The cache clean cycle versus CacheCleaner

The cache clean cycle is provided in addition to the previously used CacheCleaner, which can still be run when required (for example when reserving space for new contents to be downloaded by manually running it from the command-line). CacheCleaner deletes content based on its priority and doesn't check its last usage date. So it is possible that content which is lower in priority but has been recently used, or is used regularly (like WIM files), may also get deleted when CacheCleaner runs. This mechanism therefore acts more as an emergency option to free up disk space but may result in the deletion of content that is likely to be used (which must then be downloaded again when needed). The cache clean cycle helps to maintain the low profile of the Nomad cache by removing content that is less likely to be used.

Both of these methods are described in greater detail in the following headings.

Cleaning the cache using the cache clean cycle

From Nomad 6.3 and above a new cache clean cycle has been added that automatically runs the cache cleaner on a regular configurable interval. Nomad was designed to cache content that Configuration Manager has requested or that has been pre-cached via ActiveEfficiency. Because of this the content accumulated in the Nomad Cache can become quite extensive, with a large degree of it no longer relevant or useful. For example, a Windows software update which has been superseded by a newer update is no longer relevant or useful and can be safely removed. The cache clean cycle helps to manage the Nomad cache, remove aged and irrelevant content and minimise its impact on the host machine.

Configuration options

This option is configured by setting the following registry values or installer properties:

As you can see from the table the cache clean cycle is turned off by default, so the registry entries must be set before it will start to regularly clean the cache.

Cache clean cycle delete conditions

When the cache clean cycle runs, it will check the type of each content and the last election date associated with it. There are four main types of content and these will be marked for deletion if the following delete conditions are true:

The cache clean cycle will then also check the following, for all content other than Office 365 update byte-ranges:

1. The content should be present on at least one of the Nomad subnet peers with an equal or a higher percentage of content
2. The network has broadcasts enabled

If both of these conditions are also true the content will be deleted.

Caveats

The following points describe various caveats related to the way that the cache clean cycle works:

• If pre-cache content is deleted by the cache clean cycle, but the corresponding pre-caching job still exists, the ActiveEfficiency Server will send the download notification again to the client.

  If your Nomad policy is to not delete pre-caching jobs you should turn of deletion of pre-cached content by setting the MaxPreCacheDays registry value to 0.
• Application upgrades and uninstallation via Software Center require content to be present on the machines (the current version is required for uninstallation and the previous version is required for upgrade scenarios). If the content has been deleted by the cache clean cycle, the content would be downloaded again at uninstall or upgrade time. Because of the way that the cache clean cycle delete conditions are applied Nomad retains one copy of the content per subnet, so even if such a download request comes the content would be downloaded from a Nomad peer and not a DP. 
• ByteRange content is not checked to be present on subnet peers.
• Cache optimization does not work for content that is to be activated later on.
• Cache optimization does work on Sensitive Servers.

Cleaning the cache using CacheCleaner.exe

To maintain the cache Nomad uses a set of basic rules that lets you decide the cache's effect on overall disk usage and performs automatic cleaning when necessary. When a job is advertised, automatic cache cleaning could occur under the following circumstances:

• If the percentage of available disk space is less than the PercentAvailableDisk setting after downloading an incoming package, according to its download package size, automatic cache cleaning is initiated before the download takes place
• Nomad then assesses how much space could be cleared by deleting cache folders that are eligible for deletion. A cache folder is deemed eligible for deletion if its cache priority value is lower than or equal to the cache priority value of the incoming package. The cache priority value is set on the Nomad command-line using the pc argument, or set in the Nomad properties page when using Configuration Manager. By default, if no value is specified, packages have a cache priority of 1.
• If the assessment determines there would still not be sufficient space available: no deletion will take place, the download is aborted and an error logged, which may be one of:
  
  • Insufficient disk or cache space available to download the Program…
  • Insufficient cache space available to achieve a Max Cache Size of X
• If the assessment determines there would be sufficient space available in the cache: the folders are deleted in order according to their cache priority until the required disk space is available.

Cache cleaner and Configuration Manager

When Nomad is used with Configuration Manager the cache cleaner will also:

• Purge linked contents from the Configuration Manager cache
• Purge older versions from the Configuration Manager cache before downloading the content for a newer version

The cache cleaner mechanism also works with the priority job control to ensure that suspended lower priority job caches are not cleaned to make way for the higher priority job unless absolutely necessary.

Cache cleaner and central multicast

If you are using Nomad's Central Multicast feature, cache cleaner automatically takes into account the space required by all the concurrent jobs running before purging a cache. There is no additional configuration needed for this to happen.

Maintaining the cache manually

By default, a Nomad cache is not purged unless there is an active download. In most cases, available disk space is not really an issue but there may be circumstances where you may want to manage the cache to keep the disk usage overhead to a minimum.

You can explicitly purge the cache with CacheCleaner command-line utility which ships with the Nomad installation. Use CacheCleaner with the following command-line arguments, each preceded with '-'

-DeletePkg=PRI0000A -PkgVer=*

-DeletePkg=Content_9b9ccd56-9fbf-4587-98e7-725fcfa8e70f -PkgVer=*

-Pkgsize=1048576
-Pkgsize=1MB

CacheCleaner examples

In the following examples, there is no requirement to specify the path to CacheCleaner.exe because the path to Nomad executables is set in the PATH environment variable.

• You can set the cache cleaner to run as a Configuration Manager program and advertise it to all Nomad systems in a single operation. You can also schedule this as a regular housekeeping job, ensuring that the cache is kept under control automatically. To purge the cache when the total free disk space is at 20%, run

  CacheCleaner.exe –PercentAvailableDisk=20

• To ensure that content in the cache is not kept for more than 30 days, run:

  CacheCleaner.exe –MaxCacheAge=30

• To delete a particular package and version, run:

  CacheCleaner.exe –DeletePkg=L0A00001 –PkgVer=1

• To purge the contents of a cache with a priority of 3 or lower, run:

  CacheCleaner.exe –DeleteAll -Force=3

• To purge the contents of a cache with a particular priority and size, run:

  CacheCleaner.exe –PkgSize=10485760 –CachePriority=6