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https://jira.gcore.lu/browse/CD-2935
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dashlot authored Oct 10, 2023
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published: true
order: 20
toc:
--1--CDN Caching option: "cdn-caching-option"
--1--Query-String and Set-Cookies options: "query-string-and-set-cookies-options"
--1--Large Files Delivery Optimization: "large-files-delivery-optimization"
--1--Cache hit ratio: "what-is-the-cache-hit-ratio"
--1--Issues: "issues-with-the-low-cache-hit-ratio"
--1--Check: "check-if-slow-content-delivery-is-related-to-low-cache-hit-ratio"
--1--Causes and solutions: "causes-and-solutions-for-low-cache-hit-ratio"
pageTitle: Boost CDN Cache Hit Ratio | Gcore
pageDescription: Learn how to improve your CDN Cache Hit Ratio. Get tips on CDN integration, cache settings, and leverage Large Files Delivery Optimization.
---
# Cache percentage is low: how to solve the issue
# Cache hit ratio is low: How to solve the issue

Cached traffic is traffic that is delivered from CDN edge’s cache without request to the origin server. 
## What is the cache hit ratio?

You may track the Cache Hit Ratio percentage on the <a href="https://gcore.com/docs/cdn/view-statistics-of-a-cdn-resource" target="_blank">Report</a> tab. 
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 <a href="https://cdn.gcore.com/reports/statistics" target="_blank">Statistics</a> section within the control panel.

Cache Hit Ratio depends on the number of requests and the amount of traffic. If you have noticed that the cache hit ratio is 60% or less: 
<img src="https://assets.gcore.pro/docs/cdn/troubleshooting/cache-percentage-is-low-how-to-solve-the-issue/.png" alt="" width="80%">

1. Make sure that your website or application is integrated with CDN successfully: the static content is delivered through a custom domain, the <a href="https://gcore.com/docs/cdn/cdn-resource-options/general/create-and-set-a-custom-domain-for-the-content-delivery-via-cdn" target="_blank">custom domain is set up</a> correctly. 
2. Make sure that more than 2 days have passed since you have started to use CDN. 
3. Deliver more content through CDN, if possible. 
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.

If you already set up CDN delivery for all possible static content, but the cache hit ratio remains low, revise the CDN resource’s cache settings: 
## Issues with the low cache hit ratio

## CDN Caching option
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.

<img src="https://assets.gcore.pro/docs/cdn/troubleshooting/cache-percentage-is-low-how-to-solve-the-issue/_______________________.png" alt="CDN Caching option" width="80%">

Check if this option is added and activated. If you use this option with Origin controlled parameter make sure that origin settings are suitable for caching.

CDN caches according to Cache-Control headers from your origin. 

**Note**: Cache-control directives that are not suitable for CDN:  no-cache, no-store, private, max-age=0 

## Query-String and Set-Cookies options

<img src="https://assets.gcore.pro/docs/cdn/troubleshooting/cache-percentage-is-low-how-to-solve-the-issue/___________________________.png" alt="Query-String and Set-Cookies options" width="80%">

CDN caches content with different query-strings and Set-Cookie headers separately by default If you have such content on your website or application, we recommend you to use Ignore Query-String and Ignore Set-Cookies options for the CDN-resource. It will help to decrease the number of requests from CDN to the origin server and increase the Cache hit ratio. 

## Large Files Delivery Optimization 

<img src="https://assets.gcore.pro/docs/cdn/troubleshooting/cache-percentage-is-low-how-to-solve-the-issue/________________.png" alt="Large Files Delivery Optimization " width="50%">

Use the Large Files Delivery Optimization option for the large content caching. You may know more about the option from the <a href="https://gcore.com/docs/cdn/cdn-resource-options/optimize-large-file-delivery" target="_blank">article</a>. 

If you add any parameters (for example, a unique signature for users) to the URL, but not as a query string (```http://cdn.example.com/image.jpg?id=1```), it requires custom caching keys set up. 

Contact [technical Support](mailto:support@gcore.com) for manual settings and describe the URL scheme: which parts should be cached and which – should not.
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 <a href="https://gcore.com/docs/cdn/view-statistics-of-a-cdn-resource#statistics" target="_blank">viewing statistics of CDN resources</a>.

**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:

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

**Note**: Request content from one specific CDN edge server.

<expandable-element title="How to request content from the same server">
**Note**: Requesting content from the same server several times is very important. Normally, your request will hit multiple servers in your location, returning an unrepresentative result because one server may have cached the file and another may not.

