Cache hit ratio is low: How to solve the issue

What is the cache hit ratio?

The cache hit ratio is the percentage of traffic delivered from the CDN edge cache without requesting the origin server. You can check it in the Statistics section in the Gcore Customer Portal.

A high cache hit ratio shows the efficiency and practicality of using CDNs. The higher it is, the better. By reducing the number of times CDN edge servers request a file from the source, you improve performance, reduce the load on the source, and reduce outbound traffic costs.

Issues with the low cache hit ratio

A low cache hit ratio is a sign of something going awry, and shows that many requests are proxied to the origin instead of files being delivered from the cache.

A low cache hit ratio can be accompanied by low content delivery speed Compare the request scheme for cached file User → CDN edge → User with uncached—User → CDN edge → Origin → CDNedge → User. In the second case, more requests go to the origin, which causes longer response time simply due to a higher distance and more hops in the sequence.

Check if slow content delivery is related to low cache hit ratio

1. Check Statistics for “Cache hit ratio.” If it is low (less than 80% in most cases), also check “Traffic” and “Requests” to avoid misconceptions in determining the cause. You can learn more about where and how to view reports in the guide about viewing statistics of CDN resources.

Note: 80% is an approximate number, used for descriptive purposes, the accurate number highly depends on the use-case.

2. Check the response headers using the curl -I command; for instance:

      Copy
      
    curl -I cdn.example.com/assets/sample-image.png

Note: Request content from one specific CDN edge server.

      Copy
      
    nslookup/dig {x-id server}.fe.gc.onl

      Copy
      
    curl -Ik -H "Host: {cdn.domain}" "http{s}://{ip-address}/file"

The response will contain different headers. You should focus on the inappropriate values of caching headers “Cache-control” and “Cache”:

3. Check the CDN resource caching settings. The feature must be enabled and configured to support CDN caching.

4. Check Ignore Query String and Ignore Set-Cookie options. If it’s disabled, content with different query strings and Set-Cookie headers caches separately.

Note: If content delivery speed is not related to low cache hit ratio, check our guide to help with other causes.

Causes and solutions for low cache hit ratio

Cause	Solution
Traffic started less than two days ago, and more cache needs to be collected.	Traffic patterns and amounts dictate how long you should wait to populate the cache. Usually, waiting for around 48 hours after the CDN integration is enough to cache most resources, but the exact time is individual. You can also deliver more content through CDN.
Content is not requested enough, and the cache per server is low.	Increase the traffic to your content or use the paid Origin shielding option to accumulate requests.
A “Cache-Control” value prohibits the CDN from caching content.	Ensure that the CDN Caching option is enabled and configured in the following way. For CDN controlled: Don’t set 0 in the “Cache expiry” field. For Origin controlled: Don’t set values from the step above (max-age=0, no-cache, no-store, private.)
Responses contain query strings or a Set-Cookie header, so the files are cached as different.	Enable Ignore Query String and Ignore Set Cookie options. Then, a single file will be delivered from the cache, regardless of the request parameters and the Set Cookie header.
The Vary header is set on origins so that content is cached suboptimally.	Change the Vary header settings on your origin or use the Hide response headers feature in the customer portal. Set “Hide only,” select the Vary header and other that are necessary and save changes.