7.3 Object Size Limits

You can control both the maximum and minimum size of cached objects. Responses larger than maximum_object_size aren't stored on disk. They are still proxied, however. The logic behind this directive is that you don't want a really big response to take up space better utilized by many small responses. The syntax is as follows:

maximum_object_size size-specification

Here are some examples:

maximum_object_size 100 KB

maximum_object_size 1 MB

maximum_object_size 12382 bytes

maximum_object_size 2 GB

Squid checks the response size in two different ways. If the reply includes a Content-Length header, Squid compares its value to the maximum_object_size value. If the content length is the larger of the two numbers, the object becomes immediately uncachable and never consumes any disk space.

Unfortunately, not every response has a Content-Length header. In this case, Squid writes the response to disk as data comes in from the origin server. Squid checks the object size again only when the response is complete. Thus, if the object's size reaches the maximum_object_size limit, it continues consuming disk space. Squid increments the total cache size only when it is done reading a response.

In other words, the active, or in-transit, objects don't contribute to the cache size value Squid maintains internally. This is good because it means Squid won't remove other objects in the cache, unless the object remains cachable and then contributes to the total cache size. However, it is also bad because Squid may run out of free disk space if the reply is very large. To reduce the chance of this happening, you should also use the reply_body_max_size directive. A response that reaches the reply_body_max_size limit is cut off immediately.

Squid also has a minimum_object_size directive. It allows you to place a lower limit on the size of cached objects. Responses smaller than this size aren't stored on disk or in memory. Note that this size is compared to the response's content length (i.e., the size of the reply body), which excludes the HTTP headers.