To request the file from the same server:

1\. Request the file that appears not to be distributed from the CDN, and check the **“X-id”** header. This header shows the server that received the request. Copy that value.

2\. Run the following command to get its IP:

```
nslookup/dig {x-id server}.fe.gc.onl
```
3\. Request the file from the IP address which you got in step 2:

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

Compare the file headers with the values listed in the table below.

</expandable-element>

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

<table>
<thead>
<tr>
<td><b>Value</b></td>
<td><b>Meaning</b></td>
</tr>
</thead>
<tbody>
<tr>
<td>Cache-Control: <i>max-age=0</i></td>
<td rowspan="3">This value prohibits content caching.<br> It may cause an issue with the Cache hit ratio and reduce performance.<br> However, in some cases, this value is ok.</td>
</tr>
<tr>
<td>Cache-Control: <i>no-cache</i></td>
</tr>
<tr>
<td>Cache-Control: <i>no-store</i></td>
</tr>
<tr>
<td>Cache-Control: <i>private</i></td>
<td>This value caches content only in users’ browsers, not the edge servers’ cache.
<br><br>
<b>Note</b>: If the request contains custom headers starting with X (e.g., “X-custom-header”),<br> the CDN will treat the request as a new request and forward it to the origin (as with cookies.)</td>
</tr>
<tr>
<td>Cache: <i>MISS</i></td>
<td>This value specifies that the response is received from the origin, bypassing the cache.<br> Our CDN caches files on the first request.<br> This means that the first request for a file will result in MISS<br> (CDN had to reach the origin to pull the resource and cache it locally)<br> and every following request for the same file will result in HIT (served from a local CDN cache.)</td>
</tr>
</tbody>
</table>

3\. Check the <a href="https://gcore.com/docs/cdn/cdn-resource-options/cache/specify-cache-lifetime-on-a-cdn-resource-or-origin" target="_blank">CDN resource caching settings</a>. The feature must be enabled and configured to support CDN caching.

4\. Check <a href="https://gcore.com/docs/cdn/cdn-resource-options/cache/ignore-the-set-cookie-or-query-string-parameters-when-caching-content-on-cdn-servers" target="_blank">Ignore Query String and Ignore Set-Cookie options</a>. 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 <a href="https://gcore.com/docs/cdn/troubleshooting/content-delivery-speed-is-low-how-to-solve-the-issue" target="_blank">our guide</a> to help with other causes.

## Causes and solutions for low cache hit ratio

<table>
<thead>
<tr>
<td><b>Cause</b></td>
<td><b>Solution</b></td>
</tr>
</thead>
<tbody>
<tr>
<td>Traffic started less than two days ago, and more cache needs to be collected.</td>
<td>Traffic patterns and amounts dictate how long you should wait to populate the cache.<br> Usually, waiting for around 48 hours after the CDN integration is enough to cache most resources, but the exact time is individual.<br> You can also deliver more content through CDN.</td>
</tr>
<tr>
<td>Content is not requested enough, and the cache per server is low.</td>
<td>Increase the traffic to your content or use the paid <a href="https://gcore.com/docs/cdn/cdn-resource-options/general/enable-and-configure-origin-shielding" target="_blank">Origin shielding</a> option to accumulate requests.</td>
</tr>
<tr>
<td>A “Cache-Control” value prohibits the CDN from caching content.</td>
<td>Ensure that the <a href="https://gcore.com/docs/cdn/cdn-resource-options/cache/specify-cache-lifetime-on-a-cdn-resource-or-origin" target="_blank">CDN Caching</a> option is enabled and configured in the following way.
<ul>
<li>For CDN controlled: <b>Don’t</b> set 0 in the "Cache expiry" field.</li>
<li>For Origin controlled: <b>Don’t</b> set values from the step above (<i>max-age=0, no-cache, no-store, private</i>.)</li>
</ul>
</td>
</tr>
<tr>
<td>Responses contain query strings or a Set-Cookie header, so the files are cached as different.</td>
<td>Enable <a href="https://gcore.com/docs/cdn/cdn-resource-options/cache/ignore-the-set-cookie-or-query-string-parameters-when-caching-content-on-cdn-servers" target="_blank">Ignore Query String and Ignore Set Cookie options</a>. Then, a single file will be delivered from the cache, regardless of the request parameters and the Set Cookie header.</td>
</tr>
<tr>
<td>The Vary header is set on origins so that content is cached suboptimally.</td>
<td>Change the Vary header settings on your origin.</td>
</tr>
</tbody>
</table>

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