Wellington Chevreuil created HBASE-28596:
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Summary: Optimise BucketCache usage upon regions splits/merges.
Key: HBASE-28596
URL: https://issues.apache.org/jira/browse/HBASE-28596
Project: HBase
Issue Type: Improvement
Reporter: Wellington Chevreuil
Assignee: Wellington Chevreuil
This proposal aims to give more flexibility for users to decide whether or not
blocks from a parent region should be evict, and also optimise cache usage by
resolving file reference blocks to the referred block in the cache.
Some extra context:
1) Originally, the default behaviour on splits was to rely on the
"hbase.rs.evictblocksonclose" value to decide if the cached blocks from the
parent split should be evicted or not. Then the resulting split daughters get
open with refs to the parent file. If hbase.rs.prefetchblocksonopen is set,
these openings will trigger a prefetch of the blocks from the parent split, now
with cache keys from the ref path. That means, if "hbase.rs.evictblocksonclose"
is false and “hbase.rs.prefetchblocksonopen” is true, we will be duplicating
blocks in the cache. In scenarios where cache usage is at capacity and added
latency for reading from the file system is high (for example reading from a
cloud storage), this can have a severe impact, as the prefetch for the refs
would trigger evictions. Also, the refs tend to be short lived, as compaction
is triggered on the split daughters soon after it’s open.
2) HBASE-27474 has changed the original behaviour described above, to now
always evict blocks from the split parent upon split is completed, and skipping
prefetch for refs (since refs are short lived). The side effect is that the
daughters blocks would only be cached once compaction is completed, but
compaction itself will run slower since it needs to read the blocks from the
file system. On regions as large as 20GB, the performance degradation reported
by users has been severe.
This proposes a new “hbase.rs.evictblocksonsplit” configuration property that
makes the eviction over split configurable. Depending on the use case, the
impact of mass evictions due to cache capacity may be higher, in which case
users might prefer to keep evicting split parent blocks. Additionally, it
modifies the way we handle refs when caching. HBASE-27474 behaviour was to skip
caching refs to avoid duplicate data in the cache as long as compaction was
enabled, relying on the fact that refs from splits are usually short lived.
Here, we propose modifying the search for blocks cache keys, so that we always
resolve the referenced file first and look for the related referenced file
block in the cache. That way we avoid duplicates in the cache and also expedite
scan performance on the split daughters, as it’s now resolving the referenced
file and reading from the cache.
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