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MDS Cache Configuration

The Metadata Server coordinates a distributed cache among all MDS and CephFS clients. The cache serves to improve metadata access latency and allow clients to safely (coherently) mutate metadata state (e.g. via chmod). The MDS issues capabilities and directory entry leases to indicate what state clients may cache and what manipulations clients may perform (e.g. writing to a file).

The MDS and clients both try to enforce a cache size. The mechanism for specifying the MDS cache size is described below. Note that the MDS cache size is not a hard limit. The MDS always allows clients to lookup new metadata which is loaded into the cache. This is an essential policy as it avoids deadlock in client requests (some requests may rely on held capabilities before capabilities are released).

When the MDS cache is too large, the MDS will recall client state so cache items become unpinned and eligible to be dropped. The MDS can only drop cache state when no clients refer to the metadata to be dropped. Also described below is how to configure the MDS recall settings for your workload’s needs. This is necessary if the internal throttles on the MDS recall can not keep up with the client workload.

MDS Cache Size

You can limit the size of the Metadata Server (MDS) cache by a byte count. This is done through the mds_cache_memory_limit configuration. For example:

ceph config set mds mds_cache_memory_limit 8GB

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.

If the MDS cannot keep its cache under the target size, the MDS will send a health alert to the Monitors indicating the cache is too large. This is controlled by the mds_health_cache_threshold configuration which is by default 150% of the maximum cache size.

Because the cache limit is not a hard limit, potential bugs in the CephFS client, MDS, or misbehaving applications might cause the MDS to exceed its cache size. The health warnings are intended to help the operator detect this situation and make necessary adjustments or investigate buggy clients.

MDS Cache Trimming

There are two configurations for throttling the rate of cache trimming in the MDS:

mds_cache_trim_threshold (default 64k)

and

mds_cache_trim_decay_rate (default 1)

The intent of the throttle is to prevent the MDS from spending too much time trimming its cache. This may limit its ability to handle client requests or perform other upkeep.

The trim configurations control an internal decay counter. Anytime metadata is trimmed from the cache, the counter is incremented. The threshold sets the maximum size of the counter while the decay rate indicates the exponential half life for the counter. If the MDS is continually removing items from its cache, it will reach a steady state of -ln(0.5)/rate*threshold items removed per second.

Note

Increasing the value of the confguration setting mds_cache_trim_decay_rate leads to the MDS spending less time trimming the cache. To increase the cache trimming rate, set a lower value.

The defaults are conservative and may need to be changed for production MDS with large cache sizes.

MDS Recall

MDS limits its recall of client state (capabilities/leases) to prevent creating too much work for itself handling release messages from clients. This is controlled via the following configurations:

The maximum number of capabilities to recall from a single client in a given recall event:

mds_recall_max_caps (default: 5000)

The threshold and decay rate for the decay counter on a session:

mds_recall_max_decay_threshold (default: 16k)

and:

mds_recall_max_decay_rate (default: 2.5 seconds)

The session decay counter controls the rate of recall for an individual session. The behavior of the counter works the same as for cache trimming above. Each capability that is recalled increments the counter.

There is also a global decay counter that throttles for all session recall:

mds_recall_global_max_decay_threshold (default: 64k)

its decay rate is the same as mds_recall_max_decay_rate. Any recalled capability for any session also increments this counter.

If clients are slow to release state, the warning “failing to respond to cache pressure” or MDS_HEALTH_CLIENT_RECALL will be reported. Each session’s rate of release is monitored by another decay counter configured by:

mds_recall_warning_threshold (default: 32k)

and:

mds_recall_warning_decay_rate (default: 60.0 seconds)

Each time a capability is released, the counter is incremented. If clients do not release capabilities quickly enough and there is cache pressure, the counter will indicate if the client is slow to release state.

Some workloads and client behaviors may require faster recall of client state to keep up with capability acquisition. It is recommended to increase the above counters as needed to resolve any slow recall warnings in the cluster health state.

MDS Cap Acquisition Throttle

A trivial “find” command on a large directory hierarchy will cause the client to receive caps significantly faster than it will release. The MDS will try to have the client reduce its caps below the mds_max_caps_per_client limit but the recall throttles prevent it from catching up to the pace of acquisition. So the readdir is throttled to control cap acquisition via the following configurations:

The threshold and decay rate for the readdir cap acquisition decay counter:

mds_session_cap_acquisition_throttle (default: 500K)

and:

mds_session_cap_acquisition_decay_rate (default: 10 seconds)

The cap acquisition decay counter controls the rate of cap acquisition via readdir. The behavior of the decay counter is the same as for cache trimming or caps recall. Each readdir call increments the counter by the number of files in the result.

The ratio of mds_max_maps_per_client that client must exceed before readdir maybe throttled by cap acquisition throttle:

mds_session_max_caps_throttle_ratio (default: 1.1)

The timeout in seconds after which a client request is retried due to cap acquisition throttling:

mds_cap_acquisition_throttle_retry_request_timeout (default: 0.5 seconds)

If the number of caps acquired by the client per session is greater than the mds_session_max_caps_throttle_ratio and cap acquisition decay counter is greater than mds_session_cap_acquisition_throttle, the readdir is throttled. The readdir request is retried after mds_cap_acquisition_throttle_retry_request_timeout seconds.

Session Liveness

The MDS also keeps track of whether sessions are quiescent. If a client session is not utilizing its capabilities or is otherwise quiet, the MDS will begin recalling state from the session even if it’s not under cache pressure. This helps the MDS avoid future work when the cluster workload is hot and cache pressure is forcing the MDS to recall state. The expectation is that a client not utilizing its capabilities is unlikely to use those capabilities anytime in the near future.

Determining whether a given session is quiescent is controlled by the following configuration variables:

mds_session_cache_liveness_magnitude (default: 10)

and:

mds_session_cache_liveness_decay_rate (default: 5min)

The configuration mds_session_cache_liveness_decay_rate indicates the half-life for the decay counter tracking the use of capabilities by the client. Each time a client manipulates or acquires a capability, the MDS will increment the counter. This is a rough but effective way to monitor the utilization of the client cache.

The mds_session_cache_liveness_magnitude is a base-2 magnitude difference of the liveness decay counter and the number of capabilities outstanding for the session. So if the client has 1*2^20 (1M) capabilities outstanding and only uses less than 1*2^(20-mds_session_cache_liveness_magnitude) (1K using defaults), the MDS will consider the client to be quiescent and begin recall.

Capability Limit

The MDS also tries to prevent a single client from acquiring too many capabilities. This helps prevent recovery from taking a long time in some situations. It is not generally necessary for a client to have such a large cache. The limit is configured via:

mds_max_caps_per_client (default: 1M)

It is not recommended to set this value above 5M but it may be helpful with some workloads.