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



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


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