### Understand the alert

This alert, `memcached_cache_fill_rate`, measures the average rate at which the Memcached cache fills up (positive value) or frees up (negative value) space over the last hour. The units are in `KB/hour`. If you receive this alert, it means that your Memcached cache is either filling up or freeing up space at a noticeable rate.

### What is Memcached?

Memcached is a high-performance, distributed memory object caching system used to speed up web applications by temporarily storing frequently-used data in RAM. It reduces the load on the database and improves performance by minimizing the need for repeated costly database queries.

### Troubleshoot the alert

1. Check the current cache usage:

You can view the current cache usage using the following command, where `IP` and `PORT` are the Memcached server's IP address and port number:

```
echo "stats" | nc IP PORT
```

Look for the `bytes` and `limit_maxbytes` fields in the output to see the current cache usage and the maximum cache size allowed, respectively.

2. Identify heavy cache users:

Find out which applications or services are generating a significant number of requests to Memcached. You may be able to optimize them to reduce cache usage. You can check Memcached logs for more details about requests and operations.

3. Optimize cache storage:

If the cache is filling up too quickly, consider optimizing your cache storage policies. For example, you can adjust the expiration times of stored items, prioritize essential data, or use a more efficient caching strategy.

4. Increase the cache size:

If needed, you can increase the cache size to accommodate a higher fill rate. To do this, stop the Memcached service and restart it with the `-m` option, specifying the desired memory size in megabytes:

```
memcached -d -u memcached -m NEW_SIZE -l IP -p PORT
```

Replace `NEW_SIZE` with the desired cache size in MB.

### Useful resources

1. [Memcached Official Site](https://memcached.org/)