VMware Solutions Discussions

Partial Writes with VMWare ESX hosts


Hi all,

We are experiencing performance problems in our environment and it points to partial writes. We are seeing back-to-back CPs that is causing a spike in latency across all volumes on a filer to above 500ms.

We have contacted NetApp support and they have said yes it is partial writes and it is probably caused by ESX. The filer  is almost dedicated to ESX so it has to be ESX, we know all our VMs are unaligned but short of aligning 1000’s of VMs we want to target a few that are causing the most havoc.

How can we narrow it down to a VM level accurately, which ones are causing us the most pain?





We have 42 ESX Hosts

We have approx 1500 Hosts

I think the filer is overloaded yes, but i beleive it is because of the number of partial writes that are occuring. If all VMs (especially the highest IO ones) were aligned I would think the filer could handle the given workload quite comfortably.

We are utilising NFS datastores.

I have engaged NetApp support but i am asking here to try and get some information from people that may have experianced VM alignment problems before.

Another Note:

I have written a script to poll the filer every 15mins and get pw.over_limit stat from wafl_susp -w. I have found at times this number grows by 3000 counts /s. See attached graph over_limt. These large spikes correspond to when we see massive latency jumps on our filers (4am everyday). We are still trying to work out what happens at this time to cause this massive IO spike (and subsequent latency spike), but i still beleive the root cause is unaligned VMs. Any comments appreciated.



Here's our TR on how to align filesystems


Hope this helps.




We have the exact same problem on our IBM Nseries (rebranded Netapp)

All our ESX hosts are using FC and allmost all of our 1000+ virtual servers are unaligned.. We have approx. 60 mill pw.over_limit every 24 hours and our latency is going from a few ms to more than a second if someone is doing excessive writes.

Recently we started aligning the virtual servers using software from VisionCore, -but it's a very time consuming process and we expect to use the next 6-12 months aligning.

We qualified the top writers (LUNS) using Operations manager and our ESX guru found the virtual servers using the most busy luns.

We are about 10% done, but haven't seen any major improvements yet - but we're still optimistic..!



Hi, this thread shows up on the top of "Netapp Latency Spikes" searches.

We have a 3040 cluster hosting 11 vSphere hosts with 200 VMs on NFS datastores.

We see latency spikes 3-4 times a month as reported by Operations Manager.

We hoped our upgrade from last week to 7.3.3 would help, but we had another spike up to 1 second take out a NFS mount and all several of the VMs on Saturday.

We previously  determined the High & medium IO VMs and either aligned them or migrated them to local disk - has NOT helped - still getting the spikes.

I have another case opened with Netapp.

Following the notes in this thread, I ran the wafl_susp -w to check the pw.over_limit

Turns out ours is ZERO (is it relevant to NFS?)

I suspect an internal Netapp process is responsible for these (dedup?) - we had it disabled on - 7.3.3 was supposed to fix this (we re-enabled de-dup after the upgrade)

And the latency spike outages are back

Will share any info from the case

thanks for any tips,




We have seen excessive responsetimes, when the system did aggregate snapshots.. Try comparing the aggr snap schedule to your response time problems..

Our aggr snap problem might be related to the misalignment.

"snap sched -A"




Henrik, yes we disabled AGGR snapshots over a year ago

Still searching for an explanation for the spikes


After I took apart the 6800+ IOPS on the problem aggregate the issue turned out to be we were hitting physical limitations of the 10K RPM disks.

Further analysis (surprisingly) revealed about 50% of these IOPS were snapmirror related.

We rescheduled the snapmirrors to reduce this and have said goodbye to the latency spikes.

If interested in the details, please see:


I want to thank Netapp support - especially Errol Fouquet for his expertise pulling apart this problem and help isolating the cause.




A quick followup outlining how we currently quantify the misalignment issue:




Basic question: Do you have at least 20% free space in all aggregates?


No, we have 90% allocation in most (with aggregate snapshots disabled)



No doubt it's a problem for performance


Are you hosting the VMs on LUNs? Over NFS?

