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.


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 KVM 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.


The IP address of the host for which this test is to be configured.


The port at which the host listens. By default, this is NULL.

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 VMstext 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

Administrators of some high security VMware 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 KVM host by default.


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 communicates with every VM remotely and extracts “inside view” metrics. Therefore, by default, the Inside View Using flag is set to Remote connection to VM (Windows).

Typically, to establish this remote connection with Windows VMs in particular, eG Enterprise requires that the eG agent be configured with domain administrator privileges. In high-security environments, where the IT staff might have reservations about exposing the credentials of their domain administrators, this approach to extracting “inside view” metrics might not be preferred. In such environments therefore, eG Enterprise provides administrators the option to deploy a piece of software called the eG VM Agent on every Windows VM; this VM agent allows the eG agent to collect “inside view” metrics from the Windows VMs without domain administrator rights. Refer to Configuring the Remote Agent to Obtain the Inside View of Windows VMs, using the eG VM Agent for more details on the eG VM Agent. To ensure that the “inside view” of Windows VMs is obtained using the eG VM Agent, set the Inside View Using flag to eG VM Agent (Windows). Once this is done, you can set the Domain, Admin User, and Admin Password parameters to none.

Domain, Admin User, Admin Password, and Confirm Password

By default, this test connects to each virtual guest remotely and attempts to collect “inside view” metrics. In order to obtain a remote connection, the test must be configured with user privileges that allow remote communication with the virtual guests. The first step towards this is to specify the Domain within which the virtual guests reside. The Admin User and Admin Password will change according to the Domain specification. Discussed below are the different values that the Domain parameter can take, and how they impact the Admin User and Admin Password specifications:

  • If the VMs belong to a single domain:  If the guests belong to a specific domain, then specify the name of that domain against the Domain parameter. In this case, any administrative user in that domain will have remote access to all the virtual guests. Therefore, an administrator account in the given domain can be provided in the Admin User field and the corresponding password in the Admin Password field. Confirm the password by retyping it in the Confirm Password text box.
  • If the guests do not belong to any domain (as in the case of Linux/Solaris guests):  In this case, specify "none" in the Domain field, and specify a local administrator account name in the Admin User below.

    Prior to this, you need to ensure that the same local administrator account is available or is explicitly created on each of the virtual machines to be monitored. Then, proceed to provide the password of the Admin User against Admin Password, and confirm the password by retyping it in the Confirm Password text box.

    If key-based authentication is implemented between the eG agent and the SSH daemon of a Linux guest, then, in the Admin User text box, enter the name of the user whose <USER_HOME_DIR> (on that Linux guest) contains a .ssh directory with the public key file named authorized_keys. The Admin Password in this case will be the passphrase of the public key; the default public key file that is bundled with the eG agent takes the password eginnovations. Specify this as the Admin Password if you are using the default private/public key pair that is bundled with the eG agent to implement key-based authentication. On the other hand, if you are generating a new public/private key pair for this purpose, then use the passphrase that you provide while generating the pair. For the detailed procedure on Implementing Key-based Authentication refer to Troubleshooting the Failure of the eG Remote Agent to Connect to or Report Measures for Linux Guests.

  • If the guests belong to different domains: In this case, you might want to provide multiple domain names. If this is done, then, to access the guests in every configured domain, the test should be configured with the required user privileges; this implies that along with multiple Domain names, multiple Admin User names and Admin Passwords would also have to be provided. To help administrators provide these user details quickly and easily, the eG administrative interface embeds a special configuration page. To access this page, simply click on the Click here hyperlink that appears just above the parameters of this test in the test configuration page. To know how to use the special page, refer to PVS Write Cache - OS Test.
  • If the Inside View Using flag is set to ‘eG VM Agent (Windows)’: In this case, the inside view can be obtained without domain administrator privileges. Therefore, set the Domain, Admin User, and Admin Password parameters to none.

Report By User

For the KVM server monitoring model, the Report By User flag is set to No by default, indicating that by default, the guest operating systems on the KVM server are identified using the hostname specified in the operating system. On the other hand, while monitoring KVM VDI 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 the Report By User flag is 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.



Write cache utilization

Indicates the percent usage of the write cache.



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.