Degraded Device Handling (DDH) Revisited

Degraded Device Handling (DDH) or Dying Disk Handling as it was formerly known, is a feature that has been available in vSAN for some time. However, I regularly get questions about how it works. The DDH behavior has changed significantly over various versions. We may as well begin this post with an overview about the purpose of DDH and then get into the different sort of behaviors. First of all, the reason behind a feature such as DDH is to help avoid cluster performance degradation due to an unhealthy drive. In the early days of vSAN, we had come across…

Debunking some behavior “myths” in 3 node vSAN cluster

I recently noticed a blog post describing some very strange behaviors in 2-node and 3-node vSAN clusters. I was especially concerned to read that when they introduced a failure and then fixed that failure, they did not experience any auto-recovery. I have reached out to the authors of the post, just to check out some things such as version of vSAN, type of failure, etc. Unfortunately I haven’t had a response as yet, but I did feel compelled to put the record straight. In the following post, I am going to introduce a variety of operations and failures in my…

Component Metadata Health – Locating Problematic Disk

I’ve noticed a couple of customers experiencing a Component Metadata Health failure on the VSAN health check recently. This is typically what it looks like: The first thing to note is that the KB associated with this health check states the following: Note:¬†This health check test can fail intermittently if the destaging process is slow, most likely because VSAN needs to do physical block allocations on the storage devices. To work around this issue, run the health check once more¬†after the period of high activity (multiple virtual machine deployments, etc) is complete. If the health check continues to fail the…

Essential Virtual SAN (6.2) available for pre-order

Our friends over at Pearson and VMware Press have informed us that the second edition of the Essential Virtual SAN book (that I wrote with Duncan Epping) is now available for pre-order on Amazon. It looks like it will be available on June 13th, but VMware Press have told us that they will do what they can to pull the date in a little closer. This new edition covers all of the new features added to Virtual SAN, up to the latest (yet to be released) VSAN 6.2. Here’s some blurb on the new edition, which gives a little insight…

Datrium go GA

This week Datrium announced that their DVX system is now generally available. I met these guys at VMworld 2015, and wrote a closer look at Datrium here. If you want a deeper dive into their solution, please read that post. But in a nutshell, their solution uses a combination of host side flash devices to accelerate read I/O, while at the same time writing to the Datrium hardware storage appliance (called a NetShelf). The NetShelf provides “cheap, durable storage that is easy to manage”. The DVX architecture presents the combined local cache/flash devices and NetShelf as a single shared NFS…

VSAN.ClomMaxComponentSizeGB explained

In the VSAN Troubleshooting Reference Manual, the following description of VSAN.ClomMaxComponentSizeGB is provided: By default VSAN.ClomMaxComponentSizeGB is set to 255GB. When Virtual SAN stores virtual machine objects, it creates components whose default size does not exceed 255 GB. If you use physical disks that are smaller than 255GB, then you might see errors similar to the following when you try to deploy a virtual machine: There is no more space for virtual disk XX. You might be able to continue this session by freeing disk space on the relevant volume and clicking retry.