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[linux-lvm] Why I/O test on LVM image (KVM) works suspicious compared to raw file image via varmail.f in Filebench benchmark?

Here I compare I/O performance on LVM image and raw file image (directly generated by Linux bash 'dd'), with the virtual machine hypervisor KVM, guest and host are both CentOS 6.2, and the guest has 1 CPU, 8G memory.
I use Filebench (filebench- as one of the benchmark, I use varmail.f oltp.f fileserver.f webserver.f (unmodified from the workload in the packet) to test the macro-benchmark.

The test of fileserver.f and webserver.f show that the LVM image and raw file image performs the similar result.
But the varmail.f shows that LVM outperforms than the raw file by several times, and the table below shows the data:

fileserver.f (KB/s) oltp.f
rawfile 17318.4 76.8 256 8576
lvm 16921.6 140.8 780.8 8448

Thus, I test varmail.f by changing different parameters, and the table below shows the data:

varmail.iosize-.f 742.4 179.2 4.142857
varmail.iosize+.f 742.4 179.2 4.142857
varmail.meanappendsize-.f 51.2 25.6 2
varmail.meanappendsize+.f 2278.4 985.6 2.311688
varmail.meandirwidth-.f 729.6 179.2 4.071429
varmail.meandirwidth+.f 742.4 166.4 4.461538
varmail.meanfilesize-.f 729.6 179.2 4.071429
varmail.meanfilesize+.f 742.4 179.2 4.142857
varmail.nfiles-.f 819.2 192 4.266667
varmail.nfiles+.f 640 179.2 3.571429
varmail.nthreads-.f 486.4 153.6 3.166667
varmail.nthreads+.f 1190.4 230.4 5.166667
varmail.nofsync.f 6259.2 6272 0.997959

varmail.nofsync.f is remove these two lines:
    flowop fsync name=fsyncfile2,fd=1
    flowop fsync name=fsyncfile3,fd=1

So, the initiator is fsync, which is not existed in fileserver.f and webserver.f, and oltp.f has a dsync.
I think the function of the fsync is to force data from OS memory to disk, but how does it influence the LVM virtual I/O performance?

Could anybody told me what cause this?

Thanks very much.

Thank you!

Brian Zhaoning Zhang

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