Cache Tiering - Improve Efficiency of Read-Miss Operations¶
Suggested changes to the way read-misses are fulfilled from the cache tier to improve efficiencies.
Narendra Narang (Red Hat)
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Write operations to a cache tier are 3x replicated for durability. To fulfill a read operation not in the cache tier (aka a read-miss operation) is also 3x replicated i.e. the data for a read-miss operation is fetched from the backing tier and 3 copies of it are stored in the caching tier.
A "cache" tier would typically be configured as a 3x replicated pool. Writes to a cache tier would follow the same rules for durability and immediate consistency i.e. CRUSH -> primary OSD, secondary OSD, tertiary OSD. All 3 write ops to the respective OSD journals would need to be committed before sending an acknowledgement of commit back to the client. Then at some point, based on the LRU algorithm, the writes would be aged out to a backing (most likely configured as an erasure coded) tier.
- First, a read operation which serves the request for I/O directly from the cache tier. This "cache hit" scenario is ideal because there is no additional operation to locate and read/promote the data from the backing tier
- Second, and a not so ideal scenario, is a read "cache miss" which isn't able to fulfill the read I/O request from the cache tier. So it now has to fetch and promote the data from the backing tier to the caching tier. Additionally, Ceph first promotes the data from the primary OSD's backing tier to the cache pool tier and then also copies this data, over the network, to make 2 more copies elsewhere in the cache pool. Basically, it's promoting, copying and then storing multiple (3) copies in the cache tier, across the cluster's cache pool before it responds to and fulfills the read I/O request.
- It waits to serve the request for read I/O until 3x copies are stored in the cache tier and thereby increases response time
- It has to copy this "redundant" data over the network and thereby results in traffic overhead
- It "populates" copies of this data unnecessarily on expensive SSDs and thereby reduces efficiencies (cost/performance) of this fast tier.
For a write, storing 3x copies in the cache tier is desirable for durability. However, the same behavior is not ideal for read (miss) operations, since the read request is directed to the primary OSD anyway. In the event of a failure of either the primary OSD or the primary OSD's node, Ceph could locate and promote the data from the alternate OSDs.
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Build / release tasks