[Xen-users] Network connections stalling - Debian Linux domU's

[Henrik Størner <henrik-xen@xxxxxxx>]

Hi,

I am a long-time Linux user and mail admin (15+ years), but have only
recently been dipping my toes into Xen.

This past week I have converted my main mailserver from a setup with a
single dedicated server to a Xen virtual server configuration with
separate MySQL, Mail and web functionality. All appeared well, so I let
the users loose on the system. And immediately started getting
complaints that they could not fetch mail using IMAP.

To cut a long story short, the problem appears to be with the domU
hosting the IMAP server (Dovecot). When a user connects to the IMAP
service, the network connection quickly stalls. A "netstat" on the domU
shows a very large Send-Q (i.e. data sent, but not acknowledged by the
remote end of the connection) - usually around 25000, it ought to be
quite small. A tcpdump also shows very large intervals between packets,
10-30 seconds is not uncommon.

It is easy to replicate: Using Thunderbird, I create a new profile with
the necessary settings. The initial sync of the mailbox will trigger the
problem.


After reading a bunch of docs, blogs etc, I tried these things:
  - Dom0 memory fixed at 2 GB, ballooning disabled
  - CPU 0+1 dedicated to dom0 (pinned), the domU's configured to
    NOT use CPU 0+1
  - CPU 4.5 dedicated to the mail server domU
  - txqueuelen on the VIF changed to 1024

Neither of these appear to make any difference whatsoever (the server
was rebooted after changing the dom0 settings). And it doesn't really
seem that any of the dom's are cpu-starved.

So to at least get the users off my back, I am now running the mail
server in dom0 (mounting the mailserver volume in dom0 and running from
a chroot). This runs flawlessly, but it is obviously not how I would
like to do it...


The physical server where this happens is a quad-core/hyperthread box
(so there appears to be 8 cpu's). It has 24 GB memory - 2 VCPU's and 2
GB memory dedicated for dom0. The mailserver domU has the same
ressources - 2 VCPU's, 2 GB memory. The domU's are all PV Linuxes.


All of the software is stock Debian 7: Xen 4.1.4, Linux kernel 3.2.0.


Any ideas are most welcome!


Thanks in advance,
Henrik

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