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You may have seen some of my previous blog posts about monitoring your servers using Azure Log Analytics. If not you can find them at https://pixelrobots.co.uk/2019/06/monitor-your-servers-cpu-usage-using-azure-log-analytics/. In this post, I am going to focus on using Azure Log Analytics to monitor your servers disk space. In fact, I am going to show you two methods I use. One will be a line chart so you can see trends and the other will be a table to give you a quick glance.

The prerequisites

You will need to have the Azure monitor agent installed on the VM’s you want to monitor. You can even monitor none Azure servers too. You can find more about installing the agent at https://docs.microsoft.com/en-us/azure/azure-monitor/platform/log-analytics-agent#install-and-configure-agent

The line graph method

Let’s start with the query first.

// Chart disk if its under nnGB over the past nn days
let setGBValue = 20;
// enter a GB value to check
let startDate = ago(2d);
// enter how many days to look back on
Perf
| where InstanceName != "D:" and InstanceName contains ":"
| where TimeGenerated > startDate
| where ObjectName == "LogicalDisk" and CounterName == "Free Megabytes"
| extend FreeSpaceGB = CounterValue/1024
| extend compDrive = strcat( tostring( Computer ), "/", tostring( InstanceName ) )
| summarize FreeSpace = min( FreeSpaceGB ) by compDrive
| where FreeSpace < setGBValue
| summarize max(FreeSpace) by compDrive
| join
(
Perf
| where TimeGenerated > startDate
| where ObjectName == "LogicalDisk" and CounterName == "Free Megabytes"
| extend FreeSpaceGB = CounterValue/1024
| extend compDrive = strcat( tostring( Computer ), "/", tostring( InstanceName ) )
)
on compDrive
| make-series FreeSpace = min( FreeSpaceGB ) on TimeGenerated from startDate to now() step 4h by compDrive
| render timechart

Let’s explain a bit

// Chart disk if its under nnGB over the past nn days
let setGBValue = 20;
// enter a GB value to check
let startDate = ago(2d);
// enter how many days to look back on

For let setGBValue enter the amount of free space in GB you would be worried about. So for example, if one of my drives has less then 20GB then I would want to know. So I set the Number to 20.

Now let startDate is how many days you would like the chart to go back to. I am using 2 days in this example, so my chart will be 2 days worth of data. You could change it to say 7h to just return data from the last 7 hours.

Now for the next part.

Perf
| where InstanceName != "D:" and InstanceName contains ":"
| where TimeGenerated > startDate
| where ObjectName == "LogicalDisk" and CounterName == "Free Megabytes"
| extend FreeSpaceGB = CounterValue/1024
| extend compDrive = strcat( tostring( Computer ), "/", tostring( InstanceName ) )
| summarize FreeSpace = min( FreeSpaceGB ) by compDrive
| where FreeSpace < setGBValue
| summarize max(FreeSpace) by compDrive

What we are doing here is excluding the D:\ as its normally used for the temporary drive in Azure. And then we are getting any drive or InstanceName as its called here that contains a :.

Next, we use the Free Megabytes Perf counter and then convert it to GB. The query then creates compDrive to add the computer name and drive letter together, separated by a /. It then uses the minimum value from the counter.

The query then filters out so only servers with free disk space less than the value we set before being used.

| join
(
Perf
| where TimeGenerated > startDate
| where ObjectName == "LogicalDisk" and CounterName == "Free Megabytes"
| extend FreeSpaceGB = CounterValue/1024
| extend compDrive = strcat( tostring( Computer ), "/", tostring( InstanceName ) )
)
on compDrive
| make-series FreeSpace = min( FreeSpaceGB ) on TimeGenerated from startDate to now() step 4h by compDrive
| render timechart

The query then basically performs the same query but joins it to the previous one. It then makes a series from now to the startdate. It then does step (a point on the chart) by 4 hours. You can change this but as I am using 2 days I find this is fine. The last line renders the query into a nice line chart.

Running the query

Now if you run it on your Log Analytics Workspace you should see something similar to the image below.

You will notice that even though the server does not currently have less free disk space the limit we set it is still showing in the graph. This is by design to help you look for issues or patterns over the time period set.

The table method

Again let’s start with the query first.

https://gist.github.com/ 348dc9dec8770a9687fb2f5089756340

Let’s explain a bit

https://gist.github.com/ 348dc9dec8770a9687fb2f5089756340

So, the top bit is basically the same as the first query, but I use 30 minutes instead of days.

https://gist.github.com/ 348dc9dec8770a9687fb2f5089756340

We then have the main query where we use perf counters to check how much free megabytes are on each disk on each server. We then convert that to GB. The query then filters out all results to only display the ones that have less free disk space then the setGBValue we set.

https://gist.github.com/ 348dc9dec8770a9687fb2f5089756340

Now the last line does something cool. Without it, we would just have the list of servers and if the server had multiple drives each drive would be another entry in the list. This line basically moves the drive from a row to a column based on the server. So basically each server would just have one entry but may have multiple columns depending on how many disks it has.

Running the query

When you run the query you should have something like the below image. For me to get results I had to set the setGBValue high.

Dashboards?

It’s is very easy to pin either to a dashboard, in fact, all you have to do is click the Pin button at the top right of the query window. This will then ask you what dashboard to save the Chart to. One thing to note is you will have needed to create a dashboard and share it first. Unfortunately, you are unable to create a dashboard from the pin drop down.

And that’s it. You are now monitoring free disk space on your Azure servers.

I hope you found this article helpful. If you have any questions or comments please reach out in the usual ways.

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Pixel Robots.

I’m Richard Hooper aka Pixel Robots. I started this blog in 2016 for a couple reasons. The first reason was basically just a place for me to store my step by step guides, troubleshooting guides and just plain ideas about being a sysadmin. The second reason was to share what I have learned and found out with other people like me. Hopefully, you can find something useful on the site.

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