Poor performance with qemu and bcache FUA

[Peter Kieser <peter@kieser.ca>]

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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=3fcbc17636c83da9d85e2604de4af56c215e6e3c

Looks 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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