This blog contains experience gained over the years of implementing (and de-implementing) large scale IT applications/software.

Listing Azure VM DataDisks and Cache Settings Using Azure Portal JMESPATH & Bash

As part of a SAP HANA deployment, there are a set of recommendations around the Azure VM disk caching settings and the use of the Azure VM WriteAccelerator.
These features should be applied to the SAP HANA database data volume and log volume disks to ensure optimum performance of the database I/O operations.

This post is not about the cache settings, but about how it’s possible to gather the required information about the current settings across your landscape.

There are 3 main methods available to an infrastructure person, to see the current Azure VM disk cache settings.
I will discuss these method below.

1, Using the Azure Portal

You can use the Azure Portal to locate the VM you are interested in, then checking the disks, and looking on each disk.
You can only see the disk cache settings under the VM view inside the Azure Portal.

While slightly counter intuitive (you would expect to see the same under the “Disks” view), it’s because the disk cache feature is provided for by the VM onto which the disks are bound, therefore it’s tied to the VM view.

2, Using the Azure CLI

Using the Azure CLI (bash or powershell) to find the disks and get the settings.

This is by far the most common approach for anyone managing a large estate. It uses the existing Azure API layers and the Azure CLI to query your Azure subscription, return the data in JSON format and parse it.
The actual query is written in JMESPATH ( and is similar to XPath (for XML).

A couple of sample queries in BASH (my favourite shell):

List all VM names:

az vm list --query [].name -o table

List VM names, powerstate, vmsize, O/S and RG:

az vm list --show-details --query '[].{name:name, state:powerState, OS:storageProfile.osDisk.osType, Type:hardwareProfile.vmSize, rg:resourceGroup,, diskLUN:storageProfile.dataDisks.lun, diskCaching:storageProfile.dataDisks.caching, diskSizeG:storageProfile.dataDisks.diskSizeGb, WAEnabled:storageProfile.dataDisks.writeAcceleratorEnabled }' -o table

List all VMs with names ending d01 or d02 or d03, then pull out the data disk details and whether the WriteAccelerator is enabled:

az vm list --query "[?ends_with(name,'d01')||ends_with(name,'d02')||ends_with(name,'d03')]|[].storageProfile.dataDisks[].[lun,name,caching,diskSizeGb,writeAcceleratorEnabled]" -o tsv

To execute the above, simply launch the Cloud Shell and select “Bash” in the Azure Portal:

Then paste in the query and hit return:

3, A Most Obscure Method.

Since SAP require you to have the “Enhanced Monitoring for Linux” (OEM) agent extension installed, you can obtain the disk details directly on each VM.

For Linux VMs, the OEM creates a special text file for performance counters, which is used by the Saposcol (remember that) for use by SAP diagnostic agents, ABAP stacks and other tools.

Using a simple piece of awk scripting, we can pull out the disk cache settings from the file like so:

awk -F';' '/;disk;Caching;/ { sub(//dev//,"",$4); printf "/dev/%s %sn", tolower($4), tolower($6) }' /var/lib/AzureEnhancedMonitor/PerfCounters

There’s a lot more information in the text file (/var/lib/AzureEnhancedMonitor/PerfCounters) and my later post Checking Azure Disk Cache Settings on a Linux VM in Shell, I show how you can pull out the complete mapping between Linux disk devices, disk volume groups, Azure disk names and the disk caching settings, like so:

Useful Links

HowTo: Dynamic SQL Server Memory Change from SAP ST04

Scenario: You have a SQL Server database for your SAP system, and you know that right clicking the database server name in SQL Server Management Studio, selecting “Properties” and then “Memory”, will show you the SQL Server memory settings, but you want to know how you can see/change the same detail in SAP…

SQL Server Management Studio - Dynamic Memory

SQL Server Management Studio - Minimum memory

In SAP, you can use transaction ST04 to see the SQL Server database settings.
The memory details are visible in the “Overview” screen.
You will see the “Current Memory MB” equals the amount of memory allocated to SQL Server, and if the “Min server memory” and “Max server memory” settings have been set equal (recommended by SAP), then the overview screen will show “FIXED” for the “SQL Memory Setting”:

SAP SQL Server memory settings

It is possible to modify this setting directly from ST04.
You will need to expand the “Diagnostics” branch and then double click “SQL Command Editor”:


On the right hand side, enter the SQL Server commands to resize the memory (notice that these are slightly different to the SQL statements when using them in SSMS):

SAP ST04 SQL code execute

exec sp_configure 'show advanced options', 1
exec sp_configure 'max server memory', 4096

Click Execute:


The output window will be displayed:

SAP ST04 SQL code execute

If you noticed, I didn’t change the “Min” memory setting, only the “Max”.
When I check in the “Performance -> Overview” screen in ST04, I can now see that the “Current Memory MB” setting has not changed, but the “SQL Memory Setting” is now showing “RANGE”:

SAP ST04 SQL Server memory settings

Now if I use the SQL Command Editor to also change the “Min” memory, we will see the ST04 overview screen update:

SAP ST04 SQL execute

exec sp_configure ‘show advanced options’, 1
exec sp_configure ‘max server memory’, 4096
exec sp_configure ‘min server memory’, 4096


And the overview screen:


Well, we’re at “FIXED” again, but the amount of memory has not changed.
Yet in SSMS, I can see the allocation has changed:

SQL Server Management Studio memory settings

This is a weird, because the Microsoft documentation for SQL Server 2008R2 (my version) says that the setting should take effect straight away.
I guess there’s something within the ST04 screen that doesn’t update.
There is another way…
You can use the “Configuration -> Overview” screen and the “Configuration Options” tab to see both the Min and Max memory settings.
As per out change, these correctly reflect the current memory settings:

SAP ST04 SQL Server memory settings

WARNING: You should be aware that during testing, I was able to set the Min memory value higher than the Max memory value in ST04.
I was then unable to change this through ST04, as the store procedures just produced errors and refused to let me change the values.
In the end I had to change the Max value using SSMS.