Deployment considerations

Considerations for designating GLs and GMs

Considerations for determining whether a NetCache appliance is a GL or GM include the following:

Consider network topology. Content flows through a GL first. Therefore, when determining a GL, ensure that routing can send traffic to the appliance that you want to be the GL before sending it to a GM.
Ensure that a NetCache appliance that you want to be a GL has the capacity to handle the amount of content that would be pushed through it to GMs.
Ensure that the link speed between the ContentDirector server and a NetCache appliance that you want to be the GL supports the amount of bandwidth consumption you expect to occur during content pushing.
If your network is multicast-enabled, consider whether you want to take advantage of multicast transport from the GL to the GMs. If so, you might want a greater number of GMs per GL than if your network uses unicast.
Note
ContentDirector can distribute content on any vendor's network.

See the ContentDirector documentation for more information about CDA deployment on NetCache appliances.

Considerations about available disk space

The number of disks and size of disks are limited in some NetCache appliance models. You must consider the number of disks and disk sizes on each NetCache appliance at remote office before you start pushing content to it. See the ContentDirector documentation for additional information.

Considerations about disk space for pushed content

Consider how much disk space you want to be available for caching objects retrieved from an origin server as a result of a user request (that is, for content you did not push to the cache).

In ContentDirector, when you set up the job for content to be pushed, you can specify the period of time that you want the content to be locked in the cache of the destination NetCache appliances. Objects retrieved from an origin server as a result of a user request cannot be locked in the cache, however.

Locking of objects is significant because a NetCache appliance cannot automatically eject locked objects if it needs to free up disk space. (If disk space usage reaches 90 percent, NetCache automatically starts freeing up disk space.) Therefore, if you specify that a corporate video that you are pushing to a NetCache appliance is to be locked, the objects associated with that video are guaranteed to be available for your users during the locked period of time that you specified.

If you fill a cache with locked content, you will severely limit or eliminate the ability for the NetCache appliance to cache objects fetched from an origin server because few objects would be eligible to be ejected if the disk becomes too full. If you are locking content, set your lock times according to how long you really need specific content in your caches.

Tip:

If you decide that you no longer need specific content that you pushed to NetCache appliances through ContentDirector, you can remove that content from the appliances. To do so, delete the job from ContentDirector. This process removes the content you no longer need from the appliances to which that content was originally pushed.

After the locked time period on content pushed through ContentDirector expires, NetCache treats the formerly locked objects as any other objects and can eject that content if NetCache needs to free disk space.

Note
NetCache caches media streams by dividing them into fixed time segments. Each stream is cached as a series of objects, each containing n seconds of data. If NetCache needs to eject cached streaming media to free disk space, it does not eject an entire streaming media presentation--only the minimum number of streaming objects it needs to for the amount of disk space to be freed.

Type of content and cache disk space requirements

When you plan for content pushing, it is helpful to be able to estimate the amount of storage space that would be used by that content, particularly if you are locking the content you are pushing.

HTTP content

For an HTTP object, the size of the object on the origin server (number of bytes) is roughly equal to the number of bytes the object uses when stored on a NetCache appliance.

Streaming media content

Because of multiple-bit rate streams, estimating caching storage requirements for streaming media is not as simple as just looking at the size of a streaming media file on an origin server. NetCache dissects streaming media files, packetizes them as an origin streaming server would, and then stores the packetized data as objects in the cache. Estimating the disk space required to store a streaming media file on a NetCache appliance requires knowledge about the streaming media file.

Example 1: Calculations for a 10-minute 200-kbps single-bit rate file:

10 min x 60 sec/min x 200 kbps x 1 byte/8 bits = 15000 kB = 15 MB

To estimate the total disk space required in the cache, add about 10 percent storage overhead, which gives 16 MB.

Example 2: Calculations for a 10-minute multiple-bit rate file that contains 200-kbps and 500-kbps encodings:

For a multiple-bit rate file, you must know what has been encoded in the file to estimate the disk space required in a cache.