If you're curious about which ones are causing the most pain, collecting a perfstat and working with NetApp Support will be your best bet.



with which FAS you are working with?

how many hosts do you have?

how many vms do you have?

I did a short look over your sysstat output and read the following:

- row 2 needs to be shifted 4 columns to the right

- very high CPU load (more than 95%)

- NFS only

- very high Net in and out (in: 701142 KB/s, out: 484975 KB/s)

- very high Disk read and write (read: 991328 KB/s, write: 886479 KB/s)

are you sure that these numbers are correct?

Supposed that's correct, i think your netapp is overloaded or undersized



You might want to take a look at vscsistats and/or the nfstop tool to get an idea of which VMs are creating the highest workload.  Check out this post on Yellow-Bricks.


Hi, we are experiencing HUGE 1,000,000+ microseconds (1 second+) latency spikes on ONTAP 7.3.3 NFS volumes as reported by NetApp Management Console which is disabling VMware virtual machines (Windows SQL server needs to be rebooted, Linux VMs go into read only mode and need reboots etc)

We have a case open with Netapp (2001447643) and the latest analysis of perstat and archive stats from the spikes is being presented to us:

"Version:1.0 StartHTML:0000000149 EndHTML:0000003705 StartFragment:0000000199 EndFragment:0000003671 StartSelection:0000000199 EndSelection:0000003671   

This data is definitely good.  We are seeing the latency.
Here is what I am seeing on the filer side:
Server rpc:
calls      badcalls    nullrecv   badlen      xdrcall
232298     4294959566  0          0           4294959566

The NetApp filer is getting a huge number of bad XDR calls, indicating that the filer is unable to read the NFS headers.
We cannot determine at this time what the source of these bad calls is.
Some of the worst offending volumes during this period, regarding latency appear to be:

   Time   Time Delta   Volume   Parent Aggr   Total Op/s   Avg Lat (µs)   Read Op/s   Read Data (B/s)   Read Lat (µs)   Write Op/s   Write Data (B/s)   Write Lat (µs)   Other Op/s   Other Lat (µs)  
   Tue Jun 15 17:45:46 UTC 2010   0.00   vm64net   aggr1   311.00   6,981,691.95   35.00   336,402.00   1,336,540.48   267.00   1,561,500.00   7,953,614.68   8.00   4.29  
   Tue Jun 15 17:45:46 UTC 2010   0.00   vm65net   aggr1   115.00   6,283,673.41   0.00   2,453.00   38,863.33   107.00   1,441,475.00   6,714,803.11   6.00   12.41  
   Tue Jun 15 17:45:46 UTC 2010   0.00   vm65net3   aggr1   292.00   3,481,462.35   14.00   110,824.00   1,390,729.32   263.00   1,582,710.00   3,780,725.12   14.00   6.82  
   Tue Jun 15 17:45:46 UTC 2010   0.00   ora64net02   aggr1   17.00   3,280,731.47   5.00   92,421.00   4,776.50   7.00   24,536.00   7,710,536.08   4.00   2.77  
   Tue Jun 15 17:45:46 UTC 2010   0.00   vm65net2   aggr1   315.00   2,838,381.82   11.00   56,383.00   22,902.19   287.00   1,805,548.00   3,105,157.06   15.00   21.81 

Out best bet to track down the source of the bad calls would be to capture a packet trace from the filer when this issue is occurring."
My points I’d like to clarify:
  1. What is a bad XDR call and why are they relevant to the latency spike?
  2. indicating that the filer is unable to read the NFS headers” - need you to clarify and expand
  3. We saw another smaller spike around 2:30am today:
  4. These are all volumes on aggregate aggr1 (10K RPM disks)  is this an overloaded (IOPS-wise) AGGR issue?
We can't currently predict when these spikes in latency occur - they are random - so getting a packet capture of a random event does not seem feasible...
Any insight is welcome - we are in major pain with this for months now

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