Hello,I finally was able to track down the commit that was causing poor performance with qemu and LVM or qcow2 backed storage on virtual machines since 2013 that use barriers on their guest FS (the default.) This was most obvious with tools like apt-get and installing linux-headers (it would take ~8 minutes on my system with 2 Intel 530 SSDs in RAID1):
http://git.kernel.org/cgit/linux/kernel/git/stable/linux-stable.git/commit/?h=linux-3.10.y&id=3fcbc17636c83da9d85e2604de4af56c215e6e3cLooks like FUA implemented in bcache does not play nicely with qemu. I reverted this commit, and now a single virtual machine is no longer able to DoS all other virtual machines on the system. The symptoms would be that the cache devices would be fully utilized with a small number of writes.
Are there any plans to revisit this? I don't like the idea of leaving FUA unadvertised as data loss could occur if there is a power failure.
Thank you, -Peter
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