Forefront TMG implements a caching mechanism to improve performance and response times for Web requests from the Internet, and from published Web servers. The cache contains content that is frequently requested by internal or remote users. When users request content that is cached, Forefront TMG serves the content directly from the cache. The benefits of Web caching include:
- Faster access to requested content—Requests
are served from the cache instead of requiring a connection to
Internet servers, or access to published Web servers.
- Reduced bandwidth on the Internet connection,
and reduced load on published Web servers.
The following sections provide information that can help you plan your caching needs:
 Note: |
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| For information on how to enable and configure caching, see Caching Web site content. |
Supported caching types
Forefront TMG supports two types of caching:
- Forward caching—Caches frequently-requested
Internet content, and serves it to internal users.
- Reverse caching—Caches content that is
frequently requested from internal Web servers published by
Forefront TMG, and serves it to external, remote users. Reverse
caching is enabled by default when forward caching is enabled.
Considerations for storing cached content
Forefront TMG stores cached content in two locations:
- In memory (by default, 10 percent of the
RAM is used for caching objects).
- On disk.
Because objects that are cached to memory can be retrieved faster than objects cached to the disk, Forefront TMG stores the most popular content on both the disk and in memory. If the cache content file on the disk is too full to hold a new object, Forefront TMG removes older objects from the cache. It determines which objects to remove from the disk by using a formula that evaluates how old is the object, how often the object is accessed, and its size.
When you plan for caching, consider the following:
- More RAM provides faster performance for
serving cached content. In large deployments, it is recommended
that a high-performance hard disk is used.
- You must use a formatted NTFS file system
partition for the cache, and the cache drive must be local. When
you configure a cache drive, a cache-content file Dir1.cdat is
created in the location: drive:\urlcache.
- The maximum size for the cache file on a
single drive is 64 GB.
- Files larger than 512 MB do not remain in
cache upon reboot.
- It is recommended that you locate the file on
a physical disk other than the disk on which the operating system
and Forefront TMG are installed. This reduces contention on the
system and boot disk.
Forefront TMG cache performance counters provide information about cache memory performance, cache space, and URL handling. Based on this information, you can modify cache settings as required. For more information, see Cache performance counters (http://go.microsoft.com/fwlink/?LinkId=160063).
Caching in Forefront TMG arrays
In arrays, Forefront TMG uses Cache Array Routing Protocol (CARP) to provide a single, logical cache, for all the servers in the array. CARP allows Forefront TMG array members to efficiently balance Web-based client load, and split cached content between them. On the client side, CARP provides client computers with the information and algorithms required to identify which is the best server in the array to serve their request, thus eliminating the need for array members to forward requests between the array members. CARP also supports array server selection by the servers themselves and chained proxies.