Date: Mon, 02 May 2011 11:45:44 -0700 From: Ted Mittelstaedt <tedm@mittelstaedt.us> To: freebsd-emulation@freebsd.org Subject: Re: virtualbox I/O 3 times slower than KVM? Message-ID: <4DBEFBD8.8050107@mittelstaedt.us> In-Reply-To: <BANLkTikyzGZt6YUWVc3KiYt_Of0gEBUp%2Bg@mail.gmail.com> References: <10651953.1304315663013.JavaMail.root@mswamui-blood.atl.sa.earthlink.net> <BANLkTikyzGZt6YUWVc3KiYt_Of0gEBUp%2Bg@mail.gmail.com>
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On 5/2/2011 5:09 AM, Adam Vande More wrote: > On Mon, May 2, 2011 at 12:54 AM, John<aqqa11@earthlink.net> wrote: > >> On both the FreeBSD host and the CentOS host, the copying only takes 1 >> second, as tested before. Actually, the classic "dd" test is slightly >> faster on the FreeBSD host than on the CentOS host. >> >> The storage I chose for the virtualbox guest is a SAS controller. I found >> by default it did not enable "Use Host I/O Cache". I just enabled that and >> rebooted the guest. Now the copying on the guest takes 3 seconds. Still, >> that's clearly slower than 1 second. >> >> Any other things I can try? I love FreeBSD and hope we can sort this out. >> > > Your FreeBSD Host/guest results seem relatively consistent with what I would > expect since VM block io isn't really that great yet, however the results in > your Linux VM seems too good to be true. We know that Linux likes to run with the condom off on the file system, (async writes) just because it helps them win all the know-nothing benchmark contests in the ragazines out there, and FreeBSD does not because it's users want to have an intact filesystem in case the system crashes or loses power. I'm guessing this is the central issue here. > Have you tried powering off the > Linux VM immediately after the cp exits and md5'ing the two files? This > will insure your writes are completing successfully. > That isn't going to do anything because the VM will take longer than 3 seconds to close and it it's done gracefully then the VM won't close until the writes are all complete. > Also, it may be this type of operation really is faster on your Linux setup, > but is it representative of the primary workload? If not, you'll probably > want to arrange some type of benchmark that mimics real world IO flows as > hypervisor IO performance varies across workloads(example: it used to be > true KVM wasn't very good at concurrent IO, not sure if it's better now). > You should also ensure caches are not effecting the outcome of consecutive > benchmarks runs. umounting and remounting the file system on your SAS > controller is the quickest way to do this, but depending on your setup you > may need to reboot. If you do want to test with caching involved, you > should discard the initial run from your results. Use something like > /usr/bin/time to get consistent results of how long operations took. Ensure > each setup is running in a minimal configuration so there is less resource > contention. > > Also single runs are a terrible way benchmark things. You *need* multiple > runs to ensure accuracy. ministat(1) is a tool in base to help with this. > Here is more detail: > > http://ivoras.sharanet.org/blog/tree/2009-12-02.using-ministat.html > http://lists.freebsd.org/pipermail/freebsd-current/2011-March/023435.html > However that tool doesn't mimic real world behavior, either. The only real way to test is to run both systems in production and see what happens. I would not make a choice of going with one system over another based on a single large file write difference of 2 seconds. We have to assume he's got an application that makes hundreds to thousands of large file writes where this discrepancy would actually make a difference. Ted
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