See Block Device for additional details.
The user space implementation of the Ceph block device (i.e., librbd) cannot take advantage of the Linux page cache, so it includes its own in-memory caching, called “RBD caching.” RBD caching behaves just like well-behaved hard disk caching. When the OS sends a barrier or a flush request, all dirty data is written to the OSDs. This means that using write-back caching is just as safe as using a well-behaved physical hard disk with a VM that properly sends flushes (i.e. Linux kernel >= 2.6.32). The cache uses a Least Recently Used (LRU) algorithm, and in write-back mode it can coalesce contiguous requests for better throughput.
New in version 0.46.
Ceph supports write-back caching for RBD. To enable it, add rbd cache = true to the [client] section of your ceph.conf file. By default librbd does not perform any caching. Writes and reads go directly to the storage cluster, and writes return only when the data is on disk on all replicas. With caching enabled, writes return immediately, unless there are more than rbd cache max dirty unflushed bytes. In this case, the write triggers writeback and blocks until enough bytes are flushed.
New in version 0.47.
Ceph supports write-through caching for RBD. You can set the size of the cache, and you can set targets and limits to switch from write-back caching to write through caching. To enable write-through mode, set rbd cache max dirty to 0. This means writes return only when the data is on disk on all replicas, but reads may come from the cache. The cache is in memory on the client, and each RBD image has its own. Since the cache is local to the client, there’s no coherency if there are others accessing the image. Running GFS or OCFS on top of RBD will not work with caching enabled.
The ceph.conf file settings for RBD should be set in the [client] section of your configuration file. The settings include:
|Description:||Enable caching for RADOS Block Device (RBD).|
rbd cache size
|Description:||The RBD cache size in bytes.|
rbd cache max dirty
|Description:||The dirty limit in bytes at which the cache triggers write-back. If 0, uses write-through caching.|
|Constraint:||Must be less than rbd cache size.|
rbd cache target dirty
|Description:||The dirty target before the cache begins writing data to the data storage. Does not block writes to the cache.|
|Constraint:||Must be less than rbd cache max dirty.|
rbd cache max dirty age
|Description:||The number of seconds dirty data is in the cache before writeback starts.|
New in version 0.60.
rbd cache writethrough until flush
|Description:||Start out in write-through mode, and switch to write-back after the first flush request is received. Enabling this is a conservative but safe setting in case VMs running on rbd are too old to send flushes, like the virtio driver in Linux before 2.6.32.|