Metadata Server cache size limits

Learn how to limit the size of the Ceph File System (CephFS) Metadata Server (MDS) cache.

You can limit the size of the Ceph File System (CephFS) Metadata Server (MDS) cache by using either the memory limit or the inode count.

Memory limit: Use the mds_cache_memory_limit option. Use a value between 8 GB and 64 GB for mds_cache_memory_limit. Setting more cache can cause issues with recovery. This limit is approximately 66% of the desired maximum memory use of the MDS.
Important: Use memory limits instead of inode count limits.
Note: The default value for mds_cache_memory_limit is 4 GB. Since the default value is outside the recommended range, users are recommended to set the value within the mentioned range.
Inode count: Use the mds_cache_size option. By default, limiting the MDS cache by inode count is disabled.

You can also specify a cache reservation by using the mds_cache_reservation option for MDS operations. The cache reservation is limited as a percentage of the memory or inode limit and is set to 5% by default. The intent of this parameter is to have the MDS maintain an extra reserve of memory for its cache for new metadata operations to use. As a result, the MDS should operate below its memory limit because it recalls old state from clients to drop unused metadata in its cache.

The mds_cache_reservation option replaces the mds_health_cache_threshold option in all situations, except when MDS nodes send a health alert to the Ceph Monitors indicating the cache is too large. By default, mds_health_cache_threshold is 150% of the maximum cache size.

The cache limit is not a hard limit. Potential bugs in the CephFS client or MDS or misbehaving applications might cause the MDS to exceed its cache size. The mds_health_cache_threshold option configures the storage cluster health warning message so that operators can investigate why the MDS cannot shrink its cache.

For more information, see Configuring Metadata Server daemons.