Crash Symptoms

A CentOS 4.x or 5.x guest will crash with a message similar to the following on its console:

CentOS 4.x:

[<f883b299>] .text.lock.scsi_error+0x19/0x34 [scsi_mod]
[<f88c19ce>] mptscsih_io_done+0x5ee/0x608 [mptscsi] (…)
[<c02de564>] common_interrupt+0x18/0x20
[<c02ddb54>] system_call+0x0/0x30

CentOS 5.x:

RIP  [<ffffffff8014c562>] list_del+0x48/0x71 RSP <ffffffff80425d00> <0>Kernel Panic - not syncing: Fatal exception

A hard reset (i.e. pressing the reset button on the VM’s console) is required to reboot the guest.

Further Details

Five different VMs have encountered this issue, running at a mix of close-to-current CentOS 4.x and 5.x patch levels. Guest kernel versions when the crash occurred were 2.6.18-128.7.1.el5 and 2.6.18-128.1.10.el5 (5.x) and 2.6.9-89.0.9.ELsmp (4.x). Memory allocations on affected guests range from 512MB to 3072MB. Notably, all affected VMs are using SMP – each has 2 vCPUs – having been created before our in-house practices followed VMware guidelines and discouraged use of SMP on ESX guests when unnecessary. One VM was created via P2V; the rest were created de novo on virtual hardware.

All crashes have happened on a single node in an ESX 3.5 HA cluster composed of four Dell PowerEdge 1950s. ESX hosts have tracked the latest VMware patches closely. COS memory on the ESX host in question was increased from the default to 800MB prior to the three most recent crashes; in other words, the COS memory increase appears to have had no effect on the crashes. DRS is in use, set to “fully automated” and “apply recommendations with three or more stars” and no virtual machine rules have been created to control DRS host placement.

All guests are on the same NFS data store, served from a NetApp filer running ONTAP 7.2.x. One guest had its vmswap placed separately on an iSCSI data store; the rest have their swap stored on NFS with the VM. No log messages were seen on the filer during the event, although the a log message similar to the following has been seen several times on the ESX host:

vmkernel: 43:07:27:51.725 cpu2:2185)WARNING: NFS: 4590: Can't find call with serial number -2146566055

Curiously, all crashes have happened in the evening, in the 10 o’clock hour, after nightly backups have been completed. Backups are created using a combination of VMware and NetApp snapshots via a script similar to one detailed on vmwaretips.com. No substantial load or latency has been recorded on the NetApp during the crashes, and weeks have passed between events.

Speculation

Explanations I’m leaning towards, ranked by my judgment of their likelihood:

1) Hardware issue. Assuming a random distribution of VMs – recall that DRS is in use and no virtual machine rules are in place – the odds of all five crashes happening on one host out of four are slim: 1 in 1024. Unfortunately, by all measures we’ve used, including the VI Client’s “Health Status” and Dell OMSA, there are no hardware issues with the host.

Further, the distribution of VMs is not truly random. DRS migrations are infrequent in this cluster, and the largest determinant of guest location is migration following hosts being placed into maintenance mode for patching.

If it is a hardware issue, it’s subtle, and possibly only brought to the fore by the following issues.

2) Red Hat Enterprise Linux bug – which, by extension, is typically equivalent to a CentOS bug. In fact, this issue appears to have been raised with Red Hat already in bugs 197158 and 228108 – but, according the bug reports, the issue is resolved, and the patches have since been ported downstream to CentOS. However, perhaps the issue is not truly resolved – see comment 35 in 228108.

3) vSMP Bug. The majority of our Linux VMs are uniprocessor and appear so far to be immune to this issue; it is striking that the crash has only occurred on dual processor guests. I cannot articulate a mechanism for multiple vCPUs causing this crash, however.

4) NetApp issue. This appears to be a storage issue at some level, considering the mptscsi and NFS messages noted above, so performance of the NetApp filer would be a natural place for further investigation. However, we monitor the performance of our filer relatively closely, using the ONTAP SDK and Cacti, and nothing unusual was recorded during any crash. It seems unusual that all VMs reside on the same data store, but that data store shares an aggregate with multiple other unaffected data stores, and several LUNs are served from the same aggregate to non-ESX machines without complaint.

I have not yet opened a case with VMware on this issue – or Dell, or NetApp, for that matter – but if and when I do, I’ll update here to the extent possible.

Update 11/20/2009: Prompted by a helpful comment from nate below, I looked up and verified the NFS settings across the cluster. They are the same across all hosts, and are as follows:

NFS.IndirectSend 0
NFS.DiskFileLockUpdate 10
NFS.LockUpdateTimeout 5
NFS.LockRenewMaxFailureNumber 3
NFS.LockDisable 0
NFS.HeartbeatFrequency 12
NFS.HeartbeatTimeout 5
NFS.HeartbeatDelta 5
NFS.HeartbeatMaxFailures 10
NFS.MaxVolumes 8
NFS.SendBufferSize 264
NFS.ReceiveBufferSize 128
NFS.VolumeRemountFrequency 30
NFS.UDPRetransmitDelay 700

The only values that are changed from default are HeartbeatFrequency and HeartbeatMaxFailures, to match NetApp’s recommendations in TR-3428.

Here’s our in-house procedure for recovery; note that we do not have FlexClone licensed on our filers.

