[ceph.git] / ceph / doc / cephfs / cache-size-limits.rst

Understanding MDS Cache Size Limits
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This section describes ways to limit MDS cache size.

You can limit the size of the Metadata Server (MDS) cache by:

* *A memory limit*: A new behavior introduced in the Luminous release. Use the `mds_cache_memory_limit` parameters.

In addition, you can specify a cache reservation by using the `mds_cache_reservation` parameter for MDS operations. The cache reservation is limited as a percentage of the memory 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 consequence, the MDS should in general operate below its memory limit because it will recall old state from clients in order to drop unused metadata in its cache.

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

Be aware that 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` configures the cluster health warning message so that operators can investigate why the MDS cannot shrink its cache.
Commit	Line	Data
11fdf7f2 TL	1	Understanding MDS Cache Size Limits
	2	===================================
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	4	This section describes ways to limit MDS cache size.
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	6	You can limit the size of the Metadata Server (MDS) cache by:
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9f95a23c	8	* A memory limit: A new behavior introduced in the Luminous release. Use the `mds_cache_memory_limit` parameters.
11fdf7f2	9
9f95a23c	10	In addition, you can specify a cache reservation by using the `mds_cache_reservation` parameter for MDS operations. The cache reservation is limited as a percentage of the memory 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 consequence, the MDS should in general operate below its memory limit because it will recall old state from clients in order to drop unused metadata in its cache.
11fdf7f2 TL	11
	12	The `mds_cache_reservation` parameter replaces the `mds_health_cache_threshold` in all situations except when MDS nodes sends a health alert to the Monitors indicating the cache is too large. By default, `mds_health_cache_threshold` is 150% of the maximum cache size.
	13
	14	Be aware that 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` configures the cluster health warning message so that operators can investigate why the MDS cannot shrink its cache.