wjhypo commented on a change in pull request #11309:
URL: https://github.com/apache/druid/pull/11309#discussion_r739540220
##########
File path:
server/src/main/java/org/apache/druid/server/coordination/SegmentLoadDropHandler.java
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@@ -369,7 +392,7 @@ private void addSegments(Collection<DataSegment> segments,
final DataSegmentChan
numSegments,
segment.getId()
);
- loadSegment(segment, callback, config.isLazyLoadOnStart());
+ loadSegment(segment, callback, config.isLazyLoadOnStart(),
loadSegmentsIntoPageCacheOnBootstrapExec);
Review comment:
Hi Parag, thanks for the comment! To clarify, the point of the PR is to
provide best effort precache of segments but not 100% guaranteed precache of
segments. The reason is we don't want to sacrifice availability.
Image we have a data source with one replica configured and the host serving
it dies, then we want the missing segments to be available ASAP on other
replacement host. This is the case of loading segments into a complete new host
with its page cache empty of the segments to load. Copying segments to null
stream can take some time and if you have a lot segments, it can easily take
more than 10 mins to complete the full read into page cache which is too long
period of unavailability. In production, we tried the synchronous loading
before announcing the segment, and it was indeed too slow. Another case is if
we just restart the Druid historical process on the same host, then the process
of reading segments into null stream is pretty fast as the segments are already
cached in page cache by OS but it will still take some time compared with
announcing segments directly after download.
That said, the strategy of announce segment immediately after download and
asynchronously read into null stream afterwards still have value: without the
change, since OS will only mmap the portion of segments a query needs, even
after days of serving a segment in a historical, a different query hitting a
different portion of segment will still need to trigger disk reads. With the
change, we can ensure after the first 10 mins or so after downloading the
segments into a historical (depending on the number of segments), all
subsequent queries won't hit disk.
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