Exchange and potential Packet Loss on VMWare

technical_support_outage_advisory[1]Yesterday, I noticed a VMware knowledgebase article, updated on November 14th, which could be worth taking notice of when you’re running Exchange – or any other application – in a virtualized environment based on VMware technology.

VMware’s KB article 2039495 mentions that in VMware ESXi 4.x and 5.x, very high traffic bursts may cause the VMXnet3 driver to start dropping packets in the Guest OS. This has been observed on Windows Server 2008 R2 running Exchange 2010 with – as VMware puts it – a high number of Exchange users. What the article fails to mention is the configuration used by customers experiencing the issue. It might for example be valuable to know if a DAG was used, if the traffic (MAPI, replication) was split over multiple NICs or if it occurred with iSCSI storage. I won’t be surprised if the issue occurs with other high traffic situations as well, e.g. seeding. Luckily, Exchange is capable of handling certain hiccups so customers might not be even aware of the issue.

After some more digging I found another article, KB 1010071, which mentions a packet drop issue with VMware Guests known since ESX 3. This article explains a bit more why the issue occurs in the first place, being the network driver running out of receive buffers, causing the packets to be dropped between the Virtual Switch and the Guest OS driver.

One could argue about the impact of a few lost packets. However, as traffic increases the (potential) number of lost packets increases. Each lost packet results in retransmission of unacknowledged packets, which impacts overall throughput causing increased latencies.

VMware’s temporary solution to this problem is:

  1. Open up the Windows guest;
  2. Open the properties of the VMXNET3 NIC;
  3. On the Advanced tab, increase the Small Rx Buffers or Rx Ring #1 Size;
  4. What KB1010071 mentions and KB2039495 doesn’t, is that when using jumbo frames – not seldom used, e.g. replication  – you might need to adjust the Rx Ring #2 size and Large Rx Buffers values.

Now I say temporary, because VMware’s solution of course isn’t  a real solution; it’s only meant to – in their own words – reduce packet drops. Also, the KB1010071 article states you should “determine an appropriate setting by experimenting with different buffer sizes”. That doesn’t sound like an permanent, assuring solution for a virtualization environment running business critical applications now, does it?

All things considered, I’d recommend configuring these parameters to their maximum setting, preferably at installation time, unless anyone knows of a reason not to. In addition, this is another case for the best practice to split MAPI and replication traffic on Exchange using multiple NICs.

Finally, I already learnt of two other applications experiencing the issue. Therefor I think the problem is not Exchange 2010 specific, as KB2039495 might imply. If you have similar experiences, experienced differences between GbE and 10Ge, please use the comments to share.

11 thoughts on “Exchange and potential Packet Loss on VMWare

  1. Pingback: NeWay Technologies – Weekly Newsletter #20 – December 6, 2012NeWay | NeWay

  2. Im running Exchange 2013 in a standalone environment with just one Mailbox and CAS .. The mailbox has two NICs – for smtp relay & other for normal traffic. Should I be changing on both NIC? I have seen high volume of packet drops.


  3. I’ve got a call open with VMware for this issue with a client at the moment. Upping the buffer settings to the max has reduced the incidence of the issue but not removed it altogether. It’s 2010 CAS servers on VMware 5 – issue only happens first thing on a Monday morning interestingy enough. Causes the users to get disconnected from Outlook until the issue goes away or the servers are rebooted


      • We ended up replacing NLB on our CAS servers with a F5 hardware load balancer and it resolved all the Monday morning problems. We have a feeling that slow latency at the datastore level was the root issue but could never prove it


  4. Hi!

    I have the same issue with packets being dropped, retrans, dup acks and out of order. This is driving me nuts. We are running ESXI 5.5 with latest service pack. We have SQL talking to a few application servers HP SIM and Networking monitoring tool. We see the application disconnecting all the time from the SQL server. I am going to try to up the buffer sizes. BTW we took at the network and we don’t see any crc errors or anything out of the ordirany. Any advice would be greatly appreciated.


  5. Pingback: Exchange and VMWare Guest Introspection | EighTwOne (821)

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