Jay Kreps created KAFKA-3015:
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Summary: Improve JBOD data balancing
Key: KAFKA-3015
URL: https://issues.apache.org/jira/browse/KAFKA-3015
Project: Kafka
Issue Type: Improvement
Reporter: Jay Kreps
When running with multiple data directories (i.e. JBOD) we currently place
partitions entirely within one data directory. This tends to lead to poor
balancing across disks as some topics have more throughput/retention and not
all disks get data from all topics. You can't fix this problem with smarter
partition placement strategies because ultimately you don't know when a
partition is created when or how heavily it will be used (this is a subtle
point, and the tendency is to try to think of some more sophisticated way to
place partitions based on current data size but this is actually exceptionally
dangerous and can lead to much worse imbalance when creating many partitions at
once as they would all go to the disk with the least data). We don't support
online rebalancing across directories/disks so this imbalance is a big problem
and limits the usefulness of this configuration. Implementing online
rebalancing of data across disks without downtime is actually quite hard and
requires lots of I/O since you have to actually rewrite full partitions of data.
An alternative would be to place each partition in *all* directories/drives and
round-robin *segments* within the partition across the directories. So the
layout would be something like:
drive-a/mytopic-0/
0000000.data
0000000.index
0024680.data
0024680.index
drive-a/mytopic-0/
0012345.data
0012345.index
0036912.data
0036912.index
This is a little harder to implement than the current approach but not very
hard, and it is a lot easier than implementing online data balancing across
disks while retaining the current approach. I think this could easily be done
in a backwards compatible way.
I think the balancing you would get from this in most cases would be good
enough to make JBOD the default configuration. Thoughts?
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