Prerequisites

• An existing VMware ESX infrastructure, connected to a NetApp filer NFS datastore; SnapRestore speeds the recovery process but is not mandatory – see discussion below.
• A backup script or system which coordinates VMware snapshots with NetApp snapshots – perhaps something along the lines of Rick Scherer’s script.
• A dedicated Linux restore VM, at a similar version level to the rest of your Linux VM infrastructure. This VM should have LVM support, but should not have any volume groups (VGs) or logical volumes (LVs) configured – volume group and logical volume names on the VMDK you are restoring from must not conflict with VGs and LVs already in use on the restore system; the simplest way to guarantee this is to simply not have any VGs or LVs.

Restore Procedure

• Restore the VMDK file from the appropriate snapshot to a new location in the datastore. With SnapRestore, this can be done as follows (one line in the filer CLI, restoring from snapshot sv_daily.0 to a new file – again, be extremely careful not to overwrite the current version of the VMDK in your datastore, consider restoring to an entirely different directory in the FlexVol):

  snap restore -t file -s sv_daily.0
-r /vol/vmware04_sis/system.example.com/system.example.com-restore.vmdk
/vol/vmware04_sis/system.example.com/system.example.com.vmdk

  Follow the prompts, verifying the restore path is correct and is not the path to your existing VMDK. Do the same for the flat VMDK file (again, one line, and, as before, use caution to make sure you do not clobber an existing file):

  snap restore -t file -s sv_daily.0
-r /vol/vmware04_sis/system.example.com/system.example.com-restore-flat.vmdk
/vol/vmware04_sis/system.example.com/system.example.com-flat.vmdk

  Without SnapRestore, you can simply mount the NFS export of the datastore on a Linux machine and use “cp” to copy the files out of the snapshot. For flat VMDK files, expect this copy to run for a substantial amount of time compared the nearly-instant recovery SnapRestore offers.

• Manually edit the line below “# Extent description” in the recovered .vmdk file to match the path to the recovered flat VMDK. In this case, it would look something like this:
  # Extent description
RW 20971520 VMFS "system.example.com-restore-flat.vmdk"
• Attach the recovered VMDK to your powered-off restore host. Boot the restore host.
• Once your restore host is up, use “pvscan”, “vgscan” and “lvscan” (each without arguments) as root to examine available LVM components. Then, use the “lvchange” command to activate the necessary volume group (in this case, “VolGroup00″):
  # lvchange -ay VolGroup00
• Mount the appropriate logical volume – for example, LogVol00 in VolGroup00:
  mount -o ro /dev/VolGroup00/LogVol00 /mnt
  Restore files by copying them out of /mnt.

Cleanup

• Shut down the Linux restore host.
• Remove the recovery VMDK – the files restored with SnapRestore or by “cp” above – from the restore host in the VMware Infrastructure Client.
• Delete the recovery .vmdk and -flat.vmdk files in the NFS datastore. Don’t screw up here: Be sure to delete the recovery files only, not the working VMDK.

• ESX Swap on NFS or Not?
• VMware about ESX swap on NFS: It’s okay

After writing the second post, I thought the issue was pretty much resolved: From multiple sources, the consensus seemed to be that running ESX with VM swap on NFS would be fine.  Imagine my surprise (and disappointment) at seeing the following VMware KB article 1008091, updated yesterday: An ESX virtual machine on NFS fails with swap errors. Further details are in the article itself, but VMware’s KB site is throwing intermittent errors for me at the moment, so I’ll provide the money quote:

The reliability of the virtual machine can be improved by relocating the swap file location to a non-NFS datastore. Either SAN or local storage datastores improve virtual machine stability.

Now when did VMWare say that I’d be able to automatically Storage VMotion my VMs off those hot, power-sucking Fibre Channel drives to SATA drives so I can power down my first tier storage overnight again?

This articles describes changing the default location for your virtual machines [sic] swap files. You may need to use this if you are running virtual machines from NFS storage.
Note: VMware recommends storing your swap on a VMFS3 volume, when running virtual machines on NFS storage.

(Emphasis mine.) The article provides none of the reasoning behind this recommendation, and a search of the VMware Knowledge Base for “nfs swap” only returns this article. There are quite a few mentions of not putting swap on NFS in the VMware Communities forums, citing performance reasons, but, as far as I could find, no support for the performance issue assertion.

Personally, I’m not buying the performance claims without substantiation – NFS and iSCSI to the same filer should be nearly equally performant. I could imagine a difference in locking behavior, but I’m having trouble imagining a scenario where lock contention with a VM swap file would become an issue.

On the other side of the issue, I’ve been hearing from a few folks – mostly NetApp employees or users – that placing swap on NFS is okay. On the viops site, Steve Chambers writes in a preview of VMworld TA 2784 (which I did not attend) that “there is no need to place swap space on non IP storage.” IP storage could mean iSCSI here, of course, but in the VM /ETC write up of TA 2784, Rich Brambley’s notes state: “Do not place [swap] space on other storage – do not place the vswap on VMFS. Follow best practices and keep the swap on the NFS storage.”

So, who’s right here? VMware or NetApp? I suppose the conservative thing is to keep the swap on VMFS until VMware tells us otherwise, with the only negatives being the increased operational complexity and reduced storage utilization – that’s what I’m doing. I can’t help but wonder if this is like VMware’s Linux timekeeping recommendations, though – conflicting information subject to change – and hopefully clarification – at a later date.

Update, 11/24/2008: Keeping ESX swap on NFS with the VMhome directory is considered the current best practice.