+1 on changing no readahead to true by default for pread. Bryan Beaudreault <bbeaudrea...@apache.org> 于2023年5月31日周三 20:44写道：
> Hello team, > > I recently discovered "hbase.store.reader.no-readahead", which defaults to > false (so readahead is enabled). This only applies to PREAD reads, not > STREAM reads which always use readahead. When readahead is enabled, the > default readahead amount in the DFSClient is 4mb. In my opinion this is > extremely huge for HBase's use-case. > > Further, reads in HBase are always for a block at a time and blocks > typically have more than one row in them. So we are already reading ahead a > bit via block reads. And lastly, readahead is typically useful for > sequential read scenarios. It's unlikely for someone to do sequential IO > via PREAD, instead they would use Scans (thus STREAM). In the case where > someone is doing sequential IO via PREAD, they'd get some natural readahead > due to our reading of blocks at a time. > > I disabled readahead on about 50 servers across various clusters in our > production environment, and saw a massive (10x or more) drop in disk IO for > random read and mixed read cases. Scan workloads were mostly unaffected due > to not using this setting. I also did a targeted load test of a cluster, > with and without readahead, and was able to get double the random read > throughput with it disabled. > > I'd like to update the default for this config to "true", thus disabling > readahead for PREAD by default. I also think it's worth investigating > making readahead configurable for STREAM reads, perhaps based on the scan's > max result size or blockBytesScanned of the last next() call. > > Any objections to changing the default? > > https://issues.apache.org/jira/browse/HBASE-27896 >
