Add Redis eviction policies

docs/hypernode-platform/redis/how-to-change-redis-eviction-policy.md

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+---
+myst:
+  html_meta:
+    description: Changing the Redis eviction policy on Hypernode with hypernode-systemctl.
+      Learn which policies are available and how to verify the active Redis maxmemory-policy.
+    title: Changing the Redis eviction policy on Hypernode
+---
+
+# Changing the Redis eviction policy
+
+You can change the Redis `maxmemory-policy` with `hypernode-systemctl settings redis_eviction_policy`. This policy determines which keys Redis can evict when memory is full.
+
+## Available policies
+
+The following values are supported for `redis_eviction_policy`:
+
+- `noeviction`
+- `allkeys-lru`
+- `allkeys-lfu`
+- `allkeys-random`
+- `volatile-lru`
+- `volatile-lfu`
+- `volatile-ttl`
+- `volatile-random`
+
+```{infomation}
+By default, Redis on Hypernode uses the `volatile-lru` eviction policy.
+```
+
+## Check the current policy
+
+To view the current Redis eviction policy:
+
+```bash
+hypernode-systemctl settings redis_eviction_policy
+```
+
+## Change the policy
+
+Use the following command to change the Redis eviction policy:
+
+```bash
+hypernode-systemctl settings redis_eviction_policy <policy>
+```
+
+For example:
+
+```bash
+hypernode-systemctl settings redis_eviction_policy allkeys-lru
+```
+
+## Policy explanation
+
+### `noeviction`
+
+Redis will not evict any keys when memory is full. New write operations
+that require memory will fail.
+
+### `allkeys-lru`
+
+Redis can evict any key and removes the least recently used keys first.
+
+### `allkeys-lfu`
+
+Redis can evict any key and removes the least frequently used keys first.
+
+### `allkeys-random`
+
+Redis can evict any key and removes keys at random.
+
+### `volatile-lru`
+
+Redis only evicts keys with an expiration time and removes the least
+recently used keys first.
+
+This is the default policy on Hypernode.
+
+### `volatile-lfu`
+
+Redis only evicts keys with an expiration time and removes the least
+frequently used keys first.
+
+### `volatile-ttl`
+
+Redis only evicts keys with an expiration time and prefers keys that will
+expire soonest.
+
+### `volatile-random`
+
+Redis only evicts keys with an expiration time and removes those keys at
+random.
+
+## Important considerations
+
+- Eviction only takes place when Redis reaches its memory limit.
+- Policies starting with `allkeys-` can evict any key, including keys without a TTL.
+- Even with an `allkeys-` policy, Redis can still run into memory pressure if keys cannot be evicted fast enough to keep up with writes.
+- Setting a TTL on cache keys is still recommended, even when using an `allkeys-` policy.
+- Policies starting with `volatile-` only apply to keys with a TTL.
+- If not enough keys have a TTL, Redis may not be able to evict enough data with a `volatile-` policy, and write operations may fail.
+- Changing the policy does not remove existing keys immediately.