PVS Write Cache - OS Test

Provisioning Services (PVS) is a service utilized to stream an operating system image from a file, known as a vDisk, to a physical or virtual computer. The recipient of the stream can be a disk less computer with the vDisk acting as its hard disk drive. One of the primary benefits of PVS is the ability to utilize a single vDisk to stream to multiple computers. This type of vDisk is known as a Standard vDisk and offers increased consistency, security, and centralized management.

Standard vDisks are Read-Only. All modifications, such as application installations, are written to a temporary file known as the Write Cache. When read requests for the newly written files come in, they are read from the write cache.

The Write Cache file can be configured to reside in the following locations:

Cache on device hard drive: Write cache can exist as a file in NTFS format, located on the target-device’s hard drive. This write cache option frees up the Provisioning Server since it does not have to process write requests and does not have the finite limitation of RAM.
Cache in device RAM: Write cache can exist as a temporary file in the target device’s RAM. This provides the fastest method of disk access since memory access is always faster than disk access. This measure will report metrics only if the cache resides in the device RAM.
Cache in device RAM with overflow on hard disk (only available for Windows 7 and Server 2008 R2 (NT 6.1) and later): In this case, when RAM is zero, the target device write cache is only written to the local disk. When RAM is not zero, the target device write cache is written to RAM first. When RAM is full, the least recently used block of data is written to the local differencing disk to accommodate newer data on RAM. The amount of RAM specified is the non-paged kernel memory that the target device will consume.
Cache on server: Write cache can exist as a temporary file on a Provisioning Server. In this configuration, all writes are handled by the Provisioning Server, which can increase disk IO and network traffic.
Cache on server persistent: This cache option allows for the saving of changes between reboots. Using this option, after rebooting, a target device is able to retrieve changes made from previous sessions that differ from the read only vDisk image. If a vDisk is set to Cache on server persistent, each target device that accesses the vDisk automatically has a device-specific, writable disk file created. Any changes made to the vDisk image are written to that file, which is not automatically deleted upon shutdown. This saves target device specific changes that are made to the vDisk image.

For virtual servers, administrators typically use the server’s hard drive for storing the write cache. Storing the write cache on the target side is beneficial as it keeps the write “close” to the target and minimizes the load on the Provisioning Servers, but it requires more resources. If the write-cache does not have enough disk space resources to grow, then many modifications to the vDisk will be lost. This is why, it is imperative that the write-cache is sized right, its usage is tracked continuously, and the lack of adequate disk space for the write cache brought to the attention of administrators rapidly. This is what the PVS Write Cache - OS test does! This test monitors the size and usage of the write cache and proactively alerts administrators when the write-cache runs out of space.

Note:

This test will report metrics only if the write-cache resides in one of the following locations:

Cache on device hard drive
Cache on server
Cache on server persistent

Target of the test : A Provisioned Nutanix AHV VDI server

Agent deploying the test : A remote agent

Outputs of the test : One set of results for the provisioned Virtual server being monitored

