Re: [Xen-users] Differences in performance between file and LVM based images

[Christoph Purrucker <cp+ml-xen@xxxxxxxx>]

Hi Alex,

> Nobody seems to want to do these benchmarks, so I went ahead and did
> them myself. The results were pretty surprising, so keep reading. :)
cool you brought yourself to do the benchmark. But this test was a bit
useless:

- Dom0 has 2GB of RAM and DomU has 1GB of RAM both not running any heavy
application. And you test with 900MB of data. It's clear you are testing
caching performance but not Disk I/O since all the memory is available
for caching.

- You have overwhelming free CPU resources. 4 fast cores and nothing to
do. So you can't test, if LVM has lower or higher overhead than loop
back filing since you did not post a sum of CPU cycles consumed by both
kernels.

Conclusion:

- If you make a 10GB test, all three tests will show nearly the same
performance since there is so many free CPU time which even out any
differences.

- Normally a Dom0 has nearly no free memory since Dom0 normally does
nothing but manage the DomUs. All free memory is for the DomUs to do
their work well. So please make a test with Dom0 memory=64MB on a single
CPU environment running two DomUs (one for I/O-benchmarking and another
for running 'cpuburn').

- Since loop back files are obviously beeing fully cached by Dom0, you
can't use them in productive environment, as Andrew stated, even they
were faster. For example, a mailserver running in DomU has to be sure
that a mail is on disk before returning the remote SMTP server an OK.
But in case above the file is still in Dom0 disk cache which is bad if
the system crashes. Same with databases etc.

Tell me, if I'm wrong.

cu cp


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