10 min x 60 sec/min x 200 kbps x 1 byte/8 bits = 15000 kB = 15 MB

10 min x 60 sec/min x 500 kbps x 1 byte/8 bits = 37500 kB = 38 MB

To estimate the total disk space required in the cache, add 10 percent storage overhead to each number, which is 16 MB + 42 MB = 58 MB.

Line speed and bandwidth consumption considerations

To plan for content pushing, you need to understand the data links between your central data storage center and the devices in remote locations and how much data the links can handle. You must also make some decisions regarding the amount of bandwidth you want to be used for content pushing instead of other types of traffic. Consider the following:

Pushing your content at times of low bandwidth usage, for example, at night, reduces the impact of this extra traffic on your users.
If your content pushing must be limited to a specific time period, consider whether your line speed is fast enough to push all the content that you need to during that time period.
If, for example, you have only a three-hour period during the night in which you can push data without impacting users, do not try to push more data over your line than your line can transport during those 3 hours.
How much of your network bandwidth you are comfortable using for pushed content.
Depending on the size of your links between the central data center and the remote locations, sending a large job might consume enough bandwidth so that performance for ordinary network traffic is poor.

Scheduling content pushes for hours of low traffic will help avoid degradation of performance due to network congestion. If scheduling traffic pushes is not possible during nonpeak hours, you should weigh the advantages of sending content as a unit against bandwidth consumption. Or, schedule the pushing of interrelated content divided into smaller jobs in close time periods.

You can use the NetCache bandwidth allocation feature on a NetCache appliance configured as a GL to control bandwidth usage from the ContentDirector server to the NetCache appliance GL and from the NetCache appliance GL to the GMs. You can create bandwidth allocation rules for ContentDirector traffic to do the following:
- Reserve a portion of your total NetCache bandwidth capacity for ContentDirector traffic
- Allow, deny, or assign connections to GL based on time and interface
If your network is multicast-enabled, ContentDirector automatically uses multicast to push content from the GLs to the GMs.

Considerations about what data to push

Different types of organizations have different interests in the type of data to push to remote offices. General principles for all organizations are as follows:

Push content you know your users will access.
Otherwise, you will be filling your caches with content that will not be accessed, and as a result, reducing disk space available for other objects.
Push content that might have quality-of-service issues associated with it--larger files, especially multimedia.
Note
ContentReporter can help you identify which types of files are most frequently requested. With this information, you can use ContentDirector to push files to NetCache appliances to improve end-user response time.

Considerations for pushing content as a unit

When planning for pushing your content, you need to weigh the advantages of pushing data as a unit against the effects on bandwidth usage at a particular time. Consider the following:

If your interrelated content is divided into several small jobs, it takes longer to synchronize the data in the NetCache appliances to which the content is pushed.
From the perspective of synchronization of your content, you should push interrelated content in the same job so that when that content is first available on the NetCache appliance to which it was pushed, all parts of the content are synchronized. Otherwise, the content that has not been updated would register as a cache miss, and NetCache would attempt to fetch the content from an origin server.
The larger the job, the more bandwidth is consumed by that one job until it is completed.
You should weigh the advantages of sending content as a unit against bandwidth consumption at a given time. If you anticipate that sending a large job would affect network performance, it might be helpful to divide interrelated content into smaller jobs and push those jobs in close time periods.

Content duplication among caches

In the content push model for caching services, storing the same content in multiple locations is often a goal. Enterprises, for example, are often interested in storing the same content in many remote offices so that all users in the remote offices have fast access to the same content from the corporate headquarters. Such enterprises typically determine where to send information by considering whether specific content would be useful to users at a specific location. They then evaluate reports generated by the ContentReporter software that show what content is accessed most often at what locations. After reviewing the reports, they might then decide to push more content of the same type to the locations at which a particular type of content was popular.

ContentDirector enables you to create logical groups of NetCache appliances for content distribution that can be independent of how the appliances are grouped on their physical networks. This feature helps streamline the process of duplicating content among NetCache appliances.