Configurable parameters for the test
Parameter	Description
Test Period	How often should the test be executed
Host	The host for which the test is to be configured.
Port	The port at which the specified host listens. By default, this is NULL
Prism Element IP	If the eG manager had discovered the target Nutanix AHV server by connecting to the Nutanix Prism Element, then the IP address of the Nutanix Prism Element server used for discovering this Nutanix AHV server would be automatically displayed against the Prism Element IP parameter; similarly, the Prism Element User and Prism Element Password text boxes will be automatically populated with the Prism Element user credentials, using which Nutanix AHV discovery was performed. If this Nutanix AHV server has not been discovered using the Nutanix Prism Element, but you still want to monitor the Nutanix AHV server via the Prism Element, then select the IP address of the Prism Element server that you wish to use for monitoring the Nutanix AHV server from the Prism Element IP list. By default, this list is populated with the IP address of all Nutanix Prism Element hosts that were added to the eG Enterprise system at the time of discovery. Upon selection, the Prism Element user and Prism Element Password that were pre-configured for that Nutanix Prism Element will be automatically displayed against the respective text boxes. On the other hand, if the IP address of the Prism Element server of interest to you is not available in the list, then, you can add the details of the Prism Element server on-the-fly, by selecting the Other option from the Prism Element IP list. This will invoke the MANAGER DISCOVERY - VIRTUAL PLATFORM SETTINGS page. Refer to the Configuring eG Enterprise to Monitor Nutanix AHV topic for details on how to use this page.
Prism Element User, Prism Element Password and Confirm Password	If the eG manager had discovered the target Nutanix AHV server by connecting to the Nutanix Prism Element, then the IP address of the Nutanix Prism Element server used for discovering this Nutanix AHV server would be automatically displayed against thePrism Element IP parameter; similarly, the Prism Element User and Prism Element Password text boxes will be automatically populated with the Prism Element user credentials, using which Nutanix AHV discovery was performed. If this Nutanix AHV server has not been discovered using the Nutanix Prism Element, but you still want to monitor the Nutanix AHV server via the Prism Element, then select the IP address of the Prism Element server that you wish to use for monitoring the Nutanix AHV server from the Prism Element IP list. By default, this list is populated with the IP address of all Nutanix Prism Element hosts that were added to the eG Enterprise system at the time of discovery. Upon selection, the Prism Element User and Prism Element Password that were pre-configured for that Prism Element server will be automatically displayed against the respective text boxes. On the other hand, if the IP address of the Prism Element server of interest to you is not available in the list, then, you can add the details of the Prism Element server on-the-fly, by selecting the Other option from the Prism Element IP list. This will invoke the MANAGER DISCOVERY - VIRTUAL PLATFORM SETTINGS page. Refer to the Configuring eG Enterprise to Monitor Nutanix AHV topic for details on how to use this page.
SSL	By default, the Nutanix Prism Element server is SSL-enabled. Accordingly, the SSL flag is set to Yes by default. This indicates that the eG agent will communicate with the Prism Element server via HTTPS by default.
WebPort	By default, the Nutanix Prism Element server listens on port 9440. This implies that while monitoring a Nutanix AHV server via the Prism Element server, the eG agent connects to port 9440.
Exclude VMs	Administrators of some virtualized environments may not want to monitor some of their less-critical VMs - for instance, VM templates - both from 'outside' and from 'inside'. The eG agent in this case can be configured to completely exclude such VMs from its monitoring purview. To achieve this, provide a comma-separated list of VMs to be excluded from monitoring in the Exclude VMs text box. Instead of VMs, VM name patterns can also be provided here in a comma-separated list. For example, your Exclude VMs specification can be: xp,lin,win,vista. Here, the * (asterisk) is used to denote leading and trailing spaces (as the case may be). By default, this parameter is set to none indicating that the eG agent obtains the inside and outside views of all VMs on a virtual host by default. By providing a comma-separated list of VMs/VM name patterns in the Exclude VMs text box, you can make sure the eG agent stops collecting 'inside' and 'outside' view metrics for a configured set of VMs.
Ignore VMs Inside View	Administrators of some high security Hyper-V environments might not have permissions to internally monitor one/more VMs. The eG agent can be configured to not obtain the 'inside view' of such ‘inaccessible’ VMs using the Ignore VMs Inside View parameter. Against this parameter, you can provide a comma-separated list of VM names, or VM name patterns, for which the inside view need not be obtained. For instance, your Ignore VMs Inside View specification can be: xp,lin,win,vista. Here, the * (asterisk) is used to denote leading and trailing spaces (as the case may be). By default, this parameter is set to none indicating that the eG agent obtains the inside view of all VMs on a Hyper-V host by default. Note: While performing VM discovery, the eG agent will not discover the operating system of the VMs configured in the Ignore VMs Inside View text box.
Ignore WINNT	By default, the eG agent does not support the inside view for VMs executing on Windows NT operating systems. Accordingly, the Ignore WINNT flag is set to Yes by default.
Inside View Using	By default, this test obtains the “inside view” of VMs using the eG VM Agent. Accordingly, the Inside view using flag is set to eG VM Agent by default. The eG VM Agent is a piece of software, which should be installed on every VM on a hypervisor. Every time the eG agent runs this test, it uses the eG VM Agent to pull relevant 'inside view' metrics from each VM. Once the metrics are collected, the eG agent then communicates with each VM agent and pulls these metrics, without requiring administrator privileges. Refer to Configuring the Remote Agent to Obtain the Inside View of VMs for more details on the eG VM Agent.
Domain, Admin User, and Admin Password, and Confirm Password	By default, these parameters are set to none. This is because, by default, the eG agent collects 'inside view' metrics using the eG VM agent on each VM. Domain administrator privileges need not be granted to the eG agent if it uses this default approach to obtain the 'inside view' of Windows VMs.
Report By User	While monitoring a Nutanix AHV server, the Report By Userflag is set to No by default, indicating that by default, the guest operating systems on the AHV server are identified using the hostname specified in the operating system. On the other hand, while monitoring AHV desktop environments, this flag is set to Yes by default; this implies that in case of VDI servers, by default, the guests will be identified using the login of the user who is accessing the guest OS. In other words, in VDI environments, this test will, by default, report measures for every username_on_virtualmachinename.
Report Powered OS	This flag becomes relevant only if thereport by user flagis set to ‘Yes’. If the Report Powered OS flag is set to Yes (which is the default setting), then this test will report measures for even those VMs that do not have any users logged in currently. Such guests will be identified by their virtualmachine name and not by the username_on_virtualmachinename. On the other hand, if the Report Powered OS flag is set to No, then this test will not report measures for those VMs to which no users are logged in currently.
Exclude IP	Typically, when performing VM discovery, the eG agent automatically discovers the operating system on which every VM runs, and all the IP addresses that each VM supports. If two are more VMs on a target vSphere server are in a VM cluster, then the eG agent will also auto-discover the cluster IP address. Since the cluster IP address is shared by all VMs in the cluster, this IP address will be in the discovery list of every VM in the cluster. In this case, if the eG agent attempts to obtain the 'inside view' of each VM in a cluster using their cluster IP address, incorrect metrics may be reported sometimes. To avoid this, you may want to instruct the eG agent to not use the cluster IP address when collecting 'inside view' metrics. For this, specify a comma-separated list of cluster IP addresses to be excluded in the EXCLUDE IP text box.
PVS Write Cache Max Size	Specify the maximum size up to which the write cache file can grow. By default, this is set to 10 GB.

