YUBI LEE created IMPALA-15229:
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Summary: SHOW TABLE STATS / SHOW PARTITIONS / SHOW FILES issue one
NameNode RPC perpartition/file for the display-only EC Policy column
Key: IMPALA-15229
URL: https://issues.apache.org/jira/browse/IMPALA-15229
Project: IMPALA
Issue Type: Improvement
Reporter: YUBI LEE
{{FileSystemUtil.getErasureCodingPolicy(Path)}} issues one
{{getErasureCodingPolicy}} RPC per
path, uncached, from a serial loop, called once per partition by SHOW TABLE
STATS / SHOW
PARTITIONS and once per file by SHOW FILES — only to fill a display-only
column. Nothing
in planning, scheduling, or the read path consumes it ({{FileDescriptor.isEc}},
sourced from
the listing, covers that separately).
Production impact (non-EC HDFS cluster): one SHOW TABLE STATS on a
~105k-partition table
generated ~105k serial NameNode RPCs; audit logs showed ~952k calls in a
5-minute window,
tripping RPC-spike alerts and adding tens of seconds of statement latency.
Every call
returned NONE.
The listing that loads file metadata already carries the EC policy in
{{HdfsFileStatus}}.
Record the policy id in the file descriptor at load time and derive the column
from it;
resolve ids to names locally (system policies) or with one cached
{{getAllErasureCodingPolicies()}} call per filesystem. Fall back to the
per-path lookup only
for empty partitions and metadata written before the upgrade.
Alternatives considered:
- Removing the column introduced by IMPALA-9487 altogether. If the community
feels the
column does not carry its weight, that is also a valid resolution — but it
changes the
output schema of shipped statements that tools and tests parse, so it would
need a
deprecation discussion. Deriving the value from metadata we already have
keeps the
feature at roughly zero cost, which seemed like the less disruptive fix.
- A Hadoop configuration switch to skip the lookups (in the style of
impala.preload-block-locations-for-scheduling). This works as a mitigation
but leaves
the default behaviour expensive and drops the information where it is
disabled.
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