Online SRDF device expansion with PowerMaxOS

Beginning with PowerMaxOS on both the VMAX All Flash and PowerMax arrays, Dell EMC supports dynamic, online expansion of all SRDF pairs except SRDF/Metro (check this post for a workaround of sorts). Expansion prior to the introduction of PowerMaxOS, requires dissolving the SRDF pair, expanding each device independently and then re-creating the pair. This is impractical for production environments as during expansion, despite being non-disruptive to VMware, it leaves the environment at risk for disaster. While VMware can work around this disruption through Storage vMotion for VMFS, i.e. vacating the datastores of VMs to another protected datastore during the expansion, it is nonetheless a maintenance operation which requires both time and resources to accomplish.

I’m going to run through a typical online expansion to show you how easy it is to do. As the wizards have many screens, I think in this case a video is going to work much better. To limit the number of callouts I have to do in the video, let me run down the setup and a high level of what I will do in the video.

The environment for this example consists of a PowerMax 2000 and a VMAX All Flash 950F, both running PowerMaxOS. There is a single 1000 GB device replicating synchronously from the PowerMax (SID 083) to the VMAX All Flash (SID 047) in SRDF group 25. The storage group name on both arrays is expand_sg. The datastore is named EXPAND_VMFS. I’m going to expand the SRDF pair to 1200 GB each using Unisphere for PowerMax, then extend the VMFS datastore in the vSphere 6.5 vCenter. Why not vSphere 6.7? That will be the next topic 🙂

Here’s the video of the online expansion. I actually didn’t use any callouts because it seemed so self-explanatory to me. If that is not the case, leave me a comment please.

BTW if you prefer to use CLI, like the GUI it is a single operation to expand a device.


OK now vSphere 6.7. The previous section demonstrated expanding a VMFS datastore using vSphere 6.5. In vSphere 6.7, expanding the VMFS datastore requires different steps because of a VMware known issue. Normally when VMware scans the devices when the expand wizard is run in the vCenter, it filters out any devices that are in use as a datastore or RDM, presenting only those devices that can be used for expansion. Sometimes when the wizard is run from the vCenter, however, VMware filters out all devices and the user is presented with no choice – just any empty screen. It’s not that VMware fails to recognize the new size, it does, but the filter mechanism hits this bug sometimes. In fact, this bug has been around since ESX 4.0 if you can remember that far back! Fortunately we’ve never hit this on VMAX or PowerMax arrays until now. And it only impacts expanded SRDF devices on vSphere 6.7. Regular devices still work fine in vSphere 6.7. If you want to read more about the problem, head over here:

So you may wonder (and I certainly did) why VMware has never fixed this if it has been around so long. What VMware told me is that because changing the filters would impact arrays differently, they would need customized ones for all arrays. Then if the array changed something on their side, it would break VMware’s filter. It really is a no-win situation. BTW you can simply turn off the filters if you want (it’s in the KB), but that means VMware returns all devices and you better be sure to pick the right one, otherwise welcome to corruption. So do yourself a favor and don’t disable the filters. The workaround is easy enough – you just need to use the ESXi host vSphere Client instead. If you log in directly to any ESXi host that sees the datastore, you can expand it that way. Once expanded, all your other ESXi hosts will see the change in vCenter.

The second video here runs through an almost identical scenario to the vSphere 6.5 one. I use the same arrays, though my storage group is named expand_VMFS_sg. Otherwise everything is the same, including the datastore name and the SRDF group of 25. Once I fail to expand the datastore in the vCenter, I log into the ESXi host directly, expand the VMFS, and then go back to the vCenter to show you the result. Again, no callouts save one where I point out the blank screen in the wizard as if it weren’t obvious.

I know most customers aren’t on vSphere 6.7 so most won’t hit this issue yet, but always good to know ahead of time so you aren’t surprised.



8 thoughts on “Online SRDF device expansion with PowerMaxOS

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    1. I added in the Solutions Enabler CLI for you to the post. To expand the datastore, you would have to use either a combination of partedUtil and vmkfstools or PowerCLI has, as part of the managed object HostStorageSystem, ExpandVmfsExtent which will do it for you. Note that because of the bug I mentioned with vSphere 6.7, it causes the GPT table to be messed up and will require fixing before resizing the extent.

    1. Prior to this release, SRDF expansion requires splitting the pair, deleting the pair, expanding the R1 and R2 independently, and then re-creating the pairs. I believe you should be able to do those tasks in Unisphere.

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