[linux-lvm] Should I expect snapshot origin LV's to be 10x slower?

[Ming Zhang]

On Thu, 2007-05-10 at 10:33 -0400, Greg Freemyer wrote:
> On 5/10/07, Alex Owen <r.alex.owen at gmail.com> wrote:
> > Hello,
> > I have just been making some snapshot performance benchmarks on a
> > Debian Etch system.
> > Kernel:  2.6.18-4-686 (2.6.18.dfsg.1-12etch1)
> > dmsetup: 1.02.08-1
> > lvm2: 2.02.06-4
> >
> > I have been using commands of the form:
> >   time dd if=/dev/zero of=/dev/volgroup/test bs=1M count=100
> > to get speeds for copying to a LVM device both WITH and WITHOUT a
> > single snapshot.
> >
> > It seems that writes take >=10 times longer the first time a newly
> > snapshot origin device is written to.
> >
> > I was expecting somthing like a 2x or 3x performance loss as 1
> > physical read and 2 physical writes must occur for a single logical
> > write. I was NOT expecting there to be a 10x overhead. As I move to
> > larger devices (bs=1M count=1000) the 10x figure rises to nearer 20x.
> > This is also true on mounted origin LV's.
> >
> > Has anyone else benchmarked this? Is this normal?
> >
> > Thanks for any feedback
> > Alex Owen
> 
> I always ensure my snapshots are on physically separate drives than my
> origin.  If they are on the same drive I'm not surprised you're having
> speed issues.  You are significantly increasing the amount of disk
> seek activity.  Having in separate drives should be much better.

by putting it into separate device, you might see a 5x slow instead of
10x. still because disk seek activity 

do not use current snapshot on a write intense lv. and adjust your lv
chunk size base on your application workload can remedy it a bit.



> 
> (FYI: It has been a while since I benchmarked, so you may still have problems.)
> 
> Greg