Measurements made by the test
Measurement	Description	Measurement Unit	Interpretation
Write cache size	Indicates the current size of the write cache.	GB
Write cache utilization	Indicates the percent usage of the write cache.	Percent	The value of this measure is computed using the following formula: (PVS Write Cache Max Size – Write cache size) / Write cache size * 100 If the value of this measure is close to 100%, it indicates that the write cache may soon run out of space. Under such circumstances, you have the following options: You can increase the maximum size to which write cache can grow, or; Redirect some items out of the write cache and into a persistent drive. Before increasing the maximum write cache size, you will have to take the following into account: Basically the write cache will store all writes which would have gone to the hard disk. So if a user tends to copy large files locally to his/her desktop the write cache will grow at the same pace as the files are transferred. If there is any application which caches files or portions of a central DB locally for better performance, then the write cache will grow again. But there are some items which will always hit the write cache and these are split into two areas again. On one hand there is the user space, which contains items such as the user profile or internet/application related temp files. The user related write cache disk space needs to be multiplied by the amount of users logged on to a particular system. On the other hand we have the system space, which contains items such as logs or system temp / cache files, but we will also find files which are modified by the OS or any service for whatever reason. The system related write cache disk space is typically larger for server operating systems than for workstations. If you choose to redirect, then one/more of the following items can be redirected: Windows Pagefile. In fact the PVS Target Device driver detects if a local drive is available and redirects the pagefile automatically. Windows Event Log. While the eventlog is typically quite small (maybe 100MB or so) many customers redirect it for supportability and traceability reasons. Citrix related logs. Same as Windows Event Log. Anti Virus pattern. In case the virus scanner allows redirecting the pattern file, doing so saves some write cache space but it also saves some network traffic as it is not required to load the pattern from scratch after every reboot.
Data read rate from VHDX	Indicates the amount of data that is read from the write cache per second.	KB/sec
VHDX read rate	Indicates the rate at which the read operations are performed on the write cache.	Reads/sec
Data write rate to VHDX	Indicates the amount of data that is written to the write cache per second.	KB/sec
VHDX write rate	Indicates the rate at which the write operations are performed on the write cache.	Writes/sec
Data read rate from parent	Indicates the rate at which the data is read from the parent (base image).	KB/sec
Parent read rate	Indicates the rate at which the read operations are performed on the parent (base image).	Reads/sec
RAM cache size	Indicates the current size of the RAM cache.	GB
Data read rate from RAM cache	Indicates the rate at which the read operations are performed on the RAM cache.	Reads/sec
RAM cache read rate	Indicates the amount of data that is written to the RAM cache per second.	KB/sec
Data write rate to RAM cache	Indicates the rate at which the write operations are performed on the RAM cache.	Writes/sec