Norbert Schultz created CASSANDRA-19569:
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Summary: sstableupgrade is very slow
Key: CASSANDRA-19569
URL: https://issues.apache.org/jira/browse/CASSANDRA-19569
Project: Cassandra
Issue Type: Bug
Reporter: Norbert Schultz
Attachments: flamegraph_ok.png, flamegraph_sstableupgrade.png
We are in the process of migrating cassandra from 3.11.x to 4.1.4 and upgrading
the sstables using sstableupgrade from Cassandra V4.1.4, from `me-` to `nb-`
Format
Unfortunately, the process is very very slow (less than 0.5 MB/s).
Some observations:
- The process is only slow on (fast) SSDs, but not on ram disks.
- The sstables consist of many partitions (this may be unrelated)
- The upgrade process is fast, if we use `automatic_sstable_upgrade` instead of
the sstableupgradetool.
- We give enough RAM (export MAX_HEAP_SIZE=8g)
On profiling, we found out, that sstableupgrade is burning most CPU time on
{{posix_fadvise}} (see attached !flamegraph_sstableupgrade.png! ).
My naive interpretation of the whole {{maybeReopenEarly}} to {{posix_fadvise}}
chain is, that the process just informs the linux kernel, that the written data
should not be cached. If we comment out the call to
{{NativeLibrary.trySkipCache}}, the conversion is running at expected 10MB/s
(see !flamegraph_ok.png! )
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