[Linux-cluster] GFS and performace

Gordan Bobic gordan at bobich.net
Thu Jan 7 09:24:22 UTC 2010

Pasi Kärkkäinen wrote:
> On Thu, Jan 07, 2010 at 12:13:50AM +0000, Gordan Bobic wrote:
>> Paras pradhan wrote:
>>> I have a GFS based shared storage cluster that connects to SAN by fibre 
>>> channel. This GFS shared storage hold several virtual machines. While 
>>> running hdparam from the host to a GFS share, I get following results.
>>> --
>>> hdparm -t /guest_vms1
>>> /dev/mapper/test_vg1-prd_vg1_lv:
>>> Timing buffered disk reads:  262 MB in  3.00 seconds =  87.24 MB/sec
>>> ---
>>> Now from within the virtual machines, the I/O is low
>>> ---
>>> hdparm -t /dev/mapper/VolGroup00-LogVol00 
>>> /dev/mapper/VolGroup00-LogVol00:
>>> Timing buffered disk reads:   88 MB in  3.00 seconds =  29.31 MB/sec
>>> ---
>>> I am looking for possibilities if I can increase my I/O read write 
>>> within my virtual machines. Tuning GFS does help in this case?
>>> Sorry if my question is not relevant to this list
>> I suspect you'll find that is pretty normal for virtualization-induced 
>> I/O penalty. Virtualization really, trully, utterly sucks when it comes 
>> to I/O performance.
>> My I/O performance tests (done using kernel building) show that the 
>> bottleneck was always disk I/O (including when the entire kernel source 
>> tree is pre-cached, with a 2GB of RAM guest. The _least_ horribly 
>> performing virtualization solution was VMware (tested with latest player 
>> 3.0, but verified against the latest server, too). That managed to 
>> complete the task in "only" 140% of the time the bare metal machine did 
>> (the host machine had it's memory limited to 2GB with the mem= kernel 
>> option to make sure the test was fair). So, 40% slower than bare metal.
>> Paravirtualized Xen was close behind, followed very closely by 
>> non-paravirtualized KVM (which was actually slower when paravirtualized 
>> drivers were used!). VirtualBox came so far behind it's not even worth 
>> mentioning.
> What, you're saying VMware Server (and player) were faster than Xen PV?
> I have hard time believing that.. based on my own experiences.

Yes, that is exactly what I'm saying. But the best performing 
virtualization solution (vmware) still had a 40% performance penalty in 
disk I/O compared to bare metal. But regardless of which one is least 
slow, they are all so slow as to only be worth considering if you are 
doing anything other than consolidating idle machines. The VM may feel 
faster in terms of boot times and such-like (the second time around when 
all the data is cached in the host's RAM), but it's all smoke and 
mirrors and doesn't stand up to scrutiny.

The only virtualization solutions that deliver on the sort of 
performance claims the big vendors are making are the likes of OpenVZ 
and VServers, but those are mostly just chroots, more like FreeBSD jails 
or Solaris zones with a bit of network interface virtualization thrown 
in than what proper virtualization.

If you don't believe me, try it yourself. Do a full kernel build with 
the stock RH .config file with make -j 8 on a quad core box with the 2GB 
of RAM VM and then on the bare metal box limited to 2GB with mem= kernel 
boot parameter and see how long it takes. I make it 6.5 minutes on bare 
metal on my 3.2GHz Core2 vs about 9.5 minutes in a VM on the same 
machine (vmware, paravirtualized xen and KVM come reasonably close 
together). Each was tested multiple times, and the results were holding 


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