Hello

I just wanted to share a scenario with you and perhaps exchange more information with other people that may also have a similar scenario.

For a couple of months I have been running a oVirt Node (CentOS 7.3 Minimal) with bcache (https://bcache.evilpiepirate.org/) for caching a SSD with HDD disks. The setup is simple and was made for a prof of concept and since them has been working better than expected.
This is a standalone host with 4 disks being: 1 for Operating System, 2 x 2TB 7200 RPM in software RAID 1 and 1 x PCI-E NVMe 400GB SSD which plays the caching device for both reads and writes. The VM storage folder is mounted as a ext4 partition on the logical device created by bcache (/dev/bcache0). All this is transparent to oVirt as all it sees is a /folder to put the VMs.

We monitor the IOPS on all block devices individually and see the behavior exactly as expected: random writes are all done on the SSD first and them streamed sequentially to the mechanical drives with pretty impressive performance. Also in the beginning while the total amount of data was less than 400GB ALL read used to come from the caching device and therefore didn't use IOPS from the mechanical drives leaving it free to do basically writes. Finally at sequential IOPS (as described by bcache) are intelligently passed directly to the mechanical drives (but they are not much).

Although bcache is present on kernel 3.10 I had to use kernel-ml 4.12 (from Elrepo) and I had also to compile the bcache-tools as I could not find it available in any repository.

Regards
Fernando

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