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

[Jonathan Dill <jonathan@xxxxxxxxx>]

Christoph Purrucker wrote:
> - 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.
It might be interesting to use a multiple disk configuration with most 
of the basic OS in loopback, and any dynamic data in lvm.  A simple 
split would be to put /var and /home on lvms, although some naughty apps 
might change stuff in /usr or /usr/local tree.  This approach might also 
be convenient to copy the loopback image for multiple domUs but have 
different dynamic data in each.
There has been some previous discussion about using a "read-only" image 
then have every domU do an overlay ala UnionFS, on a loopback I'd think 
that could really take advantage of the caching in the dom0.
http://lists.xensource.com/archives/html/xen-users/2005-05/msg00463.html

However, the question is if this approach really buys you enough extra 
performance to be worth the added complexity, and especially when you 
think in terms of disaster recovery, which approach would give you the 
least headaches.
I found some other benchmarks that aren't exactly what you were talking 
about, but seem to be a good example of the procedure to follow, 128 mb 
allocated to dom0, 128 mb allocated to each domU, tests run with 1, 4, 
10, and 20 active domU, each performing the same task.  These tests 
compared using NetBSD as the dom0, Linux as the dom0 with domU on 
loopback on ext3 filesystem, Linux as the dom0 with domU on LVM.  In 
this test, the domU on NetBSD were slower, but on Linux there was not 
much difference between loopback or  LVM.
http://users.piuha.net/martti/comp/xendom0/xendom0.html

Also, those tests were done on Xen 2.0.6, it might be worth repeating 
with a newer version of Xen.
--
Jonathan Dill - The NERDS Group
Network Engineering & Resource Development Specialists, LLC
Cell: (240) 994-0012 Main: (301) 622-7995
Web: http://www.nerds.net


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