[HBASE-5979] Non-pread DFSInputStreams should be associated with scanners, not HFile.Readers

• Type: Improvement
• Status: Closed
• Priority: Critical
• Resolution: Incomplete
• Component/s: Performance, regionserver
• Assignee: Unassigned
• Reporter: Todd Lipcon
• Created: 09/May/12 22:32
• Updated: 13/Jun/22 16:23
• Resolved: 13/Jun/22 16:23

Description

Currently, every HFile.Reader has a single DFSInputStream, which it uses to service all gets and scans. For gets, we use the positional read API (aka "pread") and for scans we use a synchronized block to seek, then read. The advantage of pread is that it doesn't hold any locks, so multiple gets can proceed at the same time. The advantage of seek+read for scans is that the datanode starts to send the entire rest of the HDFS block, rather than just the single hfile block necessary. So, in a single thread, pread is faster for gets, and seek+read is faster for scans since you get a strong pipelining effect.

However, in a multi-threaded case where there are multiple scans (including scans which are actually part of compactions), the seek+read strategy falls apart, since only one scanner may be reading at a time. Additionally, a large amount of wasted IO is generated on the datanode side, and we get none of the earlier-mentioned advantages.

In one test, I switched scans to always use pread, and saw a 5x improvement in throughput of the YCSB scan-only workload, since it previously was completely blocked by contention on the DFSIS lock.

Issue Links

• is related to: HBASE-7347 Allow multiple readers per storefile
• is related to: HBASE-13291 Lift the scan ceiling