On Thu, 11 Feb 2016, Marc MERLIN wrote:
But can someone confirm that indeed if I have a single md device, and 4 CPUs, my throughput is not limited by a single CPU decrypting the device?
There have been multiple discussions about this over time, and since you didn't qualify "not good" with a number, it's hard to say if this is your problem or not.
If you're getting around 100-300 megabyte/s or something, and "top" shows high CPU utilization for a few processes and some cores idling (use "1" and "H" option after starting top to see individual cores), and "iostat -x 5" shows the drives as not topping out, then most likely you're indeed being hit by dmcrypt not being multithreaded and/or AES-NI not working.
I personally run the same kind of setup as you, I use md->dmcrypt->lvm->fs as stack. Performance isn't stellar (I just tried and I get 500 megabyte/s read on a machine I am pretty sure AES-NI is working from the start, but on another machine where I think AES-NI probably isn't loaded early enough, I only got 250 megabyte/s).
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