I was moving a lot of files (virtual machines) and during the transfer: kernel panic. This machine was totally stable until I activated lvmache. Note that it ran for weeks for the panic to happen, but it did happen eventually.
The second host was also a file server hosting huge datasets. Power failure. I couldn't remount the partition, the machine just hanged, no kernel panic, just no response at all 30 sec after the machine was rebooted. I had to boot with a liveusb, remove the cache, and reboot. Something during the activation of lvm caused the kernel to do so much stuff that it was totally unresponsive.
lvmcache is still tempting, but I will stay away from it for now until it's more stable. Maybe using a full device instead of a raid device could help ? But I don't want to make tests anymore, I expected more stability.
Any YOU, have you tested lvmcache? What are your results?
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