docs(cache): Add documentation for memcache_customprefix setting

admin_manual/configuration_server/caching_configuration.rst

@@ -187,6 +187,7 @@ The following options are available to configure when using a single redis serve
    'memcache.locking' => '\OC\Memcache\Redis',
    'memcache.distributed' => '\OC\Memcache\Redis',
    'memcache.local' =>'\OC\Memcache\Redis' ,
+   'memcache_customprefix' => 'mynextcloudprefix',
    'redis' => [
       // 'host'      => see connection parameters below
       // 'port'      => see connection parameters below
@@ -202,6 +203,7 @@ The following options are available to configure when using a redis cluster (all
    'memcache.locking' => '\OC\Memcache\Redis',
    'memcache.distributed' => '\OC\Memcache\Redis',
    'memcache.local' =>'\OC\Memcache\Redis' ,
+   'memcache_customprefix' => 'mynextcloudprefix',
    'redis.cluster' => [
       'seeds' => [ // provide some/all of the cluster servers to bootstrap discovery, port required
          'cache-cluster:7000',
@@ -374,3 +376,17 @@ Cache Directory location
 The cache directory defaults to ``data/$user/cache`` where ``$user`` is the 
 current user. You may use the ``'cache_path'`` directive in ``config.php``
 (See :doc:`config_sample_php_parameters`) to select a different location.
+
+Cache Key Prefix for Redis or Memcached
+---------------------------------
+
+By default, Nextcloud generates a semi-unique prefix for cache keys using information like instance ID, version etc. to mitigate the problem of
+collisions when using the same cache for multiple Nextcloud instances.
+If you want to make sure to prevent collisions alltogether, you can use the following
+setting to define your custom prefix::
+
+  'memcache_customprefix' => 'mynextcloudprefix',
+
+This also allows you to create ACLs in Redis and limit the keys specific users can access
+(e.g. if you want to isolate specific Nextcloud instances when using the same cache).
+This may be security-relevant for you.

admin_manual/configuration_server/config_sample_php_parameters.rst

@@ -2717,6 +2717,21 @@ Memory caching backend for locally stored data
 
 Defaults to ``none``
 
+memcache_customprefix
+^^^^^^^^^^^^^^^^^^^^^
+
+
+::
+
+	'memcache_customprefix' => 'mynextcloudprefix',
+
+Custom cache key prefix for Redis or Memcached
+
+* Used for avoiding collisions in the cache system
+* May be used for ACL restrictions in Redis
+
+Defaults to an auto-generated, semi-unique ID
+
 memcache.distributed
 ^^^^^^^^^^^^^^^^^^^^
 

admin_manual/installation/example_centos.rst

@@ -220,6 +220,7 @@ Example config::
     'memcache.distributed' => '\OC\Memcache\Redis',
     'memcache.locking' => '\OC\Memcache\Redis',
     'memcache.local' => '\OC\Memcache\APCu',
+    'memcache_customprefix' => 'nextcloud_centos',
     'redis' => array(
       'host' => 'localhost',
       'port' => 6379,