In this particular case, I did intend to deploy these instances directly to the ramdisk. I want to squeeze every drop of performance out of these instances for use cases with lots of concurrent accesses. I thought it would be possible to achieve improvements an order of magnitude or more over SSD, but it seems not to be the case (so far). I am purposefully not using O_DIRECT since most workloads will not be using it, although I did notice better performance when I did use it. I did already identify the page cache as a hinderance as well. I seem to have hit some performance ceilings inside of the kvm guests that are much lower than that of the host they are running on. I am seeing a lot more interrupts and context switches on the parent than I am in the guests, and I am looking for any and all ways to cut these down. I had read somewhere that vhost-blk may help. However, those patches were posted on qemu-devel in 2010, with some activity on LKML in 2011, but not much since. I feared that the reason they are still not merged might be bugs, incomplete implementation, or something of the sort. Anyhow, I thank you for your quick and timely responses. I have spent some weeks investigating ways to boost performance in this use case and I am left with few remaining options. I hope I have communicated clearly what I am trying to accomplish, and why I am inquiring specifically about vhost-blk. Regards, -Mike ----- Original Message ----- From: "Stefan Hajnoczi" <stefanha@xxxxxxxxx> To: "Michael Baysek" <mbaysek@xxxxxxxxxxxxx> Cc: kvm@xxxxxxxxxxxxxxx Sent: Wednesday, April 11, 2012 3:19:48 AM Subject: Re: vhost-blk development On Tue, Apr 10, 2012 at 6:25 PM, Michael Baysek <mbaysek@xxxxxxxxxxxxx> wrote: > Well, I'm trying to determine which I/O method currently has the very least performance overhead and gives the best performance for both reads and writes. > > I am doing my testing by putting the entire guest onto a ramdisk. I'm working on an i5-760 with 16GB RAM with VT-d enabled. I am running the standard Centos 6 kernel with 0.12.1.2 release of qemu-kvm that comes stock on Centos 6. The guest is configured with 512 MB RAM, using, 4 cpu cores with it's /dev/vda being the ramdisk on the host. Results collected for ramdisk usually do not reflect the performance you get with a real disk or SSD. I suggest using the host/guest configuration you want to deploy. > I've been using iozone 3.98 with -O -l32 -i0 -i1 -i2 -e -+n -r4K -s250M to measure performance. I haven't looked up the options but I think you need -I to use O_DIRECT and bypass the guest page cache - otherwise you are not benchmarking I/O performance but overall file system/page cache performance. Stefan -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
